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MG GURUyour resource for MGA, MGB, MGC and Midgets
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So i took my B out today after doing some work this labor day weekend. I love the Garmin 205, Nuvi — specifically for the screen capture function that it has built in. I’m heading East on Rt. I-70 in Colorado away from Denver. Here is a screen capture of me going almost 90 MPH.
I love this car!
THE MALFUNCTIONING SPEEDOMETER
Non-speedometer Issues
You have almost nothing to fear except fear itself. You CAN fix your own speedometer. Here is some information that may help you. The first issue is to decide whether the speedometer itself is actually the cause of the problem. Some faults that are thought to be due to a malfunctioning speedometer are actually due to a problem with the cable running to the speedometer, or with the drive gear in the transmission. Frequently the cable itself is the cause of a wavering speedometer pointer. It is unfortunately somewhat difficult to service the cable. You must be prepared to get under the car and remove the cable from the transmission. However, it is sometimes possible to service the cable simply from the speedometer end. You need to remove the speedometer, and then pull up some slack in the cable so the end of the cable is protruding slightly from the dash.
Then you may pull out the wire cable from the outer sheath. Lubricate the cable with white lithium grease or gear oil and then slide the cable back into the sheath. As you get to the last couple of inches you need to slowly spin the cable as you insert it. This will allow the square end of the cable to seat in the square orifice in the transmission drive gear (hopefully). If, after multiple attempts, you cannot get the cable to seat, then you will have to get under the car, and unscrew the cable from the transmission. Then press the cable fully in the sheath, and attach the speedometer. Then, back under the car, you must gently seat the cable into the drive on the transmission and screw it down securely. Test the speedometer with the newly lubricated cable. Test this before fully re-installing the speedometer in the dash.
Other causes of a wavering speedometer needle lie inside the speedometer itself. I have seen binding of the input shaft cause wavering as it slows down, then breaks free and turns faster briefly. Binding can also occur between the shaft and the retaining flange. I have also seen binding in the odometer wheels (particularly the “old” style) cause cyclic resistance against turning, resulting in wavering. There can also be a dirt or lack of lubrication in the needle bushing between the magnet wheel and the pointer spindle. Binding odometer wheels and needle bearings often will cause speedometer wavering that is proportional to road speed.
PARTS EXCHANGE GUIDE
Repairs of the speedometer and odometer sub-sections can be made by exchange with intact/functional parts from other Smiths or Jaeger speedometers. Many parts will be common across a broad range of models and years. There are four main variations (that I know of) that will influence the possibility of exchange. Within a specific type, parts seem to be completely interchangeable. “Old” models have all metal construction except for the worm gear and also have separately driven main and trip odometers. “Intermediate” have plastic odometer wheels, and the trip odometer wheels are more widely spaced. The “new” models have mostly plastic construction and a gear drives the trip odometer from the main odometer, so there is only one worm and pawl. The spindle bearing in the magnet wheel is more shallowly set in the “new” type of speedometers. Triumphs seem to have had a slightly different variant speedometer than the MGs. The primary difference is that the spindle to which the pointer is attached is longer (.180″ vs. .150″) and has a somewhat narrower taper (.035 to .032 vs .035 to .030). This makes it less than optimal to move the works from an MG to a Triumph because the Triumph pointer fits slightly loosely.
The move of a Triumph works to an MG is even less possible due to the MG pointer being too tight to fit on the TR works. The diameter of the base of the long and short spindles is approximately the same, so the longer one reaches a narrower tip. It is possible to shorten the MG spindle to the Triumph length and thereby have the diameter correct for the Triumph pointer. Use a file or rotary stone on a dremel tool to shorten the spindle by a 30 thousandths or so, and try refitting the pointer. If it will not slide on, there may be a burr on the tip, so use a fine file to chamfer the edge.
Depending on the calibration required, the worm on the input shaft may have 20, 25 or 32 teeth (there could be others but I have not seen them). It appears that 32 teeth were very commonly used on the “old” and “intermediate” versions, with 20 and 25 are also seen. 20 and 32 teeth were used on the “new” styles. There was a wide variety of gears used on the odometer wheels to provide the final calibration. The calibration of the odometer is the number of teeth on the worm gear multiplied by the number of teeth on the odometer wheel gear. This gives the number of input shaft turns for each odometer shaft turn.
Of course parts are completely interchangeable between identical units, but many parts are carried across a broad range of speedometers, and will be completely interchangeable. For instance there are only two types of magnet wheels that I have identified.
One type has a shallowly set spindle bearing, and the other is more deeply set, so the magnet wheel can be interchanged quite freely with a similar type from any source. The main speedometer frame is identical across all models as far as I can tell, and are completely interchangeable. In the “old” and “intermediate” units, the spindle/main odometer frames are interchangeable as long as the pointer fits properly. In the end, it is usually possible to obtain sufficient parts to repair your speedometer without great difficulty or expense by visiting the tables of instruments at flea markets. KPH and MPH speedometers are essentially the same and parts exchange guidelines apply here as well. As far as I can tell, the actual speedometer function is exactly the same.
Figure 1.Cut away view of a typical speedometer.
Figure 2. Exploded diagram of a typical speedometer
Only the printing on the dial face is different. The odometers are also essentially the same. The KPH units have 62% fewer teeth on the gear mounted on with the odometer wheels so there will be more turns of the odometer for the same distance travelled. It is fairly easily to convert a KPH speedometer to an MPH unit. All you need to do is exchange the dial face and install the proper odometer gear to set the desired calibration. To convert from KPH to MPH, you can calculate the desired calibration by multiplying the calibration printed on the dial of the KPH speedometer by 1.609. Dividing by the same number may make the reverse calculation. The resulting number usually will not correspond to an actual calibration that was available on speedometers. You need to round to the nearest 20, 25,or 32 (the number of teeth on your worm gear). For example, a common TR6 KPH speedometer has a calibration of 740. This corresponds to an MPH calibration of 1190.6. This is just about centered between the two possible calibrations of 1180 and 1200. To settle the issue of the what calibration you REALLY should have, you ought to calculate your ideal calibration as described later in this manual, then translate that into a MPH/KPH calibration and then look for the best possible calibration available.
