Why odometer gears break

[mxfrank]

My car had a broken odometer, so I disassembled it and installed a new drive gear. The old gear was almost ok, just one tooth broken near the tip. My odometer worked for a while, then stopped again. When I disassembled it, I found the new gear was fine, but the mechanism was jammed. I fiddled with it a minute, and the mechanism became free, but I couldn't see what caused the jam. Put it back together, happened again. I finally realized that there was too much end shake in the mile wheel set. The zinc drive gear was shifting away from the number gears, causing the follower to sometimes not land correctly on the number gear.

Unfortunately, I didn't take any photos, but I annotated this photo from a tech article here on Pelicanparts (apologies):



This photo is somewhat different from a late 944 odo, but the concepts are the same. In a 944, the odometer motor drives a train of gears which in turn drive the zinc wheel on the right. Above this wheel is a follower gear which has two sets of teeth. One set engages the zinc wheel, the other engages the plastic number wheel to it's left. For every full turn of the zinc wheel, the follower will cause the number wheel to move 1/10 of a revolution. Each number wheel has a matching follower which works the same way, rotating the wheel to it's left 1/10 rev for every full rev.

The problem with my odometer was that there was too much endshake in the stack of number gears, allowing the spacing between the wheels to become sloppy. The follower would engage incorrectly, and stick. From that point, it's a question of whether the torque of the motor can overcome the interference, or break a gear. Otherwise, the motor stalls and burns out (which would have happened with mine, if I had let it go too long). The solution was to drive out the shaft and reassemble with a washer to shim out the endshake. It's a delicate operation, but the odometer now seems stable and fully operational. If you're killing drive gears, there's your solution.

[AKCJ]

Thanks for posting.

My odometer sure acts like it's jamming as you describe. I've had it "repaired" twice. Looks like I may have to perform surgery some day.

Is it the same for an early and a late? I understand the picture is for an early?

[fmartenies]

I haven't gotten into the Porsche odometer but I have worked on old Honda motorcycle guages which has basically the same mechanism. What I have found is there is often a metal clip pressed on the end of the shaft that tends to get sloppy with age. I have also seen this as a plastic clip/spacer in the area you labeled end shake that breaks and falls off.

Bonus points on the washer. Working on guages is thankless work, which is why the trip meter on my bike always reads 666.

[sney]

I've heard it can also fail if you try to reset the trip odo without coming to a complete stop. That probably causes the exact same thing as you just described.

[fmartenies]

It shouldn't. The piece that pushes each gear forward should be hook shaped so one direction it catches and the other direction it drages over the teeth but doesn't catch. Hondas look like a hurricane symbol with the shaft running through the eye if that makes sense.

[mxfrank]

I haven't been around these cars long enough to know the variations. My car is an '88, and doesn't have the gear on the left side of the shaft. Instead, the zinc gear on the right is geared to the odometer motor.

I would guess that there was originally some sort of spring or retainer in there to prevent the gears from getting sloppy. But I didn't find any likely looking parts or debris. The amount of endshake was at least 2mm, more than enough to allow the wheels to separate and get out of synch. Installing the washer was delicate surgery, but it works great. You might be able to get the same effect by using a small tie wrap to keep the wheels from drifting, but the washer is really the way to go.

[smokin_944]

Originally Posted by fmartenies

It shouldn't. The piece that pushes each gear forward should be hook shaped so one direction it catches and the other direction it drages over the teeth but doesn't catch. Hondas look like a hurricane symbol with the shaft running through the eye if that makes sense.

Check out the site the 944.com for odometer repair early car. Or, reset the odo when you're doing 30MPH. That will answer your question and you can report back to us.

[911rudy]

The ododmeter on my '87 just failed when I reset the trip meter while moving. I didn't know about being still when resetting it but I do now!

[wild man]

The failure mode on the very early ones was a plastic (nylon?) gear that was too tight of a press onto the steel shaft. The gear would end up splitting, and no longer have grip on the shaft. I've rebuilt several of them, so I know.