Power to stereo drops dead when turning on headlights

[TellarHK]

It's safe to say the subject summarizes things pretty well, but I'll elaborate a touch here. When running my '90 NA 300Z, and cruising around during the daytime, the stereo runs just fine. But, if I turn on the headlights - power to the stereo drops dead instantly. Turn the lights back off, and the stereo fires right back up. I suspect it's probably a short someplace, as the car was heavily molested by a prior owner with the electrical skills of a ... Okay, I've got no joke for that. Damn. No electrical skills, let's just say it like that nice and clean.

It doesn't matter if the car's moving or parked, and the stereo still drops dead with lights on when things are just in "on" at the ignition and the engine itself isn't running, which leads my roommate and I to assume it isn't directly alternator related.

Any thoughts out there?

[BigTDogg (MA)]

Any thoughts out there?

From the pics I saw yesterday, it looks like they completely destroyed the factory harness. So the switched +12VDC could be coming from anywhere, possibly the clock harness. I attached my radar detector hardwire to my clock harness and it would shut off and turn back on with the headlights.

I'd disconnect the stereo, and check the wires with a multimeter while switching the lights on and off to see what changes. Only thing that should change is the dimmer switch. Factory side stereo harness color coding shown below.

[TellarHK]

I'd disconnect the stereo, and check the wires with a multimeter while switching the lights on and off to see what changes. Only thing that should change is the dimmer switch. Factory side stereo harness color coding shown below.

Thanks, Corvidae's out there now digging into the wiring with a multimeter and a scowl. He thinks he's found something, but I don't know yet.

[evildky]

sounds liek they grounded the stereo to the illumination circuit, this happens way more often than ti should, runt he ground wire directly to the chassis

[Corvidae]

Well, sort of. I'm not sure what you meant by the "dimmer switch," but I'm assuming you mean pin 2. That's consistent with what I found:

Pin 2
Lights off: 0V
Running lights only: 11.52V
Full lights: 11.30V


I checked pins 1 and 3 , as well:

Pin 1
Lights off: 11.96V
Running lights only: 11.81V
Full lights: 11.68V

Pin 3
Lights off: 12.19V
Running lights only: 11.95V
Full lights: 11.82V


Pin 6 was always 0V. 7 went from 0 with lights off to 0.34V when running lights were on, but that looks like just bleedthrough and not intentional voltage. Pin 8 shouldn't matter.

I also ran tests with the engine running; the 12V went up to 14V, but the behavior was the same. Good to know the alternator's working, at least, but the issue happens when the car is on, as well. This leads me to believe that the issue is somewhere else and not just a voltage drop to a level that the stereo can't operate on.

I think the issue might not be with the factory wiring, but in the harness itself. Since the only thing that changes significantly is pin 2, the only way I can see this situation making sense is if the stereo is turning itself off in order to keep from damaging itself by receiving 12V on a pin that's not supposed to get that much. I'll admit I don't know that much about car audio, but that could make sense from an engineering standpoint.

Do the voltages above make sense, given that you guys know far more about how these cars are wired than I do, or is there an issue somewhere I don't see?

[Ba11erz]

I concur with previous comments.

Based on your description I would suspect somewhere behind your radio, there is a terminated connection to your dim/illumination wire. (this could have been performed manually by previous owner)

Why: The dim/illumination wire is a negative output (-) and it would be very easy to tap, especially when doing radio installs, because most vehicles (in stock configuration) have this wire somewhere in the radio harness. It's there to provide illumination capability to the stock radio when the lights come on, but it should NEVER be used as a ground. It can be used with an aftermarket radio equipped with a (-) dim input for illumination capabilities.

I've done hundreds of radio/security/aftermarket stereo installs and seen this common mishap of owners tapping into factory harness. The preferred method is to use a factory harness adapter and ABSOLUTELY check every wire before connecting it to the back of the radio when performing a radio install.

RECOMMENDATION: Use a voltmeter and physically check the ground wire connected to the radio harness. The radio does not have to be connected. Turn the lights on and see; first, if there is any fluctuation in voltage, second, switch your voltmeter to "ohms" setting and attach one lead to the radio harness ground, and the other to a true ground. Adjust the dim switch located above light switch, up and down, and see if your impedance on the voltmeter increases or decreases. There are other things to test, but I recommend you try this first to see what you come up with. If your impedance fluctuates at any point, I would be convinced this is your problem as the guys suggested earlier (above). Otherwise, your impedance reading should remain constant while you adjust the dim/illumination switch.

Give some feedback on your next objectives of what you intend to try next. I'm fairly confident this can be solved by tracking down the gremlin in your electrical system.

Good luck!

[Corvidae]

Is the ground wire on the factory harness pin 6 (labelled "joint harness")? It seems that that pin is grounded, as I got no voltage there regardless of light status. I did all my tests against true ground, not the harness ground, though, so something like that could be the problem.

I'll test the impedance tonight and will let you know. Just want to make sure that I'm looking at the right place.

Thanks for the info!

[BigTDogg (MA)]

Is the ground wire on the factory harness pin 6 (the joint harness)? It seems that that pin is grounded, as I got no voltage there regardless of light status. I did all my tests against true ground, not the harness ground, so yeah, something like that could be the problem.

I'll test the impedance tonight and will let you know. Just want to make sure that I'm looking at the right place.

Thanks for the info!

No. Pin 6 goes to the joint harness, which is essentially a powered shield for the preamp signal, from what I gather from the factory service manual.

The ground on the stereo harness should be directly connected to the chassis, scraped bare metal chassis connection. The joint harness doesn't connect to any pins in an aftermarket stereo.

Pin 2, illumination control, should only be used if your stereo has a dimmer input. Otherwise, it should be left capped off and free. If it's not capped off, and shorts to ground, it will blow your clearance illumination fuse. How do I know? You're not the first one to buy a Z with shoddy audio installs

If you're running an amp, the only wires you need hooked up to your head unit would be pins 1, 3 and 8 on the 10 pin block. Run your HU ground to the chassis as mentioned and everything should work fine. If it does have an illumination input, either 2 or 7 goes there (I forget which, sorry).

[Corvidae]

Okay, then I screwed up the stereo harness connections. So much for color matching.

I'll look at it tonight and let you know how it goes. Thanks again.

[BigTDogg (MA)]

Okay, then I screwed up the stereo harness connections. So much for color matching.

I'll look at it tonight and let you know how it goes. Thanks again.

Yeah, I don't think the industry standard was adopted until 1995, which is also the same year the Z32 stereo harnesses changed.

Match the descriptions, not the colors, to the diagram I provided for a 1990-1994 Z32.

[Corvidae]

Match the descriptions, not the colors, to the diagram I provided for a 1990-1994 Z32.

Okay, I got it. Ignored the joint harness and the illumination leads, connected the others up, ran the ground wire to the chassis, and boom, it all works great.

Thanks for the tips, BigTDogg(MA) and Ba11erz! Now I just need to finish ripping the doors apart to replace the blown speakers, then we're getting somewhere.

[BigTDogg (MA)]

Match the descriptions, not the colors, to the diagram I provided for a 1990-1994 Z32.

Okay, I got it. Ignored the joint harness and the illumination leads, connected the others up, ran the ground wire to the chassis, and boom, it all works great.

Thanks for the tips, BigTDogg(MA) and Ba11erz! Now I just need to finish ripping the doors apart to replace the blown speakers, then we're getting somewhere.

Excellent, glad to hear it!

Definitely run fresh wire from your amps to your new speakers, that factory wiring is FUBARed