car: 1992 740 wagon, 8cyl, base, US/California.

Problem: Speedometer cuts out at around 40 mph and sometimes fluctuates before cutting out completely (needle bounce), odometer no longer works, ABS light illuminates immediately upon ignition, does not clear. No OBD1 ABS codes on port 3. (no codes anywhere, for that matter)

Hello.

I have to think these symptoms are related. Visual inspection of the speedometer sensor in the diff looks good. The security box in the left rear was sealed. I opened it, that connection is solid.

I removed the instrument cluster, removed the speedo itself but didn't go through the whole speedo dissection. Visual inspection (as much as possible) looked ok. No signs of corrosion. The caps I could see did not look to be bulging or corroded. All contacts seemed just fine.

I tried to repeatedly hit the service light reset stalk and the trip meter clear to see if the odometer failure was mechanical. No change.

I checked fuse 12 on the main fuse bus, it's fine.

I read there was a relay to the left of the steering wheel shaft, by the footwell. There is supposedly a relay with a fuse in it? I have not located this.

I've not completed a full continuity test from the speedo sensor to the dash. I am wondering what the best way to test the speedo sensor is though. I'd like to rule that out. I've seen multiple suggestions on how to test, but it's still not 100% clear to me what the most conclusive approach is.

It seems like this is all related and since the ABS takes input from the sensor in the diff, and so does the speedo.... I thought this would be easier with the three clues I have.

I'm finding great information about this less then obscure problem but I'm still struggling to find a solution.

Thanks in advance,

KlausU