List of Smiths/Jaeger types:
Old Style: Separate main & trip odometer frames and drives (all 120 mph?) TR2 thru 3A
Type 1: 25 tooth worm gear
Type 2: 20 tooth worm gear
Intermediate style: Separate main & trip odometer frames and drives (all 120 mph?) All 32 tooth worm
gear (?) TR4 and TR4A (probably TR3B)
Type 1: Narrow trip odometer wheels (early)
Type 2: Wide trip odometer wheels (late)
New Style: Single main & trip odometer frame and drive (TR5/250? and TR6)
Type 1: 32 tooth worm gear (all 100, 120 mph) (MG only?)
Type 2 20 tooth worm gear (all 140 mph?) (TR only?)
Note: This summarizes the extent of my experience with TR and MG speedometers
Any additional information will be greatly appreciated and incorporated into the next edition of this manual.
THE SPEEDOMETER: Mechanical Description
The speedometer (speed indicator, not odometer) functions in just the same way as a tachometer. The cable spins a thin bar magnet. Just in front of the bar magnet is a disk mounted on a spindle. Also attached to this, on the same spindle, is the pointer that is visible over the dial face. When the bar magnet spins, it causes the disk (drag cup) just in front of it to try to spin as well. The amount of twisting force (torque) imparted by the magnet to the disk is proportional to the rotational speed of the magnet. If the magnet spins twice as fast, the torque is approximately twice as great. The spindle is attached to a flat coiled return spring to resist rotation. The amount the spring winds is proportional to the torque. In this manner, the pointer moves progressively farther as the magnet spins faster.
There is very little that can go wrong with the speedometer. The places where a problem can occur are the following.
1) The magnet wheel may not spin. In this case all functions cease and the drive cable (or angle drive) will break.
2) The spindle may not move freely due to the disk binding against an obstruction. In this case the pointer is stuck in one place or will not rise above a certain level. The spindle pivot may lack lubrication and this will cause the needle to jump from one speed to the next rather than move smoothly. Lack of lubrication between the spindle needle tip and the bearing in the center of the magnet wheel may cause chatter at certain speeds.
3) The return spring (hair spring) may be broken. In this case the pointer will wind completely around until it hits the stop. A professional repair is required.
4) If everything moves normally, but it indicates the wrong speed, it is probably out of calibration. This is unlikely unless the pointer has been touched directly or you are using tires that vary substantially in rolling diameter from the originals. Also, violent swinging of the pointer may cause it to shift if it hits the stop forcefully.
5) If there is still error at other speeds after calibration, the hair spring may have weakened. A weakened spring will move more for a given speed than expected by the markings on the face. While you can calibrate it for a certain speed, it will register low below that speed and high above the speed. It is possible to unsolder the return spring, pull it tighter, and re-solder, but it is probably best to leave this to a professional speedometer repair shop. Alternatively, you may be able to swap the spindle/main odometer section of another unit. I have heard that it is possible for the bar magnet to lose its magnetism over time. If this happens, it is as if the hair-spring is too strong, and you will see the opposite problem than the weak spring described above. Professional shops are supposedly able to re-magnetize the bar magnet.
SPEEDOMETER: Inspection
To remove the speedometer works, first remove the metal bezel and the glass. Remove the two screws on the back and, if necessary, the tiny screw holding the reset cable (TR2-4). Then press the reset shaft (TR2-4) into the case and then push the threaded end (where the speedometer cable attaches) inward. The works should slide forward out of the case. There may be some adhesion to the rubber gasket inside the case, If the works do not move freely check the reset shaft and be sure that it is not catching on the case. Use a screwdriver to press it free.
Figure 3. This shows the works of the three major variations of speedometers. “Early” on the left, “Intermediate” in the middle, and “Late on the right.
Figure 4. Now the odometer wheel frame has been removed from the magnet wheel frame. This is a view of the underside of the odometer wheel frame with the attached drag cup. In the center of the drag cup is the thin spindle which fits into the bushing in the center of the magnet wheel.
Figure 5. This is a view of the front of the magnet wheel after removal of the odometer wheel frame. In the center is the brass bushing in which the spindle of the pointer rides.
When the works are free of the case you can now inspect them. The disk to which the pointer is attached should move freely. Twist the entire works back and forth. The pointer should move. You can use your finger to gently move the silver disc (drag cup). That should make the pointer move as well. Turn the speedometer drive at the attachment point of the cable by hand. This can be achieved by inserting a screwdriver into the square end of the input shaft and twisting counterclockwise. You should see the worm move and after 32 turns (or 20/25) the pawl(s) should have gone through one complete cycle. As the pawl cycles, it should push the 1/10’s wheel ahead by one tooth. The wheel ought to have a ratcheting action to prevent the wheel from rolling backward.
SPEEDOMETER: Dismantling
Make a scratch on the drag cup in line with some obvious landmark on the frame while the pointer is resting at zero. This will allow you to reinstall the pointer without any significant loss of calibration. Remove the pointer by gently turning the drag cup until the pointer is at 60 mph. Then gently hold the drag cup in place. Hold the pointer by the hub and pull and twist the pointer until it pops free of the spindle. Now remove the two small screws attaching the face to the frame.
