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2003 duramax no brake lights

TJ1978

TJ1978

I have MANY questions
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driving today I was informed I have no brake lights. All the other lights work. No brake or the lights on the top back of the cab.
I checked the fuse (25) it looked good. This all happened after off roading today. I checked the junction box by the spare tire and nothing looks bad..

All the bulbs work
I can't find the ground for the rear. Is it above the spare tire and if so how do I get to it to clean it?

rear lights work when alarm is pressed

What else could cause this?

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TJ1978 said:
driving today I was informed I have no brake lights. All the other lights work. No brake or the lights on the top back of the cab.
I checked the fuse (25) it looked good. This all happened after off roading today. I checked the junction box by the spare tire and nothing looks bad..

All the bulbs work
I can't find the ground for the rear. Is it above the spare tire and if so how do I get to it to clean it?

rear lights work when alarm is pressed

What else could cause this?

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There is a connector towards the back in the harness, check to if it's pulling apart.
 
yes that 4 plug device is a solid state lamp control module they do fail. Check that you have power in at stop lamp switch on brake pedal, power out when pedal depressed.
 
There are a couple fuses. I believe the turn signal flasher module in the left side of dash fuse panel also have issues on these trucks. I highlighted the brake light circuit yellow. Start at the fuse for the brake switch and work your way through.
 

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  • Exterior Lights - Exterior Lamps Circuit (Lighting and Horns) - ALLDATA Repair.pdf
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This thing is the issue. Usually when I brake the red light illuminates on it, now nothing. If I press in the button the break lights work. I tried unplugging it from the fuse box by the brake pedal but the brake lights don't work that way either.

Is there a way to fix this thing or work around it to bypass this trailer brake assist thing?

If the fuse is blown can it still work by me manually depressing it to light up the brake lights?
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Also, normally when I start the truck the headlights come on and now they don't
 
Weird, the brake controller just gets a signal from the brake switch to send juice to the trailer brakes. Just unplug it. Maybe bad trailer plug.
That's daytime running lights. Different issue, burned out bulbs? Blown fuse?
 
Truckman4life said:
Weird, the brake controller just gets a signal from the brake switch to send juice to the trailer brakes. Just unplug it. Maybe bad trailer plug.
That's daytime running lights. Different issue, burned out bulbs? Blown fuse?
Click to expand...
Right... when I press the brakes with it connected I get no brake lights. When I disconnect it no brake lights. But when I manually press the controller button the brake lights work.

I'm assuming the brake lights should work regardless if that's plugged in or not. But They do not. And the fuse for the brake lights should, pictured above looks good.
 
The trailr control is supposed to activate the brake lights when you use it w/o stepping on pedal. It allows you to brake using the trailer brakes only. Wise move when going down steep grades.

Check the brake switch at pedal i suspect that is where the problem lies.
 
Wes Harden said:
The trailr control is supposed to activate the brake lights when you use it w/o stepping on pedal. It allows you to brake using the trailer brakes only. Wise move when going down steep grades.

Check the brake switch at pedal i suspect that is where the problem lies.
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Ok. I see video how to test it.... I think
 
Wes Harden said:
The trailr control is supposed to activate the brake lights when you use it w/o stepping on pedal. It allows you to brake using the trailer brakes only. Wise move when going down steep grades.

Check the brake switch at pedal i suspect that is where the problem lies.
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So why was it lighting up when I pressed the brake pedal and now it doesn't?
 
TJ1978 said:
So why was it lighting up when I pressed the brake pedal and now it doesn't?
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It's supposed to.
I think the trip you did jarred the brake switch and most likely got out of adjustment.
Check it out like @Wes Harden said.
The brake pedal is supposed to activate the controller and that will get the brakes on the trailer.
I believe your switch is bad or out of adjustment. Because obviously when you apply it manually the light works
 
imiceman44 said:
It's supposed to.
I think the trip you did jarred the brake switch and most likely got out of adjustment.
Check it out like @Wes Harden said.
The brake pedal is supposed to activate the controller and that will get the brakes on the trailer.
I believe your switch is bad or out of adjustment. Because obviously when you apply it manually the light works
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I'm sure it was last trail I did. It was heavily pot-holed and wash boarded.. I'll check it out tomorrow.
 
I think the brake controller lighting up your brake lights when manually pressed is normal. It acts just light a brake switch when doing it manually. It will backfeed into the output side of your brake light switch and light up the lights letting someone behind you know you activated your electric brakes. If it doesn't function with the pedal then either your brake switch is bad or the fuse before the brake switch.
 
This morning I started it up and the trailer brake was not connected and the brake lights worked like two times then stopped working.

This is the switch, correct? How do I check it? And what is the best way to remove it? This thing is a very inconvenient spot and very difficult to see hahahHa

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The two wires on the left are for the brake lights. Orange is power coming from the fuse (hot all the time) and the farthest left is white and that is output from the brake switch. If you can get a test light in there you can just probe them from the back on that style terminal.
 
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