I had some spare odometers around and thought I'd see if I could get some working - I needed one, a friend needed one ... I ordered some new gears from eEuro.

One by one I took them apart, reflowed the 3 solder connections as indicated in one the post "Art Benstein on Fri Jul 2 09:12 EST 2010" and an earlier Art B post with pics. I blew the dust/pieces out with air and mechanically removed some bits with a small pick.

I ended up getting a few units to work. Test bench is running a car in gear on a lift.

These three stand out (dates on back of odo unit)

1. A 1989 odo would not work with a new gear but did work with an used gear.
2. A 1988 worked with an old gear and a new gear
3. A 1990 worked with a new gear but not an old gear.

By "gear" I mean the small 25-tooth inner gear. I had one new and several used outer gears and swapped them around above with no difference eg. (3) worked with a new 25-tooth gear in multiple large gears.

I infer gear & odometer manufacturing tolerances are at work here, however, I noted the following differences between the new and old gears.

Using a calipers, the center spindle of a new gear appeared slightly thicker and taller than some older gears. (You'll have to trust me on this - even on macro, my camera won't show the difference).

So, if a new replacement gear is slightly thicker, it's possible that it would bind inside some units. This doesn't explain (3) of course - and, yes, the used gear from (3) worked in another unit.
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240 drivers / parts cars - JH, Ohio