The works are now free to work on. Much of the service can be done with no further disassembly.
Removing the odometer wheel sets will allow access to the magnet wheel and the seat of the needle pivot. Depending on the vintage of the speedometer the two wheel sets may be separate (early) or connected (late). You can tell if they are separate by looking for two separate gears driven off the worm on the shaft of the magnet wheel. The wheel sets and their frames are held in place by 4 small screws oriented on the top, bottom, left and right sides. These may be removed. If it uses a separate frame, the trip odometer may be lifted free after removing the small spring retaining the pawl. Remove the spring holding the pawl on main odometer. The spring is not present on the “new” style units.
Gently turn the works upside down while supporting the upper part of the frame and then lift the bottom half of the frame free. You will have to move the main odometer pawl to allow the parts to separate. You may rest the upper frame and the main odometer wheels out of the way canted to one side upside down. Just do not allow it to rest on the spindle where the pointer attaches. To do so may stretch out the flat coil return spring.
NOTE: IT IS ESSENTIAL TO AVOID DAMAGE TO THE RETURN SPRING. WHEN THE SPEEDOMETER IS DISMANTLED THE DRAG CUP AND THE RETURN SPRING ATTACHED TO IT ARE POORLY SUPPORTED. IT IS EASY TO DAMAGE THE SPRING UNLESS YOU ARE CAREFUL TO SUPPORT THE DRAG CUP AT ALL TIMES.
The older speedometers had two screws holding the retaining flange of the magnet wheel and the input shaft to the frame. The newer ones are riveted. If you strongly suspect a problem with the input shaft bushing lubrication, you could drill out the rivets and then maybe it is possible to tap threads into the frame to replace the rivets with screws. I have never done this, so I can not comment on its feasibility. You will need to be very selective about which brass (non-magnetic) screws to use to re-secure the flange as protruding heads will interfere with the magnet wheel. If you have screws, they may be removed and the magnet wheel and shaft may be withdrawn from the frame. Clean the highly polished input shaft and the bushing in the frame. You may use light grease to re-lubricate it. Apply ample grease in the narrow mid-sections to provide lubrication for the long term. Once greased, shaft can be replaced in the frame. Try spinning the magnet wheel. It should move freely, but the close tolerance of the input shaft as well as the grease do not allow it to spin multiple revolutions without constant pushing.
Nevertheless, the resistance to movement should be very small. The speedometer pointer spindle rides in a small bushing in the center of the magnet wheel. I do not know what sort of lubrication was used originally. They may have used none. The bushing may be cleaned with a jet spray of electrical contact cleaner. I got some from Radio Shack that included mineral oil lubricant. I think that “Brakleen” could also be used, and then you may need to use a very light machine oil. Engine oil will be too thick. Since the magnet wheel is constantly spinning, you want to have as little transfer of torque to the pointer spindle as possible. Viscous lubricant would cause significant torque on the spindle. This is the reason that I suspect that the spindle originally had no lubricant between the steel needle-tipped spindle and the brass bushing.
Now you may reassemble the lower frame to the odometer frame(s). Be sure to avoid damage to the spindle as you refit the frames. Also you need to have the lower frame rotated to the proper position on the upper frame. If you use the wrong orientation, only the upper and lower screw holes will line up. Once the frames are properly aligned, they may be secured with one screw below the main odometer. If your odometer has a separate trip odometer, it may be installed now as well. Use the other three screws when it is in place. Now install the two return springs on the pawls. A fine forceps will make this job much easier!
Replace the face and pointer and reindex the pointer to the proper location based on the scratch made in the beginning. If necessary calibrate the speedometer as listed below.
SPEEDOMETER: Calibration
One very observant person pointed out to me that there is a small dot at about -5 mph on the dial (on tachometers too). He found that when he pulled out the stop post on which the pointer rests when reading “zero”, the pointer came to balance pointing at the dot. MG speedometers have a line in about the same position. If you are restoring a speedometer without appreciably changing its calibration, then you can probably use that dot to set the speedometer. Unless the hair-spring (return spring) or the magnet have lost strength over the years, then that dot ought to provide a fairly accurate initial calibration. If you are recalibrating the speedometer, then that dot will not be accurate at all and you will have to use the calibration technique outlined above.
One note, if you are recalibrating the speedometer to a setting considerably different from the original calibration, there will probably be some residual error particularly at the low speeds. At zero the pointer will have significantly more or less tension against the stop post. This may result in considerably more or less tendency for the pointer to move at low speeds resulting in low speed error. You can see that this is already an issue for the speedometer by looking at the noticeably smaller space between 0 and 10 than between 10 and 20. This is due to the rest pressure of the hair-spring on the pointer. At zero mph, there is NOT zero force on the pointer. This requires the magnet to have to turn at some speed just to get the pointer off the rest post. (about 5mph). Due to the error you might experience, it is best to calibrate the pointer to the speed you are most concerned about, such as the national speed limit. When calibrated to give no error at a certain speed, that is the ONE speed you can be positive about. Speeds close to that will have minimal error as well.
Driving Calibration
Calibration of your speedometer is easy. If you have not serviced it, it ought to be close to the correct speed unless you have installed tires of a significantly different rolling diameter. To fine-tune your calibration, you need to drive a measured mile at exactly 60 mph indicated on the dial (you may use any other speed as well, but 60 is a nice mid-range number to use). Use a stopwatch to time your mile trip. Your actual speed is 3600 divided by the number of seconds it took to drive the mile. Now remove the speedometer from the dash, and remove the works from the case. Do not remove the pointer or face. Move the pointer to the indicated speed you used in the mile trip (60mph). This is done by gently holding the drag cup in the works. Double check that the pointer is still at the indicated speed. Now gently push the pointer to the calculated speed while being sure that the disk does not move. Now pull up some slack speedometer cable into the dash and reconnect the works of the speedometer without reinstalling the case. Carefully perch the speedometer in the dash opening without allowing the dash to contact any moving parts. Time another measured mile at exactly 60. If the time is not very close to 60 seconds readjust the speedometer. You should be getting very close to perfectly calibrated with one or two re-settings of the pointer.
