The odometer and tripometer in my 1990 740 GL wagon only works sometimes. It works just as it should for a while and then just stops rolling over for another while, no clue why... anybody have any ideas?

 

You can try tightening the ground for the pod (behind the right passenger side kick panel), better yet refloat the solder points on the back of the pod's circuit board as well. This may work if the capacitors in the speedo head are not shot.

 

odos can also fail from broken teeth on the plastic gears.

 

but would that lead to intermittent operation, or outright NO operation?

 

I had this problem on my 740 but it only happened when I reset the tripometer . I read that sometimes this happens when the gear hangs up. I solution that fixed my problem was to reset the service light . This is the little hole in the center of the cluster with the rubber plug in it. I would just using a nail push in the reset button and the problem was solved. Seems these gears would hang up once in a while and this is the way to clear them. I don't know if you have the same problem but this worked for me.

 

If it's intermittent then it's not a gear...

 

that’s why i asked.

 

The odometers on these are driven by a stepper motor. Sometimes the solder joints on the stepper motor develop cracks that can cause intermittent connections. Pull your cluster and take a look. You may need to use a magnifying glass to check for cracks. Re-flowing with a soldering iron would cure this kind of problem.

Here's some links regarding diagnosing and repairing 240 speedometers. There are a few pics in there that show the solder mounts of the stepper motor:

Electronic Speedometer Notes
cleanflametrap.com
1989 Volvo 240 Speedometer Diagnostic

 

1990 740 is, i believe, a one yeah instrument panel. I know 91-92 are unique, too and not interchangeable with later or earlier