How to Fix Your S3 Odometer

Norman Angerhofer writes: I too have a defective trip odometer. It won't reset to 0, and the numbers are generally up or down half a digit. Once I got it to partially reset, but to my consternation, it jammed the total mileage odometer, which quit increasing until I jabbed the reset shaft a time or two. I won't touch that reset shaft now, as "total miles" on the car is more important than "miles this trip". I'd love to know if the unit can be repaired.

At least how I did my 1986 North American car.

This seems to work fine for me and I think it is easy enough that anyone with enough patience and experience with precision parts can handle it. Be advised: it is easy to break the internals of the speedo so be careful!

First, remove the speedo from the dash. This is easily accomplished by pressing the outside ring firmly into the dash and rotating the entire unit counter-clockwise about 1/8 turn till it stops. The speedo should come out easily at this point. Pull the instrument lamp out of the back and disconnect the 7-pin connector to free the unit from the car.

On a clean and uncluttered work surface, carefully remove the outer ring from the front of the can by gently bending the 6 metal tabs out with a small screwdriver and needle-nose pliers. Disconnect the wiring harness from the three nuts on the back, noting which wire went where and pull out the turn-signal lamp and the ever-elusive bulb failure warning lamp.

The guts should come easily out of the can and you can see how this thing works. Very gently rotate the speed pointer counter-clockwise to loosen it from the shaft. It is press-fit on like a clock hand and should click loose and then pull off easily. If it does not, I would recommend stopping right here, as repairs to the movement will require a professional shop and it is somewhat delicate. Assuming that the pointer came off, remove the two black screws holding the face-plate on and lift it, the plastic light diffuser, and the two colored bezels off. The reset pin should be lifted off and the reset bar under the pin removed by lifting it up towards you.

Since cleaning this thing requires using a chemical contact cleaner, I decided to remove as much of the plastic as I could to avoid possible damages. At this point, release the two chrome light guides and the piece supporting them by pushing the hook-tab located under the "tens" wheel and sliding the assembly down and away from the face of the odometer. The two light guides lift out easily and the support piece can be removed by rotating mad pulling away.

Using a good grade of electrical contact cleaner (I used Contact Clean 2000 by Chemtronics) thoroughly wash the number wheels, advance pawls and gears to remove all grit. If you have compressed air, blow the thing dry or use the two-lips method. Once it is all dry, using a tooth-pick or other small tool, apply a tiny drop of thin SILICONE BASED lubricant to the cams between and to the left of each wheel. Petrol based lubes like WD-40 will eat the plastic and the wheels will never reset.

Replace the light diffusers (note the o-rings), support bracket and reset bar. I added a minute bit of silicone grease on the reset-bar guides to ease it's movement. Replace the dial face and plastic light diffuser with the colored lenses. Be sure the turn-signal arrow points the proper direction. Holding the unit so that the face is vertical and oriented as it would be in the car, firmly press the speed pointer back on the shaft so that it points to "0".

Dust the interior, clean the plastic lens and re-assemble as you took it apart. It is a good idea to wipe the rubber seal clean and totally clean the lens and metal rings before re-assembly. Drop the guts back into the can, re-attach the wiring harness and place the lens and ring back in place. Put the speedo face down on a firm surface protected with a soft rag (I drilled a hole in the bench for the reset pin to protrude into) and bend the tabs back down by hand pressure with a blunt ended punch. Re-install it in the car taking care to check which lamp goes where and enjoy your trip odometer.

Next time this thing dies, I think I am going to design a little digital unit to drop in the speedo as a replacement. I have a display that fits the hole size, just requires a bit of faceplate trimming, and the circuit is a cinch since the Jag uses a pulse to determine speed/distance. I might even design an interface to make the dash computer work for us lumpers! Anybody interested in a stop-watch integrated into the computer?