Bench Calibration
If your speedometer has been serviced, you can do a bench calibration quite easily. You need the speedometer, a tachometer, a speedometer or tachometer cable, and a drill that will turn about 1000 to 2000 rpm. First you need to find out exactly how fast your drill turns. Attach the drill to the drive end of the cable and attach the tachometer to the other end. Turn on the drill to maximum RPM in REVERSE. Watch the pointer of the tachometer. It should be quite steady. If it wavers significantly, either the drill does not turn a constant speed or the cable is binding. You can lube the cable by removing the center wire and greasing it lightly with lithium grease. Once you achieve a fairly constant reading, you can determine your drill RPM by dividing the reading by two (the ratio is indicated on the tachometer dial face: “2-1”). For instance my drill showed exactly 2400 rpm, so it was turning exactly 1200. This happens to be the maximum speed listed on its label. Now you need to calculate what the speedometer OUGHT to be reading at that RPM. You need to know one of two things. One is the calibration number printer on the dial face of the speedometer just above the “MPH”. This is the cable RPM that it takes to indicate 60 MPH. If you know that your tires are substantially different from those originally supplied with the car, you can not use that number as your final setting. If you DO know the engine RPM that corresponds with 60 MPH, then divide that by 2.5 (known correct for the TR2-6 series, other transmissions may differ. I suspect that Spitfires used 3.5) to determine the cable RPM that corresponds to 60 MPH. If you are not sure of this data, then set the speedometer to the printed calibration and then do the driving calibration discussed above. As an example, you want to set your speedometer to 1152 calibration. You need to calculate what MPH should be indicated when you run the drill at its maximum speed. The MPH will be 60 * RPM / 1152. So, in this case the MPH = 60 * 1200 / 1152 = 62.5. Now turn on the drill and look at the indicated speed on the speedometer. Turn off the drill and hold the disk with the pointer at the observed speed. Gently push the pointer to the calculated speed (62.5) and retest. Adjust the pointer until it reads exactly the calculated speed.
Now you can attach the speedometer to the cable in the car and drive a measured mile and adjust as outlined above. These adjustments will set the speedometer only. It will not set the odometer, which is gear driven as stated below.
THE ODOMETER: Mechanical Description
The odometer is gear driven. The cable turns a worm screw which turns a 32 (sometimes 20 or 25) tooth gear. This gear is directly attached to a pawl via an eccentric pivot. Every turn of the gear will pull the pawl once. The pawl turns a gear at the end of the odometer wheels. This gear has a different number of teeth depending on the calibration of the odometer. This calibration is written in small letters on the dial face above the “MPH”. 1152 and 1184 are the most common on the TR4 series, Other speedometers may use a 20 or 25 tooth gear and more teeth on the odometer wheel gear. Fewer teeth on the worm gear makes the movement of the odometer wheels much smoother. The calibration is always a whole multiple of the 32 (or 20,25) tooth gear. For instance, 1152 is 32*36. There are a few places where problems can occur.
1) The worm may not turn, and this is the same problem as #1 in the speedometer section.
2) The plastic 32 tooth gear can be stripped, or the clip that holds the pawl to the eccentric pivot may have fallen off allowing the pawl to fall from the eccentric.
3) The spring pulling the pawl to the gear on the wheels may be weak or missing. This may prevent the pawl from touching the gear and thereby prevent any motion of the wheels.
4) There is the very unlikely possibility that the wheels themselves no longer index properly.
ODOMETER: Calibration
Calibration of the odometer is not as simple as making an adjustment in the works somewhere. Being completely gear driven, you need to replace the gear on the odometer wheel axle that is moved by the pawl. To do this you need the appropriate gear from an otherwise identical speedometer with the proper calibration. The simplest method to do this is to determine what calibration you need. This can be calculated by driving a measured distance of road.
Figure 6. Diagram of “old-style” main odometer components
Figure 7. Diagram of “intermediate-style” odometer components.
Figure 8. Diagram of “new-style” odometer components
Figure 9. The main frame of the “Early” speedometer with the main odometer disassembled and the trip odometer frame removed. The “hair spring” is visible. On the other side of the frame is the drag cup. This frame is from an MGA and has the trip reset wheel entering straight into the frame. The metal fingers which allow indexing of the odometer wheels are visible on the lower section of the frame.
Figure 10. The disassembled parts of an “early” main odometer. The order of parts is from left to right and top to bottom. The brass keyed washers rest on top of each odometer wheel. These drag on the wheels and allow each to index when the wheel to the left releases the finger underneath.
Figure 11. An “early” trip odometer incorporating wheels identical to the main odometer. The trip odometer is mounted on its own sub-frame. The metal fingers are shown holding the wheels. It is clear how each wheel can release the finger restraining the wheel to the left.
Figure 12. This is from a TR6 (New-style). Viewed from the rear with the case removed. You can see the input shaft in the center. You also see the grey plastic gear, which is driven off the worm. Also visible here are the magnet wheel and the drag cup (silver). These will be shown in greater detail in upcoming photographs.
Figure 13. This is the same TR6 speedometer with the dial face removed. You can see the all plastic construction, and the single orange odometer drive gear on the left of the lower odometer set. You can also see the flatcoiled brass return spring in the center.
