Gauge cluster woes

[E36kid]

1985 325e coupe, motometer gauges. Just recently bought this car, it's my first E30. Just about worked out all the Kinks but I'm having some problems with my cluster. First, just the speedometer was messed up and everything else seem to function fine. Now the tachometer is idling at about 5k rmps. Like I said, it was functioning fine in the beginning. After I went for a drive and maxed out the RPMs it wants to stay high. When you turn it off the gauge goes down to zero. Then when you start it, it goes right up to about 5k. As far as I can tell they are the stock gauges. I did read about people swapping gauges from four to six cylinders and it giving them problems but I don't think that's my problem. Any guesses?

[cory58]

I'm not an expert, but I've read that a lot of crazy things start happening when the PC board batteries are failing.

Cory

[E36kid]

How do I change those batteries?

[jdt10768]

so RPM tach is just reading incorrectly? Reading way high, when you're simply idling at 750 rpm?

[E36kid]

Haha yes, it was. Started it the other day and it was working normally again.

[cory58]

How do I change those batteries?

The official way is to replace the pc board, but E30 folks are notoriously cheap and have figured out how to just solder in new batteries.

Google will get you a bunch of results and DIYs. Probably on Youtube also.

Cory

[JohnnyO]

I did my own batteries

I read a bunch of threads and watched a couple of videos. Pretty easy but my speedo was jumping then quit. Changed out speed sensor in Differential but think it might be for a later model. Will try a sensor for my 88.
I got my batteries at a local commercial battery store. They even soldered on the leads for me. $8.00.