1990 GMC Truck Brake lights, reverselights problems

JAHUNGER

MEMBER
1990 GMC TRUCK
V8
4WD
AUTOMATIC
200,000 MILES

My front headlights and parking lights and hazzards work fine, but I have no Tail Lights, Reverse Lights Or Brake Lights. The Hazzards work in the back, but only if the headlights are turned off. When I push the brake pedal, the dash lights and radio turn on!
I can rebuild the 350 TBI in my sleep, but this puzzle has got me completely confused. I've changed out all of the relays and turnsignal fuses that I could find, and still no help.
Is there some sort of sensor on the brake pedal lever that controls the rear lights?
I can't drive for fear of getting rear ended, and i've tried every thing I can think of. The local shops have benn no help, but costly.
I'm seeing many posts with this samr problem so it has to be a single part, i'm thinking, that is faulty or wears out easily.

3 Replies

BUDDYCRAIGG

MECHANIC
2,262 POSTS

The circuit board the lights plug into have problem with corrosion. You can get replacements at the auto part stores.

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Thursday, April 23rd, 2009 AT 5:05 PM

CARADIODOC

MECHANIC
33,873 POSTS

Check your trailer wiring harness to see if it's chewed up or if you have the U-Haul type with three red leds in it. If the leds short, which was common, voltage to the brake lights was applied to the running and tail lights which is tied to the dash lights, (and radio lights).

Also check for a bad ground on the rear bulb sockets. Current will try to go through the brake light bulb, can't get to ground, but will find a path through the other filament, (tail light), through the tail light on the other side, then to ground from that socket. If the bad ground is in a single wire for both the brake / tail light and the backup light, the brake light current will find its way to ground through the backup lights too, so they will also be on dim.

Thank goodness you don't have computer-controlled lights like on the newer trucks. This over-engineered mess is one reason GM, (everyone) can't sell vehicles anymore. People have been bitten too many times with huge repair bills for what used to be simple problems.

Caradiodoc