Figure 14. A view of the left side of the same speedometer. You can see the orange speedometer drive gear, the grey plastic retaining washer, the white plastic pawl that pushes the drive gear, and the dark metal retaining clip on the base of the pawl.
Figure 15. A close up of the front of the odometer drive gear. The yellow plastic legs to the right of the gear are attached to a plastic that has a splined surface and provided a ratcheting action to the gear. The legs prevent rotation of the yellow . Just to the right of the metal arm of the frame you can see the egde of a second grey plastic retaining washer.
Figure 16. A close up of the worm on the input shaft. The white gear is the 20-tooth worm gear. On the opposite end of this gear is the eccentric pivot to which the pawl attaches. The silver rim visible to the left of the gear is the drag cup that is turned by the magnetic action of the magnet wheel. The magnet wheel is not clearly visible here as it is recessed into the drag cup.
Figure 17. A view of the front of an “Old-style” speedometer. Note the separate odometer drive gears for each wheel set. Also note the all-metal construction. The trip odometer wheels are set immediately next to each other in contrast to later speedometers such as the new-style one in the earlier photographs (fig. 2). The shaft on the right is the trip odometer reset. The return spring is not in the same place as the later speedometer. It is just above the pointer under the odometer wheels.
Figure 18. A view of the rear of an old-style speedometer. Note the twin 32 tooth worm gears. One for each wheel set.
Figure 19. A view of the left of an old-style speedometer.. Note the all-metal construction. The shiny thin metal against the left side of the odometer drive gear is the combination retaining washer/ratcheting lock. The number “37″ written (by the manufacturer) on the frame is the number of teeth on the odometer drive gears.
The longer the better, ten miles minimum. Then compare the actual mileage to the indicated mileage. The calibration you will need will be: Old Calibration * Indicated Miles / Actual Miles. If you drive 20 miles and read your odometer to about 1/2 of a tenth, then you will get your correction factor to better than a half a percent and probably close to a quarter of a percent. This is certainly a better calibration than the car had originally.
You can calculate the THEORETICAL calibration you need by finding the “turns-per-mile” (TPM) specification of your tires. See APPENDIX I & III for calculating and measuring the TPI for your tires. Your driveshaft RPM (at 60 mph) is TPM * Differential Ratio. The differential for the TR2-4 series is usually 3.7. It will vary in other cars. You also need to know the number of driveshaft turns per cable turn. On the TR2 through early 6 series it is 2.5 drive shaft turns to 1 cable turn (possibly 3.5 for Spits). For a TR2-4, the odometer calibration you will need is calculated by TPM * 3.7/2.5. This is a very theoretical number and you are much better off by driving a distance on a marked highway (at any speed) and comparing the indicated milage to the real mileage. Unless you are very lucky, you will not be able to find a speedometer with exactly the required calibration. See APPENDIX II for a chart showing odometer calibrations. You can calculate the closest POSSIBLE (but not necessarily available) calibration by dividing the calibration you think you need by 32 (or 20, or whatever the number of teeth of the worm gear) then rounding to the nearest whole number. For instance, if you find that your speedometer reads 11 miles when you drove 10, and the calibration is 1184, then you need a new calibration of 1184*11/10 = 1302. When divided by 32, this is 40.7. This rounds to 41, which tells us that the closest possible calibration is 32*41= 1312. You need to look for a real speedometer with a calibration of 1312, or at least as close as possible to the calculated 1302. Any Smiths or Jaeger speedometer of the same vintage ought to be very similar in the works and may be able to be swapped. For example, I needed a 1280 speedometer and found one from an MGB of the 70’s vintage. It would have been a perfect match for a TR6 speedometer and was a serviceable match for a TR4 speedometer. One from the 1960’s probably would have been a perfect match. I will keep looking. After I did a 30-mile odometer calibration trip, I found that I need a calibration of 1344. This just happens to be a whole multiple of 32 (32*42), and it might be possible to find an odometer gear to give the exact calibration. Between all the cars that used these basic styles of works, there is a wide variety of calibrations to be found at flea markets. You should keep looking, but if you find a unit that has an odometer gear that is within one tooth of your “ideal” calibration (around 2.5% variance), it will probably be quite sufficient. Another option you have is to have an adapter made. It will have gears with a certain number of teeth to convert a certain input cable RPM into a certain output RPM. For instance, if I need a 1312 and I have a 1184 speedometer. They would probably make an adapter with 41 teeth on the input gear and 37 teeth on the output wheel. One manufacturer of these is APT Instruments in Bloomington, MN (612-881-7095). The cost for one is reported to be about $40. I spoke with them about the construction of an adapter. It seems that there is difficulty due to the difference between English drive cables and US types. They made it sound rather complicated, and I did not spend the time (I was paying long distance charges) to resolve the details of the difficulty. They suggested that the speedometer “head” be recalibrated. I am not sure that the guy I talked to understood the exact construction of a vintage Smiths/Jaeger speedometer. He did say that if I were to give him the true miles and odometer miles reading they would be calibrate it perfectly for $120, and for that price, throw in a cleaning too! They would also then check, calibrate, and if necessary repair the speedometer as well as to calibrate the odometer. With an adapter, you do not need to look for scarce or impossible speedometer calibrations and still keep your original equipment completely original. Of course, during the calibration tests you need to be using the tires you plan on keeping on the car and they need to be properly inflated. As the tires wear, the speedometer calibration will vary. It is not worth getting too picky about the exact accuracy of the calibration since tires of the same nominal size may differ by many tenths of a percent in their “turns-per-mile”.
ODOMETER: Inspection
Remove the works from the speedometer as described in the prior section. Turn the input shaft by hand as described previously. You should be able to twist the magnet wheel easily. As you turn the wheel, it should drive one or two gears from a worm. As each gear turns it should move a pawl via an eccentric pivot. The pawl should be seen to advance the 1/10’s wheel of the odometer one tooth for each pull.
