Brake light issue

[Bagd72Chevy]

67 GMC, factory wiring. Brake lights function normally if the ignition is off. If the ignition is on and headlight switch off the brake lights are off. If the headlight switch is on and ignition on brake lights are on. If igntion is off and headlights on the running lights are on and brake pedal operates the brake lights. Ignition off and headlights off the brake lights function with pedal.

Hopefully someone can make sense of this, I cant wrap my head around it. Thanks
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[LockDoc]

Originally Posted by Bagd72Chevy

67 GMC, factory wiring. Brake lights function normally if the ignition is off. If the ignition is on and headlight switch off the brake lights are off. If the headlight switch is on and ignition on brake lights are on. If igntion is off and headlights on the running lights are on and brake pedal operates the brake lights. Ignition off and headlights off the brake lights function with pedal.

Hopefully someone can make sense of this, I cant wrap my head around it. Thanks
Posted via Mobile Device

Usually these kinds of gremlins are caused by bad grounds, but there are other things that can cause them too. The first thing I would do is to read through this thread and make sure you have all of the ground straps listed. Also remove any that are there and clean the terminal ends and the metal where they attach as shown in the pictures.

http://67-72chevytrucks.com/vboard/s...d.php?t=422234

I know it is a lot of reading and tedious work but it is cheap to do and needs to be done to eliminate the possibility of bad grounds. If you still have a problem after you do that, then we can move on to other problem areas.

LockDoc

[Bagd72Chevy]

Yes, I put extra time into grounds when I first put the truck back together. I have grounds from bed to frame, cab to frame, engine to frame. Even taillight and marker housings to frame. I thought this problem just showed up but that's not the case. The floating grounds on these old trucks bit me on the led tails I have. Guess I've had this problem longer than I realized, just didn't drive at night. With my led tail lights I had to put a pull-down resistor on the brake light wire to ground. That got me fixed up.

[franken]

The little parts needing grounds is just part of the whole system needing grounds. Or?
There are about 8 ground straps, few of which make a difference here, that are normally there. Think electrical path from the alt/batt all the way around back to the alt/batt. It's a circle--all the current that goes out comes back.

[Andy4639]

Originally Posted by Bagd72Chevy

67 GMC, factory wiring. Brake lights function normally if the ignition is off. If the ignition is on and headlight switch off the brake lights are off. If the headlight switch is on and ignition on brake lights are on. If igntion is off and headlights on the running lights are on and brake pedal operates the brake lights. Ignition off and headlights off the brake lights function with pedal.

Hopefully someone can make sense of this, I cant wrap my head around it. Thanks
Posted via Mobile Device

The brake lights and all other lights for the rear of truck and front run through the turn signal switch. I would start by checking the connector under the dash for a bad connection.
Also the brake light switch get power from the dome light circuit.
If you save the wiring diagram to your computer you can make it larger and trace the wiring out. The orange wire is feeding the brake light switch then goes out on the white wire to the turn signal harness.