crazy tach question

[AriBerberyan]

Hi, my tach moves erratically when the key is out of he ignition. The car has been in winter storage with the negative lead of the battery disconnected and the battery on a smart charger (the battery shows good voltage). I noticed mice in the glove compartment and thought they chewed up the wiring, so I took the instrument cluster, radio and everything else on the dash to find a faulty wire but all wiring looks perfect (including under the hood and everywhere else where I can see exposed wiring). I have not started the car yet since last autumn. Anyone experienced this? I the cluster supposed to get electricity with the key out?
Video here: https://www.reddit.com/r/MechanicAdv...ach_gone_crazy

[scabzzzz]

I've seen something weird before like that on mine and I assumed the cluster just needed a rebuild. Sounds like you'll need to break out the multimeter

[ZeKahr]

Might have something to do with the SI board in the cluster. The tach in my 86 325 was doing the same thing when I shut off the car and the cluster had a bad SI board. The board uses soldered batteries for memory power (dumb design) and causes all sorts of problems with the tachometer once the batteries die. On the M10 318i and 325e cars you can permanently fix these kinds of tach problems by swapping in a 1984/85 cluster (tach on those has its own circuitry and can operate independently of the SI board) but the 325i, Super Eta, and M42 318 cars migrate the tachometer circuit from the tach itself to the SI board so you need to get the board rebuilt unless you’re okay with having an M10 or 84/85 Eta cluster with an incorrect redline in the car.

[AriBerberyan]

Thank you for this, after reading your post I found an article on Pelican explaining how to change the batteries and in the comments section many people have the same symptoms. I will attempt to change the batteries now and hopefully that will fix it. I guess I spent two days looking for a short for nothing...oh well.