3-Series (E21, E30)Chat relating to the BMW 3-Series from 1975-1983 and 1984-1991 line. Specific models: BMW 315, BMW 316, BMW 318, BMW 318i, BMW 320/4, BMW 320i, BMW 320/6, BMW 323i, BMW 320i. E30 Family models include: BMW 325e, BMW 325i, BMW 325is, BMW 325ix.
Does your tach work? Failure of the MPG gauge and/of the tack could indicate a problem with the SI (Service interval) pc board located in the instument cluster. Condsider changing the batteries on the board. Not a big job, and you can find the aa size ni-cads (or 3.3volt ni-cads depending on your car) at radio shack. This will require soldering. It could also be a bad MPG gauge.
Before you do anything to anything, you need to know if you have a VDO or moto-meter tach. Even though the face of the cluster may say VDO in the center, it may have moto-meter guts. The VDO tach has a resting pin for the needle, the moto-meter has no resting pin.
So long as you can ensure that you SI board is ok. And that your 13 button OBC isn't FUBAR (the OBC uses the MPG input to guesstimate range, if shorted it'll screw with the mpg guage, test this by clicking range if you get a reading it's your MPG guage if not unplug the OBC and see if your MPG guage comes back to life) And if you have the moto-meter its a good chance that the tach is bad itself, rare but good chance. The VDO runs off the main cluster board where the moto-meter has its own electronic board. If you have the 7000 rpm tach I have one moto meter you can buy and also a VDO.
P.S. I had one instance where an aftermarket raido installer probed wires looking for a alternate power and probed hole shorted out on another wire causeing a bunch of hoaky stuff.
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