Brake lights do not work?

CESAR SAMANIEGO

MEMBER
1997 GMC SUBURBAN
5.7L
V8
2WD
AUTOMATIC
320,000 MILES

I bought this truck dirt cheap, runs and drives like a dream. The wiring for the rear lights was all cut up and all over the place, the actual plugs are gone, so I had to splice everything to make it work. And eventually after a few fuses and such I got the running lights, the signals/hazards and the reverse lights working. But I can’t seem to get the brake lights to work. I’ve been going crazy. All the fuses are okay, and I even changed out the brake switch but nothing. I’m about to lose my head over this. Any help would be appreciated.

3 Replies

JACOBANDNICKOLAS

MECHANIC
109,867 POSTS

Hi,

No problem. My first question is, does the third high-mount brake light work? If not and there is no power to it, I need to know if there is power to the brake light switch.

At the brake light switch, there will be an orange wire that should have voltage at all times. When you depress the brake pedal, the switch should close and send power via a white wire to the third brake light and to the turn signal/hazard switch.

Since the hazards work, I know the fuse is good and there is power available. We just need to determine where it is lost. If the third brake lamp works, the signal switch is the likely issue. If it doesn't, then either the brake lamp switch is bad or there is an open circuit to or from the switch.

Let me know what you find.

Take care,

Joe

See pics below. Note, that was one page. I had to cut it in half to make it readable. I did overlap them and highlight all relevant wiring for the brake lamps.

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Monday, October 31st, 2022 AT 7:06 PM

CESAR SAMANIEGO

MEMBER
3 POSTS

Also, I feel like I should give a full background on the issues. So, when I bought the truck the third brake light didn’t work and only the left brake light worked. And none of the rear running lights worked. And I know this wasn’t because of the bulbs because I bought new ones all around. So, then when I check the wiring under the rear bumper and it was all garbage, somebody tried to jerry rig it with extra pieces of wire and tape and it was a mess, so I went out bought new wire and got rid of the burn off parts and all the old and practically ran new wire from each light and a segment on the main harness from the fuel tank towards the back. I connected everything with clips and shrink wrap and tan a new ground cable where the old one was because it was all torn up also and after that I got the running lights to work, the hazards, the turn signals and the reverse lights. But not the brake lights. I bought a new brake switch but didn’t seem to do anything, I checked the orange wire and yes there is power to it. When I close the switch on the brake switch nothing happens, I checked the white wire and there is power, but I’m lost on what to do next. Like I have said this electrical issues have me pulling my hair.

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Tuesday, November 1st, 2022 AT 12:33 PM

JACOBANDNICKOLAS

MECHANIC
109,867 POSTS

Hang in there. We'll figure it out.

If you have power to and from the brake light switch, the third brake light should work. Does it?

If it does, chances are the turn signal/hazard switch is bad. Power goes to the switch from the brake lamp switch and is distributed via the same wires are the turn signals. So, confirm there is power at the switch itself (white wire) and power out from the signal switch to the yellow and dark green wires.

Let me know.

Joe

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Tuesday, November 1st, 2022 AT 7:18 PM