Repairing Jaeger and Smiths Speedometers
An Article by Anthony Rhodes Copyright 2000
Page 4
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.
- The magnet wheel may not spin. In this case all functions cease and the drive cable (or angle drive) will break.
- 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 pivots 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.
- The return spring may be broken. In this case the pointer will wind completely around until it hits the stop. A professional repair is required.
- 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.
- If there is still error at other speeds after calibration, the return 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 he 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 return 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.
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. You should see the worm move and after 32 turns (some TR6 models have 20) 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.
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