My 89 325i has a tach that usually works, but sporadically doesn't; a speedo that usually doesn't, but sometimes does. I have tried other used instruments in my housing, have a new "rebuilt" SI board, a new sender in the dif., but now I'm thinking the main circuit board called the "middle board" might need replacing. Any similar experiences out there?
A common issue is with the traces on the Printed Circuit Board. A fix that has worked for many is to touch up the solder joints on the PCB. My suggestion instead of touching up would be to remove the old soder and replace with fresh solder. I really need to do this because my tach usually doesn't work immediately unless I literally bang on the plastic cover for the cluster.
Having a crack in one of the printed circuit board solder traces is certainly a possibility. However, I also have heard that just retightening the nuts holding together the contacts at the back of each gauge can solve it too. Likewise, there are some spring-loaded contacts between boards or cable connectors which might be dirty or loose. There are all kinds of places where there could be a loose connection. Clearly there is a loose connection somewhere. These things are always tough to isolate to a specific thing. Usually you do a bunch of stuff and mysteriously it all starts working again. It might be a good first step to borrow an instrument cluster to isolate the problem to the cluster itself before you start the head banging exercise.
