Electrical problem
1997 Volvo 850 Front Wheel Drive Automatic 120000 miles
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My 1997 Volvo GLT 5-cyclinder had the odometer and trip mileage counter stop working at around 120000 miles.

I have 25 years as a military aircraft mechanic and am highly mechanical. However, I can't find a repair manual that identifies how to fix this problem.

The local library didn't have repair manuals listing the odometer operation.

Is this some thing I could do on my own? Or which repair manual might show me a diagram?

I had the same problem with my Volvo 850 at about the same 120000 miles. I managed to find the problem and also fixed it.
The problem is situated in a small rubber gear that drives both the odometer and mileage counter at the same time. You just need to replace this little gear.
In order to do this, you need to remove the facia cover (follow the instructions in the Haynes Manual)and then take out the VDO. You dissassemble the VDO and you will have access to the odometer and mileage counter which form one block.
The odometer and mileage counter are driven by a step motor which you need to remove before finding the small rubber gear.