Trailer stop light problem

[CCrev440]

My first post. Hopefully someone can help me.

I just bought an '04 Avalanche about 6 weeks ago. Its got 60,000 miles on it, looks like mostly highway. I don't know if it towed at all.

Anyway I bought the truck to tow my snowmobile trailer (just a 4 pin). I hooked up a few days ago and all the lights worked execpt the left brake light. It blinks with the turn signal however.

I then took a test light to the plug on the truck and it was the same. No brake light, so the problem is in the truck. I checked the fuses and the only ones I found are trailer stop &turn so if that was blown it wouldn't blink, right? I swapped them anyway but it didn't change anything.

Is there another fuse somewhere or a relay, or even just where the junction of the brake and turn signal is for the trailer harness. Any other thoughts?

I may have to bring it back to the dealer, I just don't want it to be something stupid and look like a dope.

Thanks in advance,
Chris.

 

[ygmn]

fuse box under hood... for trailer lights.

Check connector under rear bumper.. might be corroded?

 

[mpingree]

I then took a test light to the plug on the truck and it was the same. No brake light, so the problem is in the truck. I checked the fuses and the only ones I found are trailer stop &turn so if that was blown it wouldn't blink, right? I swapped them anyway but it didn't change anything.
Chris.

Had the same problem with my old '04. Fuse is under the hood.

 

[CCrev440]

Thanks for replying.

Ok, I looked in the main fuse box under the hood again, the only fuse in there for the trailer is a 15 amp and its trailer park lamps. One side stop light works so there can't be just one fuse.

On the panel inside the door there are 2 10amp fuses for left and right trailer stop and turn. I've switched both of those and it didn't help.

Am I just blind and I'm not seeing a left trailer stop fuse? Or is there anpther fuse box under the hood?

Chris

 

[AVid]

The fuse your looking for is under the hood. The legend is in the owners manual and also on the underside of the cover to the UBEC. The fuses inside the IP are for the truck, the trailer circuit is separate from those.

AVid

edit: I just re-read your post and you said that the turn signal worked, yet not the brake light. They are the same wire on the truck. It sounds like you may have a ground problem in your trailer, or in the trailer tail light assembly itself. 

 

[CCrev440]

It is definetly a problem with the truck, I put a test light right on the trucks'  plug and the right side works and the left side doesn't. The trailer works fine with my Tahoe.

In my truck, the fuses for the trailer are in the IP. (trailer turn/stop rt and lft) Only trailer running lights are under the hood, unless they are named something else. Doesn't matter as I have switched all trailer related fuses, they were all good and nothing fixed the stop light.

I'm wondering now if it could be the switch on the brake pedal or the plug into the fuse box is screwed up. . .

 

[Maverick1974]

I would check the wiring under the truck, see if something might have been modified in the past.  Also check the wiring behind the brake lights.  You might have gotten a truck that had the all four brake & turn mod done to it, and it was not removed correctly. Check this thread

 

[AVid]

Good idea. My next thought would be to crawl under the truck and unplug the harness from the plug. Check for corrosion and voltage at the proper wire.

AVid

 

[Maverick1974]

Because the trailer is getting a turn signal and not a brake signal on the left side sounds like the left side is wired into the turn signal wiring of the truck not the combined brake/turn wiring in the trailer harness.  I think you need to pull all the trailer fuses out and see if any of the lights work on the trailer, that is an easy way to find out if the trailer harness is hooked up correctly in the truck.

 

[CCrev440]

I tried pulling the fuse marked "left trailer turn/stop" and it didn't blink as it shouldn't. And each of the other lights when the trailer fuses where removed didn't work.

I also unplugged the harness from the plug and it is the same as the plug.

I'm wondering if I can pull the fuse and jump it with my test light and see if the test light works when the brake is on?


My supercomputer won't link me to that 4 way brake thread so I'll read that later on to see if its any help.

Thanks for the ideas!

 

[ygmn]

under truck somewhere in rear is harness for truck tail lights & trailer plug thingy..

some people have had issues with a connector semi hidden corroding and messing up the tail lights.. but I suppose it could effect trailer harness plug thingy...

 

[bandt0913]

if crawling under the truck, disconnecting and reconnecting doesn't help, try replacing the 4 pin connector on the back of the truck. I had same problem and it was fixed when I replaced the connector.

 

[CCrev440]

Update!



I took it to the dealer, took an hour ($85) to trace it to the flash module ($70) which they didn't have.

I do have an account at another dealer, which had the part. ?I took the upper and lower dash panels off and after a bit of a fight I got the flasher module out of the back side of the fuse panel. ?I put the new one in and it works!

Thanks again.
Chris