speedometer only works when headlights are on

[tewhalen]

...in my (new to me) '87 325i convertible. This is just one of many very vexing problems that probably make it not worth fixing up, but here we are.

Anyway, one fun and entertaining thing about my car is that the speedometer only works when the headlights are on. Can I rule out a problem with the instrument cluster and just concentrate on the rats nest of wiring the previous owner left me with?

Relatedly, the aftermarket stereo only works when the headlight switch is in either park or headlight mode, and the ignition position has no effect. I'm not sure whether this is evidence that a PO did something insane to the wiring, or what.

[325isman]

Sounds like a wiring issue.

[superj]

ha ha ha /\

it sounds like something is grounding through your light switch instead of its normal ground. I can offer no other help though, I don't know the electrical system well enough. google e30 headlight wiring diagram?

[tomstin]

Anyway, one fun and entertaining thing about my car is that the speedometer only works when the headlights are on.

Relatedly, the aftermarket stereo only works when the headlight switch is in either park or headlight mode, and the ignition position has no effect. I'm not sure whether this is evidence that a PO did something insane to the wiring, or what.

I think you need to start with a VERY careful evaluation of what works and doesn't. Particularly with the instrument cluster. If its ONLY the speedometer, I would look at the sensor circuit. If its more than that not working, I'll agree with an earlier post about a bad ground. But a careful list of what works and doesn't will be a good first step.

Then you need to dig into the wiring diagrams.

I doubt if you will get a quick fix on this one! You have a headache for sure!

[tewhalen]

Thanks for the advice. Once I get deep in there and start checking stuff, it's going to be a pain to test it. Can anyone think of a way to test the speedometer that doesn't involve actually moving the car?

[superj]

jackstands under the back and chocks in front

[earthwormjim]

First place I would check, is the grounding for your whole cluster. Just take it out, and inspect the wiring on the connectors, and look for the big ground point under the dash.

Ensure every ground wire is actually common with your chassis. It sounds like someone rerouted a ground to your headlight switch.

[tewhalen]

jackstands under the back and chocks in front

My friend Ferris also recommended this method for working on some odometer issues I am having. If we’re careful, I don’t see how it could go wrong.

[tewhalen]

I've investigated the ugly Kenwood aftermarket head unit that the PO "installed" - they apparently decided to splice together the "accessory, run, or start" and the "when the headlights are on" power wires before connecting them to the head unit. I'm no expert, but this seems like a bad idea. Seems like it would cause everything upstream to get power when either of the conditions is true. I guess it might have other weird effects.



I'm going to properly wire my new head unit (a super-cheap, but OEM-looking-ish "Bravo View ind-400bt") and see if this helps anything with my speedometer problem. My quick perusal of the wiring diagram doesn't obviously indicate that this would cause the speedometer to act weird, but we'll see.

Edit: extended perusal reveals a potential source of issues caused by the dumbass splice:

[tewhalen]

As I suspected, Fuse 12 was indeed blown. Which means that speedometer was getting power from the weird radio splice when the headlights were on. I've undone the splice, put in the new radio, and replaced the blown fuse. Will test the speedometer later, but I'm assuming that it works now.

[earthwormjim]

Wow, nice job tracking that down. I would have wasted a ton of time inspecting grounds.