No brake, running, reverse nor tail lights! Help!

Okay I’ve been going crazy and figure one of you fine folk can help me.

1976 gmc c15 with the 350 and auto trans.

Off truck with power and ground straight from battery, all lights work. Plugged to truck, only headlights (low and high), blinker/flashers work.

No parking lights or running lights of any kind front nor back, which also means no tag light, no brake lights, no reverse lights.

I have replaced the headlight switch (which caused the headlights to function after not working before), the high/low kick panel switch, the fuses, the ignition switch thing on top of the steering column, I have unplugged the brake pedal switch and plugged the wire into a new button and tried pressing and unpressing it and nothing.

I don’t know what else to do or how to look for my problem. I have the neutral safety switch as well which I figure might address the reverse but still no idea about the brake lights nor the amber running lights.

Any ideas? What to look for? I have a voltage meter and limited understanding of it if there’s anyone who can help me figure this out and get my damn truck on the road!

Thanks in advance

There is a round plug up under the bumper in the back and it lets you access the taillight wiring. It’s the place they plug in an adapter for a four wire trailer lights plug. That loom also has a white ground wire near the center, which is grounded to the body. Remove the ground wire, sand the ground point to bare metal, and reinstall. Check for 12 volts between the ground wire and each power terminal (IIRC, running lights, left turn and right turn, 4th terminal is the ground) in the plug.

What @75gmck25 said above. Three rules for electrical issues with these trucks.
1. Check ground
2. Check ground
3. Check ground!

Report back @hscott771

Usually parking/running and dash lights are tied together. They do this so when the fuse blows you know you don't have outside lights.

Turn signal switch? Brake lights run through it.

75gmck25 said:

There is a round plug up under the bumper in the back and it lets you access the taillight wiring. It’s the place they plug in an adapter for a four wire trailer lights plug. That loom also has a white ground wire near the center, which is grounded to the body. Remove the ground wire, sand the ground point to bare metal, and reinstall. Check for 12 volts between the ground wire and each power terminal (IIRC, running lights, left turn and right turn, 4th terminal is the ground) in the plug.

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Hey bud,

So, previously did have a ground issue. Added an additional ground and ran power from the battery through the intermediate harness through the rear and got all lights (except one reverse, don’t care about that right now) working. So harness is grounded and there’s no faults in the positive lines or the lights wouldn’t have lit from direct 12v. And ground works or nothing would have.


Really scratching my head here. Seems to be Something between firewall and foot but I’m not seeing it.

Randy De Oreo said:

What @75gmck25 said above. Three rules for electrical issues with these trucks.
1. Check ground
2. Check ground
3. Check ground!

Report back @hscott771

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I have removed all grounds I’ve found and sanded both the contact and the body sides. Hasn’t solved anything yet.

Scott91370 said:

Usually parking/running and dash lights are tied together. They do this so when the fuse blows you know you don't have outside lights.

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And with a newly replaced fuse (old one seemed fine), what’s your next step?

squaredeal91 said:

Turn signal switch? Brake lights run through it.

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But my flashers work… different circuit?

Maybe try Turning on flashers and unplug turn signal switch and see if they keep flashing or not.

Harley take pic of fuse box. Our C10 that fuse was labeled courtesy lights. I think 15A. I kept blowing it because PO tied an instrument cluster light with another light from God knows where. When we did Vintage Air install just needed to tap into dash lights for the A/C panel light which was a single lead LED. So we found the dash back lights wire (grey) and tapped in and thought all was Good. Turns out when you rotated dimmer and power increased it would go to ground and short out Blowing that fuse. Took me several days to find PO splice and correct it.

Randy

My next step would be to remove tail light assembly and get my probe out and see where I can find power On the wiring harness on drivers side rear where it splits to passenger and driver clusters (check grounds inside the rear taillight assemblies as well. . If nothing that far back keep moving forward or vice versa. And headlights and front lights all work correct?

Randy

Harley, check this post… headlight switch was causing this guy a problem as well….

hscott771 said:

And with a newly replaced fuse (old one seemed fine), what’s your next step?

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Several have said headlight switch so I would follow their advice - they know these trucks better and I do. I would have grabbed a test light or volt meter and started at the fuse box, traced wires until I found where the power stopped.

If I remember correctly, the taillight harness feed is the smallest of the connectors at the firewall electrical connector. Since you said basically nothing works at the back of the truck, start there.

Turn on the headlights, use a test light to backprobe the brown wire. Hopefully you have power. Next, have a helper push the brake pedal, check for power on the yellow and dark green wires. Hopefully you have power. Next, release the brakes, turn on the ignition and try the turn signals. Yellow is right turn, dark green is left. Lastly, put the shifter in reverse, check for power on the light green wire.

If everything checks out there, the problem is between the firewall connector and the taillights. First place to check is behind the bumper, look for a hacked up trailer light connector.