OK Dave, that helps to know you have used a soldering iron before. I've seen some of these boards look like that iron was a 300-Watt American Beauty. :) The heat used to "reflow" the joints melted the plastic inside the motor and, essentially, the winding was open circuit as a result.

I'm not too hopeful a gear is the problem here. You can push the rotor around with a pick through the top, if you're careful, to see that it is not bound up, and with persistence, see that it is turning the odometer display. And, you can measure the resistance of the windings with an ohmmeter -- I see about 80 ohms, or if you measure end-to-end from the two pins shown in my photo -- 160 ohms.

So, you're confirming two new pieces of evidence. One, the odometer hasn't worked since you got the car, and two, you didn't zap it with the tach wire. If you suspect it was, before, you can check the resistance to ground from either of those two motor pins I referred to above. With the gauge in your hand (disconnected from the power) using the low ohms scale (i.e. where you'd measure 160 ohms) you should not see any short circuit. If you do see anything less than 1000 ohms, the chip is suspect.

Be aware the whole shebang (board, motor) is all held in place by the pins on the motor and the plastic pins melted over on the solder side of the circuit board. That's what supports the weight of the board and vibration movement such as that imparted by the wiring harness connection at the L-shaped plug. That's a lot of mechanical stress on those motor pins. That's why these things need to be reflowed after having it apart to change the gears. Also, it is easy to seat the motor screws with the rotor not fully seated and the gears engaged, so, again, stress is placed on the motor mounting pins. Devil's in the detail.

The replacement instructions available from the gear suppliers is woefully inadequate.
--
Art Benstein near Baltimore

Some mistakes are too much fun to only make once.