To further test the odometer wheels, you will need to be able to spin the works at a reasonable speed. The only way to do this without crashing your car is to use a spare tachometer or speedometer cable and a drill set to reverse. Run the drill about 1000 rpm. As the drill turns the cable, you should see the worm gears turning and the pawls advancing the 1/10’s wheel every few seconds. As the 1/10’s passes 9, you should see the miles wheel advance by one.
If the gears and pawls are moving the wheels correctly, but the wheels fail to advance, then there is a problem with the wheels themselves. The best correction is to swap the entire wheel sets with a different speedometer of the same calibration. If that is not possible, then the wheels themselves may be changed by dismantling the wheel sets. You will need a wheel set from a similar speedometer, though the calibration will not matter.
Odometer: Mechanism
The “old” style odometers work by friction trying to turn all the odometer wheels and then a restraining clip underneath the wheel prevents motion except at certain times. The drive gear is keyed to the shaft and there are keyed washers between each of the wheels. The wheels themselves are not keyed and can turn freely. As the drive gear turns, it turns the shaft. The shaft turns the washers between the wheels. By friction, the wheels try to turn, but the clips prevent turning. The restraining clips underlie two adjacent wheels; so one wheel can disengage the clip under the next wheel to the left. The left and right edges of each wheel have a thin metal edge with notches. These notches engage the clips. On the right side of each wheel the edge has ten notches. The left side of each wheel has one notch. When looking at two wheels, as the right wheel turns one entire revolution, it disengages the clip under the left wheel once. The right wheel moves ahead by one notch, and then the clip re-engages and prevents further forward motion until again disengaged. The “intermediate” and “new” main and trip odometers function similarly. The “new” type has a different layout for the drive gear, but the odometer wheels function similarly. The drive gear spins the shaft, then the shaft turns the 1/10ths wheel. The main odometer has a 1/10ths wheel that is fairly narrow and has no lettering. The odometer wheels turn freely on the shaft and are indexed by a cog between adjacent wheels. The main odometer has the cog between the wheels so there is no gap between the wheels. The trip odometer has more widely spaced wheels and the cogs are external to the wheels. These odometers turn much more freely than the “old” types as they do not rely on friction to make any movement.
Odometer: Dismantling
It is usually only necessary to remove the pointer and face in order to access the odometer wheels. Further disassembly usually does not improve access to the odometer wheels, and exposes the pointer spindle and the coiled hair spring to potential damage. The worm-driven gears are held in place by a spring clip (see the exploded diagrams). Remove this clip and then the gear may be withdrawn from the frame. Clean the bushing in the frame and the gear. Lubricate it with light grease and reinstall it. The spring clip is difficult to install, but with patience it will snap in place. Damage to the odometer works is most likely to occur to this gear as it is the only plastic part in the pre-1968 (or so) units. In later units, the entire works is plastic and damage can occur anywhere.
The odometer wheels turn on an axle. This axle is held in place by a spring clip on the left side of the wheels. In the older units this clip is a metal snap that has a finger which locks into a recess in the yoke holding the axle. The top edge is lifted away from the yoke to unlock the snap, and then it is pulled upward to remove. Then the axle may be slid out to the left or right. Depending on your needs one direction may be more useful than the other. For instance, if you simply need to replace the gear to the left of the wheels, then pressing the axle to the right slightly will allow the gear to be removed without disturbing the odometer wheels themselves.
In newer units, a split nylon washer retains the axle. This washer is more difficult to unlock from the axle. You need to use a fine screwdriver to displace the leading edge of the washer out of its groove in the axle and then continue the displacement around the perimeter of the washer. Once it is out of the groove, the axle may be slid out through the washer. Again, depending on the direction you need to move the axle, you will need to displace the washer one way or the other on the axle.
When the axle is free, you will see that the gear is pressed against a washer by a spring. This washer has indentations that match similar ridges on the gear. This causes the gear to “ratchet” forward and inhibits motion of the gear without the pull of the pawl. When reinstalling the gear, washer, and spring, the spring tends to fly across the room if given the opportunity. Be very careful then compressing the spring and attempting to reinsert the axle through the spring. The wire of the spring will tend to snap into the groove for the locking snap/washer. A little fiddling with the axle and spring will allow the axle to move past the spring.
It is possible to change the odometer reading by rotating the wheels. The wheels have a locking mechanism that needs to be addressed in order to make the adjustment. Early speedometers have brass fingers under the wheels between the frame and the wheels which prevent forward movement, or backward movement past zero. You can rotate the wheels backward by simply turning the wheel, but you cannot back up past zero.
Figure 20. Exploded diagram of “Old-style” trip odometer.
Figure 21. Exploded diagram of “Intermediate style” trip odometer. The “New-style” trip odometer is essentially the same in a different frame.
To rotate forward or backward at will, you need to press the brass finger underneath the wheel to disengage the lock and then spin the wheel Newer odometers have cogs between the wheels. It is necessary to completely remove the wheels from the axle, then turn each wheel individually to the proper orientation, then re-stack the wheels and remount them on the axle. The trip odometers are essentially the same except that only the gear on the end (in this case the gear is to the right of the 1/10ths wheel) will ever need to be changed unless there is a damaged wheel. Newer odometers do not have a drive gear for the trip odometer. Instead there are gears in the works between the two-odometer mechanisms to drive the trip 1/10ths wheel off the unlabeled 1/10ths wheel of the main odometer. This completes the article about speedometer calibration, maintenance, and repair.
First of all, we’ll talk through Electricity & Magnets before we get into what’s the difference between Generators & Alternators.
