Brake lights not working properly

RONTALLEY

MEMBER
1997 CHEVROLET TRUCK
5.7L
V8
4WD
AUTOMATIC
189,000 MILES

When you first hit the brake pedal the brake lights come on, if you push until brakes apply brake lights go off. Have replaced brake light switch with two after market and a GM switch. Still having the same problem. If you hit the brakes with the truck not running, they work fine but if you hit the pedal with the truck running, they go out.

1 Reply

STEVE W.

MECHANIC
13,445 POSTS

Okay. So, the lights work until you start the engine. I think you have a bad ground from the engine to the truck's body, common on GMs actually. To test this, take a jumper lead and connect it from the engine block to a clean metal area on the body and another to the frame. Do the lights now work properly? If so, you can replace that ground with a new one. It also grounds a lot of other items and can cause charging and other odd problems. It normally goes to the front of the engine block on the passenger side. The other ground that likes to fail is the one at the rear of the engine on the same side. That one may have failed as well. Replace that one as well.

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