Brake lights/wiper issues?

Tiny
TEXAS DOZER
  • MEMBER
  • 1994 CHEVROLET 1500
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 20,000 MILES
Hello,

I have the truck listed above that has all three brake lights working when the truck is off. When I turn on the key the cab brake light works, but the back two don't. The windshield wipers also don't work, but if I hit the "sprayer" button, I can hear it running. I just replaced the wiper motor, the turn signal wiring harness, and the brake switch on the pedal. The only thing I can think of now, is it's the actual turn signal switch itself? Help!
Wednesday, May 10th, 2023 AT 4:57 PM

1 Reply

Tiny
STEVE W.
  • MECHANIC
  • 13,575 POSTS
Looking at the wiring I would test the stop lamp relay, if it lost the ground connection with the key on the top light would work but nut the main lamps. The ground it uses is a common failure point as well. The one at the rear of the right-hand cylinder head, which is also the one the wipers use. Could be a corroded connection and these are the first signs of failure. It's also possible that the engine ground is corroded and causing the ground to allow some current flow which stops the relay. The relay is mounted to the cowl panel on the right-hand side, basically above the ground connection it uses. In the third image you can see how the power from the stop light switch goes directly to the CHMSL but passes through the relay to power the other lamps. A fast way to tell if the relay is the problem or it's ground would be to find the relay mounted to the firewall and run a second ground to it as a test. If the lights now work and the wipers work, go over the grounds located on the rear of both cylinder heads.
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Thursday, May 11th, 2023 AT 12:26 AM

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