Electricity & Magnets
This stuff is basic to any kind of electrical charging system, so you should understand this first. The only test will be if you know enough to do what you want to do without messing anything up. 
When you put electricity (current) down a wire, the wire will have a magnetic field around it. Conversely, if you move a wire through a magnetic field, a small current (electricity) is created in the wire. The more wires you use and/or the greater the strength of the magnetic field, the greater the effect becomes. These two inverse principles are the basis for electric motors, generators, alternators, and even things like the solenoid inside of a relay. If you have one item (movement or electricity), you can convert it into the other. Also tied in here is the fact that magnets repel and attract each other - that’s part of how you make an electric motor move. You can use more turns of wire (windings) to generate a stronger effect.
What about voltage vs. current? Well, current is a measure of how much stuff is flowing down a wire - kind of like the number of gallons of water that are flowing down a pipe every second. Voltage is a measure of pressure - like how many pounds per square inch (PSI) of air are in your tires. They measure different things, but they can be confusing since you can’t “see” electricity.
What about AC vs. DC? These stand for Alternating Current and Direct Current. AC is the stuff used in your house. DC is the stuff used in your car and what you get out a battery. The difference is that in DC current always flows in the same direction - from positive to negative (or, if you’re a real physics geek, from negative to positive) - while AC alternates the flow of current between the two directions at some rate. This rate is expressed a cycles per second, or Hz (pronounced “hurtz”). In the USA, the electricity in your house is changing directions at 60Hz - 60 times a second.
The final tid-bit of information is that when you spin wires and magnets near each other, you create AC in the wire. This is because the wire and magnets are continuously moving closer to and farther away from each other in a repeated cycle. As they move closer together, the current moves one way. As they more farther apart, the current goes the other way. If you’ve ever seen the typical “sine wave” graph of AC power, that exactly what I’m talking about here. This is important because you need some way to make that AC into DC to use it in your car. The process of charging AC into DC is called rectification. How you choose to do that is the key design difference between an alternator and a generator.
Generator
First up is the generator, also known as a dynamo. I explain it first because it functions in a more basic way and is easier for many people to understand. These are the original electrical generation units used on automobiles - it was much later on that alternators were invented and car manufacturers switched over to them. To understand alternators, you should make sure you have a basic understanding of generators as many of the pieces and basic theory are the same.
The generator is like an electric motor in reverse. Instead of applying electricity to it to make it spin, when you spin it, it makes electricity. It does this by spinning a series of windings of fine wire (called the armature) inside of a fixed magnetic field by connecting them to a belt and pulley arrangement on the engine. As the armature is spun by the rotation of the belt and pulley, it gets a current and voltage generated in those windings of wire. That current and voltage will be directly proportional to the speed that the armature spins and to the strength of the magnetic field. If you spin it faster, it makes more and if you make the magnetic field stronger it makes more current. The speed of the spinning is controlled by the speed of the engine - that’s why you need to rev the engine up to help charge the battery faster. The magnetic field is controlled by an electro-magnet, so by changing the amount of current supplied to the electro-magnets that make up the field you control the strength of the magnetic field. This current is referred to as the “field” current and it is controlled by the regulator in response to the electrical needs of the automobile at any given time.
The voltage of the generator is controlled by the number of windings in the armature. The current output varies widely from zero if the battery is perfectly charged and nothing is using any power up to the maximum rated output of the generator. The current output is controlled by the field current, but also by the speed at which the armature is spinning. This is important because a generator can only put out it’s maximum rated current at or above some speed - at lower speeds the output drops off very quickly. This is why a generator-equipped car will not charge (or even maintain!) the battery at idle and this is one of the main reasons for the development of the alternator.
The current generated in the armature is AC - not DC. To get it converted to DC so it can charge your batter and run your headlights, a device called a commutator is used to “rectify” this situation. It is on the armature and has a series of contacts along it’s outer surface. Two spring-loaded brushes slide on the commutator - one brush is connected to ground and the other is connected to the main output of the generator. As the armature and commutator assembly rotates, the brushes come touch the different contacts on the commutator such that the polarity of the current moving in the armature is always connected to the correct brushes. The net effect of this is that the generator output is always DC even though the current inside the armature windings is always AC.
A generator has to be “polarized” after the system is connected and before it is used. This is typically done by momentarily connecting the main output terminal of the generator to the battery with a jumper wire. This allows things to be set up so that the generator produces power of the correct polarity due to residual magnetism in the generator. For a simple visual image, imagine trying to jump start a car and reversing the jumper cables on one vehicle. It’s not something you really want to do - unless of course you like sparking, arcing, and possibly burning out electrical components… This is important if you ever disconnect a generator or regulator - you must polarize it (follow the instructions in the manuals for your car!) before starting the engine.
A generator will have three connections - the field, the armature, and ground, although the ground is sometimes an “implied” connection because everything is metal and is bolted together. The field terminal is the smaller of the two main connections and is typically labeled “F”. The armature is the bigger of the two main connections and is typically labeled “A” - this connections carries the main power output of the generator. Consult your manual for the specifics. All three connections go directly to the regulator and there will be a separate output on the regulator for the battery. The OEM regulator is almost always a mechanical device, although some aftermarket replacement units could be solid-state. (I don’t know of any myself, but it is theoretically possible to build one.) A typical generator wiring diagram from a 1958 Buick is below for reference - click on the image to see a larger view.
