Hello. I have a buddy that drives a 1991 m30. recently, he came out and noticed the speedo was pinned all the way up. Iam not familiar with these cars and can find very little info about them. Iam a heavy truck mechanic so i am familiar with a few different set ups. I was hoping to get an opinion on the matter before I go look at the car tomorrow. In my mind the speed sensor is out because in most vehicles the sensor is a magnet/coil and makes a/c voltage to make the gauge go up. I also read that on this model the a/c signal goes directly to the speedo head before the ecm, kinda backwards from what i normally see. Does this speedometer head unplug from the gauge cluster? Is this a common problem on this car? do i need to bring any special tools with me for disassembly? Any help would be greatly appreciated. Thanks a bunch.
Nick

I had the same issue and swapped the cluster.....which fixed it.

I believe there to be a cracked solder joint or a single bad component.

A few days before i replaced it, the original speedo needle jumped around a bit and went back to normal for a couple of trips before failing as you described.

What i didn't try was checking the connectors as i was sitting in a 500 degree car and decided replacement was more fun than troubleshooting it.

Just like HowlerMonkey I had the same issue and swapped the cluster, also....
There is an unmarked IC in the back which looks like a 3-terminal Voltage regulator - or like a transistor. My guess is that item is faulty. I got my replacement luster from a wrecker yard. The mileage dial unit is held in with a couple of screws and can be swapped to keep the car mileage correct.