Brake lights dont work with DRLs on..

[Kpawl89]

1988 GMC C1500.
Okay, so I just noticed when my DRLs are switched on, my brake lights don’t work. Everything else works as designed. With the DRLs OFF, the brake lights work. When I have a turn signal on, the brakes work as well. Hazards work. Signals work. Literally everything except when DRLs are on the brakes don’t. Please help!

 

[DeCaff2007]

Didn't know an '88 even had DRL's. Do you have 4 headlights?

 

[Kpawl89]

Didn't know an '88 even had DRL's. Do you have 4 headlights?

Okay parking lights would be the correct term I guess. When you turn on the ambers and rear tail lights but not the head lights

 

[GoToGuy]

Headlights off foot brake, no brake lights? Headlights off, no turn signals?
Just parking lights on , turn signals? Just parking lights on, brake lights?
The turn signal and brake function on the same bulb filament, the brighter one.
You started the truck, step on brake, have brake lights?
Turn headlights on, tail lamps on? Then step on brake, no brake lights?
This is a two person test, unless you have a large mirror to see the rear end.
Brake lights and headlights are two different circuits.

 

[Kpawl89]

Headlights off foot brake, no brake lights? Headlights off, no turn signals?
Just parking lights on , turn signals? Just parking lights on, brake lights?
The turn signal and brake function on the same bulb filament, the brighter one.
You started the truck, step on brake, have brake lights?
Turn headlights on, tail lamps on? Then step on brake, no brake lights?
This is a two person test, unless you have a large mirror to see the rear end.
Brake lights and headlights are two different circuits.

Headlights off, brakes YES.
Headlights off turn signals YES.
Parking lights on turn signals YES.
Parking lights on brakes NO.
Start truck brake lights YES.
Headlights must go on with tail lights, brake lights NO.

 

[DeCaff2007]

Kpawl89 may be on to something here. Tail lights and brake lights are dual filament 2057's. If you look close, there are offset nubs on the stalk of the bulb. They are meant to allow the bulb to be installed only one way. Well, over 30 plus years, the bulb sockets are prone to let the bulbs install either way. Check them. There's a 50/50 chance the bulbs are simply installed backwards - which means you'll get tail lights, but no brake lights, because when installed backwards you'll get both filaments lit or none at all.

I went through this same crap on my T/A and something similar on my 88 RCSB.

 

[95burban]

Is everything oe replacement or do you have aftermarket led stuff?

 

[Kpawl89]

Is everything oe replacement or do you have aftermarket led stuff?

All oe original as far as I’m aware

 

[Kpawl89]

Kpawl89 may be on to something here. Tail lights and brake lights are dual filament 2057's. If you look close, there are offset nubs on the stalk of the bulb. They are meant to allow the bulb to be installed only one way. Well, over 30 plus years, the bulb sockets are prone to let the bulbs install either way. Check them. There's a 50/50 chance the bulbs are simply installed backwards - which means you'll get tail lights, but no brake lights, because when installed backwards you'll get both filaments lit or none at all.

I went through this same crap on my T/A and something similar on my 88 RCSB.

Hmm this makes sense! I’ll check when I get home and get back to ya !!

 

[Kpawl89]

Hmm this makes sense! I’ll check when I get home and get back to ya !!

So the bulbs were indeed installed incorrectly, and noticed they were rusted on the inside, so I went ahead and replaced them.

Now, the driver light works perfectly as it should. The passenger light, the brake light is dim, and when the running lights are on and I press the brakes, the lower running light turns off and I don’t have brakes, but only on the passenger side.

I noticed a red neck job of splicing the trailer connector so I went ahead and ordered a new wire harness for the lights and will remove the trailer harness to see if that works.