The Stovebolt Site Forums The Shop Area The Electrical Bay 71 GMC Brake Lights // 1 ton dump

Only one brake light is working. Bulb and harness check fine. Trace good all the way back to the connector that in in the rear frame. No juice going to the right side when brake is applied. Here is where it gets complecated......the right turn signal is working fine!! When brakes apply both elements in the bulb light up. The turn signal does the same but interrupts the curcuit with a flasher. My understanding is that the feed for the brake light and turn signal go down the same wire along the frame. Seems strange the turn signal works and the brake light does not. Thinking my issue may be under the dash. Any words of wisdom out there?? I will eventually find the problem but a hint in the right direction can save a bunch of time.

yes, the turn signal uses the brake circuit, interrupting the selected side, whether the brakes are applied or not .... both elements should not activate at the same time, ever .... check your ground connections at the rear lights, even if they're well grounded to the bed, the bed might not have a good ground to the frame [greasy dump hinges] .... next check would be th power supply for the lights, the brake and tail circuits should be separate at the switch .... did this just start on its own or have you been doing some work on any part of the wiring?

Bill

Yep. Start with checking your grounds.

Thanks for both replies. Bill, to answer your question.....The rear wiring was a total mess when I bought the truck. many lights not working. I "cleaned" up the mess when I added a trailer harness. When I finished in March of last year, everything worked, even the back up lights. This situtaion just now happened.

Howdy,

Always check your grounds first! The dark green wire is your right rear turn and brake light. Double check the dark green wire all the way back to the connector on the right side of the fire wall. From there, dark green and yellow, (left turn/brake,) go to the turn signal assembly.

The fuse box feeds brake light switch (orange wire) and from there a black wire goes to the turn signal assembly to feed the green and yellow.

What I'd look at is 1. corrosion and grounds between to lamp and the firewall. Don't overlook the socket. 2. Wires from the turn signal assembly to the firewall and the assembly itself. 3. fuse box to brake light switch to turn signal assembly.

Make sure all connections are good and there is no corrosion.

Your problem pretty much can't be anywhere else!

best wishes,

Les

All is well now. I disconnected the truck battery. Disconnected the brake switch connector and installed a jumper wire. With a spare battery I back fed electrical current on the dark green wire from the rear of the truck. The problem was in the half moon connector under the dash. Just as "Roadmarks" suggested, there was minor corrosion. I cleaned the terminals with a detail brush and applied a skim coat of dielectric grease. Ready to roll.