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Weird electrical issue with headlights

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I had the whole interior taken out when having the car painted. Prior to, headlights and everything worked fine. I've since put the dash back in and got the car running and everything works fine except for the headlights and turn signals. Both the combo switch and dimmer switch are rebuilt and seem to be working fine. I do have a couple of individual wires with no home in the steering column that I can't identify on the wiring harness and have no power or continuity to ground (seen in the pic below).


1. Tested 12V at both HL fuses (upgraded MSA blade fuse panel) when switched on


2. Tested dimmer switch and see ground being applied to different wires when high beams are switched on/off


3. I have 12V to the headlight plug black wire (shouldn't this be ground?) I also have ground continuity check fine at the headlight plug on the red/black wire


4. When I plug the physical headlight into the plug, the 12V black wire becomes <1V and stays as such until I cycle the combo switch


5. Front turn signals do light up and have power, but do not flash and the hazard switch does not work...not sure if they are related and I haven't tested the blinker.


Both headlights are H4 housings which use all the stock wiring, upgraded by previous owner, if that makes any difference.


Is there a grounding issue somewhere? Car starts and runs fine, all other lights, including brake lights, are fine.


Wires with no home

1. Red wire out of the harness with aftermarket female connector

2. black wire out of the harness with aftermarket male connector

3. black/yellow wire out of harness with female connector

4. blue/white wire out of steering column with male connector



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You didn't mention the basics:


  • What year is your Z?  Wiring changes from year to year.
  • What modifications have been made to the electrical system?
  • Do you have the Factory Service Manual for your z?
  • Have you downloaded electrical schematics for your Z?  Members have made some very well done schematics in color in pdf format.  Search.
  • Do you understand how the headlight system is wired?  May be different than you are used to.
  • Do you know that black wires on the Z are grounds?

Suggest that you get a schematic and identify the wires "with no home".


Google switch repair: there are many "how to" write ups and YouTube videos explaining, in detail, the combo switch lighting/turn signal switch wiring.


Until you understand what each wire does you will be at a disadvantage troubleshooting the electrical system.

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It's a '72 with factory wiring (from what I can tell) for all of the lighting, switches, etc. except for the conversion to MSA blade style fuse box. 


I do have a service manual and have looked through the diagrams and tested to my ability the switches which appear to be functioning properly at the switch level. The fact I had power to the ground (black) wire on the headlight plug is what was confusing to me and I hadn't seen anyone else in a search with similar issue. I'm reduced to guessing, and not having a thorough understanding lead me to the question. I manually traced all the wires involved with the headlight circuit behind the dash, but outside of that it's all in the original loom. I'd rather not tear that apart if someone had something else I should look at per the symptoms.


Both switches were rebuilt and working previously, The "loose" wires I have don't appear to be in the diagram from what I can tell (except for the blue/white) and that's part of where I'm at a loss (if they are indeed even necessary or part of my issue). Unfortunately they were not labelled when the dash was removed by someone else and left in my lap...but that's another issue altogether...

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It ended up being the ground wire from the dimmer switch to headlight switch, it was connected and tested continuity fine, but the connection wasn't strong enough for the headlights. When I ran an independent ground wire from the dimmer switch to the chassis they came on. I put on new connectors on the original wire and the headlights work finally.

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The  stock headlight circuit is  prone to overheating the headlight switch.


Do the headlight relay harness developed by  Hybridz  member  HLS30-08077  (Dave).


Allows running higher wattage headlights without burning up the stock harness/switch.







Seem to recall that  MSA also sells Dave's harness.

Edited by Miles

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