740 dead odometer/trip meter

The odometer and trip meter (VDO speedometer) in my 90 740GL quit recently and I found the small 26 tooth gear was broken. I replaced both plastic gears and it worked for about 300 metres and quit again. Pulled it apart and found what looked like a couple of dodgy solder joints on the small circuit board for the motor that drives the gears. I resoldered them and everything else looked fine so put it back together and still no dice.

The speedo is working fine so I don't think it could be the diff sensor or wiring or could it? Are there any other possible causes? Is there a way of testing the trip meter motor out of the car?

Thanks for any input.

Update:
I managed to get the odo/trip meter working intermittently by using a small screw driver and physically pushing the numbers on the odo as described here under the heading "Still not working?".
It would work for a few hundred metres then stop again but the turning of the numbers was jerky. Pushing with the screwdriver had it working for another few hundred metres.
I pulled it apart again, sprayed the contacts on the stepper motor with contact cleaner, checked the numbers could turn smoothly and reassembled. I've yet to test again but will report back.

I bought another cluster off ebay, but the soldering was the root cause for my 87 and 88. I used some flux and redid every joint, and everything worked ever since.

+1 for bad solder joints probably are your problem. This happens to every unit, just a matter of time.

Thanks guys
Still haven't tested it yet but will see how it goes. I've done a few resoldering jobs on Volvos over the years so I know they have a tendency to go bad.