Alternator
The more modern and more capable alternator is explained here. Every modern vehicle uses an alternator - and for good reasons. It is more complicated than a generator, but that added complexity brings a few very good features that you will most certainly want on your vehicle - mainly the fact that it will charge the battery at idle and can support the higher amperages needed to run all of the electrical equipment on a modern vehicle. Alternators tend to be more reliable than a generator and have fewer “hard to diagnose” problems as the system ages - particularly the internally regulated models. The internally regulated models are also very easy to service if something goes wrong - there is only one part to fail (the alternator itself) and replacing it is a simple 30 minute job. This all adds up to the performance and reliability that is expected in a modern vehicle.
The key different between an alternator and a generator is what spins and what is fixed. On a generator windings of wire (the armature) spin inside a fixed magnetic field. On an alternator, a magnetic field is spun inside of windings of wire called a stator to generate the electricity. This allows the wires to be directly and easily connected to their outputs without the need for sliding contacts to carry the relatively high output current. The magnetic field is still generated via electro magnets mounted on a rotor, and the relatively small field current that powers them is supplied to the rotor by two small brushes that each ride on a separate and continuous slip rings. These smooth slip rings (unlike the comparatively rough contacts on a commutator in a generator) and the fact that the relatively heavy windings are fixed instead of rotating allows the alternator to be spun to much higher speeds. This allows it to reach it’s maximum output sooner and to be spun fast enough at engine idle speeds to produce enough electricity to power most (if not all) of the needs of the car without relying on the battery.
There are typically three separate windings of wire in the stator that are all set to so that the AC current that is generated is slightly out of phase in each one. The peaks and valleys of the rising and falling current do not happen at the same time, rather they are staggered a bit. This increases and smoothes the electrical output of the alternator much the same way that a 8 cylinder car runs more smoothly than a 4 cylinder one does - there are more power pulses happening in each revolution allowing more total power and better smoothness.
The process of rectifying the AC current into DC current is handled inside the alternator by something more complex than a commutator - diodes. A diode is a “solid state” device that allows current to flow in one direction only - “solid state” means it does this without any mechanical or moving parts. It relies on the different electrical properties of the materials it is made of to act as a one-way valve for current. By arranging diodes so that current from each of the three stator wires is only allowed to pass in one direction, and by connecting the three outputs together, you get a reasonably smooth and stable DC output without any moving parts. (This arrangement is typically manufactured as a single part and is referred to as the diode pack or diode trio.) This lack of moving parts makes the alternator not only very reliable - but also comparatively inexpensive to build and repair. That diode trio costs well something trivial like $1 to produce in large quantities.
Alternators do not need to be polarized after installation. You mount them to the engine, plug them in, and go. This is an advantage for not only manufacturing the car but for servicing it as well.
On externally regulated models, there are typically four connections on the alternator - the large output terminal (BAT), the ground terminal (GRD) which may be “implied” though the metal mountings of the alternator, the field connection (F), and terminal #2 on the regulator is a separate connection to one of the three poles on the stator (R). Unlike on a generator, the BAT terminal is directly connected to the battery and the rest of the cars wiring system, while only the F, R, and GRD connections will connect to the regulator. Also, terminal #3 on the regulator (if present) is connected to the main junction block for the wiring system and serves as a “remote voltage sensing” wire. Terminal #4 on the regulator will be connected via small wires to the charge indicator light on the dashboard of the car and the charge resistance wire. The regulator itself can be a mechanical or solid state device. A typical externally regulated alternator wiring diagram from a 1963 Buick is below for reference - click on the image to see a larger view.
On internally regulated models, there are also four connections on the alternator, but there is no separate regulator in the system - it is inside the alternator and constructed of solid-state components. The connections here are the large output terminal (BAT), the ground terminal (GRD) which may be “implied” though the metal mountings of the alternator, and two connections typically labeled simply 1 and 2. Terminal #1 on an internally regulated alternator is the same as terminal #4 on the regulator of an externally regulated system - it connects to a small wire that is goes to the charge indicator light on the dashboard of the car and the charge resistance wire. Terminal #2 on an internally regulated alternator matches terminal #3 on an external regulator - it is connected to the main junction block for the wiring system and serves as a “remote voltage sensing wire”. If you are comparing to the externally regulated wiring, then you will note that the F and 2/R wiring connections are done inside the alternator. A typical internally regulated alternator wiring diagram from a 1973 Buick is below for reference - click on the image to see a larger view.
When using a hydraulic jack to raise all 4 wheels of your B or Midget, put the car in neutral (be sure you are on a level surface!), disengage the parking break, raise the front of the car first, place jack stands under the left and right A arms, then raise the rear of the car and place the jack stands under the left and right leaf springs where the springs bolt to the body forward of the rear wheel well openings. An alternative position is to place jack stands on both sides of the rear axle. Lower the car in reverse order of the above process.
NEVER WORK UNDER A CAR SUPPORTED ONLY BY A JACK!
Make sure each jack stand is secure and that it will not slip from the position you have placed it before you get under the car!
When jacking the front of the car, place the jack paddle teeth in one of the two slots found in the front cross member. when jacking the rear of the car, place the jack paddle directly under the differential. If the differential drain plug extends below the differential take a 2×4 block of wood, drill a hole the size of your drain plug and position it between the differential and your jack paddle to avoid damage to the drain plug.
Never attempt to jack your MG by placing the jack under the floor panels or any other metal that is not supported by a structural beam or cross member.
My advice, BE SAFE by WORKING SAFE!
To check belt tension on the fan belt to your alternator: with the engine off, if you can turn the alternator pully counter-clockwise the belt is too loose. Loosen the bolts securing the alternator at its various mounting points and increase the tension on the belt by lifting up on the alternator. Do not pry the alternator with a pry bar as you may damage the alternator housing. Have an assistant hold the alternator in place while you tighten the bolts. Recheck to make sure you have the proper tension. Do not apply to much tension to the belt or you can damage the alternator and/or the water pump.
