Electrical gremlins (e30)

Never had any real electrical issues in the rallycross e30, but this weekend some odd things popped up, which may be related, or maybe coincidental:

1. Last month installed a VDO water temp gauge (with a new VDO sender to the stock port in the head) and an Autometer fuel gauge (digital, run to stock sender). These two gauges were wired to the same switched power source and grounded to the main chassis interior ground. They worked fine for the first month they were in. Then a few days ago the temp sender went all the way to the right and stayed there (car was not overheating), and the fuel gauge calibration seemed off. Then coming home from the rallycross yesterday, the fuel gauge went blank (coolant temp gauge still pegged to the right).

2. Once it got dark on the drive home, noticed that my factory gauge cluster lighting was dead...BUT the lighting for the 3 gauges in my dash pod and for my aftermarket tack were all still workign fine (wired to the interior lighting loom).

3. On the way home I noticed my elec fan stayed on once car was turned off at a gas station. But it turned off once I turned the headlights off. I can't find any relation of the headlight circuit to the elec fan circuit.

4. note that my elec fan is wired to a thermostatic switch in the radiator for automatic operation when the coolant gets above 210 (or so), plus it has an over-ride switch using the original wiring from the OEM a/c aux fan/compressor button on the dash. But again, no idea how this could be affected by the headlights?

Any ideas are welcome before I start tearing things apart.

Btw, this is an '85 318i (with M42 swap, but that shouldn't affect any of this) using mostly the original wiring harness.

Sure sounds like a grounding issue. Weird-doesn't-make-sense stuff like that usually is. Make sure you have a heavy ground cable(s) going from chassis to engine block to battery negative. Good luck.

Putting my vote in for a grounding issue as well. You may have some circuits "backfeeding" through others due to not having a proper ground, hence the crazy, haunted bullE36 M3 like having your lights control the fan.

Pretty much ripped half the dash apart tonight, and it looks like at least some of the problem is with the main light switch, and the fact that the two gauges are wired to the interior lighting power that has a dimmer, rather than a solid switched. I usually have the dimmer knob at "full power" but looks like it has no resistance now and just spins around....thus making the gauges attached go wacky.

Still haven't figured out the fan thing, but it's not doing it with the override switch detached, so I'm thinking it's a switch issue.

Will double check my engine ground though. Battery is in the trunk.

If you hard-wire the dimmer to "full on" how much does it help? If things are powered off of/grounded through that and it has failed that could be your whole problem.

Yeah, if that is failed, it may have introduced some ground loops that have changed reference ground enough to screw with gauge readings. If the electric fan has a relay, it could have also changed the reference enough to fire the relay, thus turning the fan on. Grounding can be a real pain in the ass sometimes, but it is usually responsible for strange electrical issues.

Did you add the new digital Autometer that uses the existing temp sender in addition to the wiring already present? If so you have changed the signal by placing a 2nd load in parallel to the other.

If it worked fine for a while before ... clean the grounds and/or change the sending unit in the radiator. Also look at the AC wiring as the thing that turns that fan on high as an over-ride is the AC button on the dash. If you removed and taped all that back pulling off the condenser and so on... check the bundle of wires.

Do you have a volt meter? If the system is in a low voltage condition under load or at certain RPM because vreg/alt is failing < ... <11v or so odd stuff happens. Like turning off the headlamps raises the voltage so things that switch off come back on and sensors that read 0-10v start reporting higher values. Relays seem to toggle randomly. Stepping on the brake makes the dash dim... etc. Check the charging system if the grounds are good.