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Posted by: scottthephotog May 15 2020, 05:29 PM

So I've been chasing electrical gremlins with the rear lighting since I bought the car a year and a half ago, and from what I can tell, the previous owner was as well (but just didn't mention it during the sale). The front lighting works as it should.

Here are the symptoms:
with the parking lights/headlights off

• The turn signals operate correctly (no pedals or other switches engaged)
• Pushing the brake pedal causes both brake lights and turn signal lights to flash simultaneously
• No reverse lights when in reverse
• turning on the emergency flasher lights flash the licence plate lights.

with the parking lights/headlights on

• Either direction on turn signal flashes both turn signal and reverse lights simultaneously
• pushing the brake pedal causes both brake lights, both turn signal lights, and both reverse lights to flash simultaneously
• turning on the emergency flasher lights cases license plate lights, turn signal and reverse lights to flash
• No reverse lights when in reverse

Things I've tried:

• Cleaning all ground points on the entire car (double checked locations with diagrams on 914World)
• Taillight bulb sockets and connectors are clean
• Checked taillight wiring against diagrams on 914World and shop manuals (I found previous owner had printed out those diagrams as well) - They are correct
• Checked headlight and hazard light switch against diagrams on 914Wolrd and shop manuals - They are correct
• New headlight switch (which it looks like the previous owner tried as he included a spare)
• The previous owner also tried different turn signal flasher relays as I have 2 spares
• The rear wiring looks like it is in good condition. I don't see any splices or breaks.

I know that the hazard light switch is also part of that system. I'm at the point of trying a different one of those, but I don't want to just throw money at random parts.


Any thoughts or suggestions? The front lights work perfectly fine, so I'm leaning towards a wiring issue. Are the front and rear lights split at the hazard light switch that part of the switch could have failed? I also can't figure out why the licence plate lights, reverse lights and brake lights are affected by this.

Posted by: framos914 May 15 2020, 08:54 PM

I had strange things happen with my lighting that didn't make sense because the circuits weren't related. Found a couple of wires on my light switch had bare spots and were touching. This was after I had been doing some work with the wiring.
Start with a good wiring diagram and try to isolate the circuits. A few circuits have the emergency flasher switch in common. Make sure all are connected to the correct terminal.

Posted by: scottthephotog May 15 2020, 09:15 PM

I'll double check again tomorrow, but I didn't see any bare spots on the wires anywhere. I unplugged and cleaned all the connectors on switches and lights and double checked the all of the connections based on the wiring diagrams in workshop manuals and the wiring diagrams I've seen here on 914World for the switches and taillights.

Posted by: Sway Bar May 15 2020, 10:51 PM

I'm thinking a short/ground somewhere in or near the tail light assembly that makes it all behave weird.

I've been working on my car which was a parked roller for decades and it displayed all the same systems...front works fine but rear behaved funny. Some lights worked then turned on park lights and then didn't work etc etc.. Then one side worked so swapped bulbs and all of a sudden it didn't work. Everything cleaned, new bulbs/old bulbs, correct wiring etc.

In my case it was 1 or 2 of the bulbs and sockets...clean as could be but if the bulb was turned/locked all the way funny results but if I twisted back the bulb a fraction all good and everything works great....so some are shorting slightly somehow in the socket.

It will be something simple.

Posted by: framos914 May 16 2020, 04:52 AM

I agree with Sway-Bar, it does sound like an issue with a short in the rear assembly bulbs or wiring.
Was looking at 73 wiring diagram and all those lights share the same ground point. But voltage for turn and flashers come from Emergency flasher switch. Stop lights, license plate lights, tail lights and back up come from their own separate switches and fuses.
Maybe a good way to troubleshoot is to remove all rear light bulbs and reinstall one at a time. Rear side marker lights also use same ground.
This old wiring can be a pain

Posted by: ejm May 16 2020, 06:00 AM

Make sure the correct bulbs are installed. A single filament where there should be a double will cause issues like this. Sometimes double filament bulbs short internally. Replace all of them or test by removing one at a time. The reverse light problem is most likely the switch.

Posted by: scottthephotog May 16 2020, 11:05 PM

Update:

Bulbs were correctly installed (single vs double filament) for those who were wondering. That wasn't the source of the problem.

At some point this afternoon, I got the brilliant idea of bypassing the original ground point and wire and just grounding the taillight directly to the negative terminal on the battery. No idea why I didn't think of this earlier... Instantly the lights were way brighter, and lighting up one at a time, although not correctly.

The first problem was of course a bad ground. I'm not sure if it was a break in the existing ground wires, or a failure of the ground point. The ground point was clean, so I'm guessing the wires broke somewhere along the way. I tested various places in the trunk to find a good place to ground and created a new ground point by the taillights. I then grounded the taillights and the old ground lines to that new point.

Finally, after having a good ground, I got to the fun of determining which wires did what. Even though it matched multiple diagrams, it still was illuminating the wrong bulbs. After a bunch of back and forth from the dashboard to the rear of the car, I managed to get it all sorted out. I have no idea what caused it to become such a mess, but it is finally fixed!


Thank you all for the suggestions. My brain kept coming back to grounds because all of the lights were affected, but with the ground point clean, I kept ruling it out. I don't know why it never occurred to me that perhaps the ground wire itself was faulty. I think seeing the responses about grounding/shorting made my brain finally think about just grounding it directly to the negative terminal for testing.