Trailer brake lights not working but turn signals do. 2005 Yukon - SOLVED

[rcecarbldr]

Just recently I lost the brake lights on any trailer I hook up to my Yukon yet the turn signals work, running lights and hazards. It has a 4 pin connector. Yet the trailer brake lights use to work. Any ideas?

 

[Big Mama]

Most likely corrosion in one of the pins or receptacle if they both went out at the same time. Do the brake lights on your truck work? Do your reverse lights work?

 

[rcecarbldr]

Most likely corrosion in one of the pins or receptacle if they both went out at the same time. Do the brake lights on your truck work? Do your reverse lights work?

Everything on my Yukon works. And only the brake lights on the trailer don't work. All else does. Tested two different trailers, same results. I was told this could possibly have something to do with it? I replaced the OEM on last September with one from Amazon but all trailer lights worked till just recently.

 

[Fless]

FWIW the trailer lights go through the turn signal flasher, if that's any help. You can find electrical diagrams on www.charm.li.

Is the 4 pin connector a factory connection? Many of these came with the factory 7 blade trailer connector - yours didn't?

 

[Big Mama]

As @Fless stated if you’re using an adapter the trouble may be there. Your test light can verify. You also may just have a frayed wire there.

 

[GMCChevy]

Start by checking if the brake light pin works at the connector. If your 4 pin was added and you're not using an adapter to go from 7 to 4 start there.

 

[Fless]

Remember that the trailer light circuits are fused separately from the truck body light circuits, but they share some functions with/through the flasher. I'm not saying it's a fuse issue because the turn signals work, and those are the roughly the same as the brake lights.

Are the body turn signal light bulbs different from the brake light bulbs? They are on my '04, but I'm not sure on your '05.

On my body the turn signals and emergency flashers are amber, but the flasher needs to combine the trailer brake light function since the trailer uses the same bulb filament for both turn signals / emergency flashers and the brake lights. It's a trailer light converter of sorts for TV systems that have separate turn signals and brake lights. The flasher works like this 3-to-2 converter:

Tail light converter

 

[rcecarbldr]

FWIW the trailer lights go through the turn signal flasher, if that's any help. You can find electrical diagrams on www.charm.li.

Is the 4 pin connector a factory connection? Many of these came with the factory 7 blade trailer connector - yours didn't?

I bought this used 2 years ago with a hitch and 4 pin connector already on it but everything worked fine up till recently. And the '05's have the same tail lights as the earlier models. The two pins that send power to the Left and Right trailer lights supply power for both which is why this is confusing but I haven't tested for voltage at the pins yet while not connected to a trailer so I'll be doing that later today.

 

[Fless]

The 3-to-2 circuit conversion is done either by the SUV's flasher, or by an aftermarket converter that was installed to get the 4-pin plug in place. You have one or the other.

Get underneath and trace the 4-pin harness to see if it simply connects to a body harness, or if there's a converter installed.

 

[rcecarbldr]

Yesterday I removed the 4 prong plug off the OEM wire harness where it had been spliced on by a previous owner and tested for power with the brakes applied and nothing at the harness which certainly tells me it's up front somewhere. Whoever spliced on the 4 prong plug left a whole lot of excess wire so I shortened it up some, spliced it all back together with heat shrink butt connectors and tested again. Everything works as it should except no brake light power. Does anyone know if the brake light switch under the dash could be the culprit. Again all brake lights on the Yukon work fine but there is no power at the trailer plug.

 

[Big Mama]

Does your cruise control work? It’s any easy way to check that switch.

 

[Fless]

The 3-to-2 circuit conversion is done either by the SUV's flasher, or by an aftermarket converter that was installed to get the 4-pin plug in place. You have one or the other.

Get underneath and trace the 4-pin harness to see if it simply connects to a body harness, or if there's a converter installed.

Is there an aftermarket converter or incorrect aftermarket wiring? If so, that's the culprit. Otherwise change the flasher.

 

[rcecarbldr]

Well I decided today to dig back into this issue and see if I could discover the problem and I finally did. Last June I installed an LM487 turn signal flasher relay to stop the hyper flash with LED lights. So after replacing my brake light switch thinking it could be that I got to thinking about what FLESS said and decided to swap in my original flasher and BOOM! I have trailer brake lights again. So I guess I'm just going to wire in resistors to the turn signal sockets on my tail lights and leave the OEM flasher in. The flasher was a cheap one I got off Amazon. Higher quality ones are available from NAPA and other parts stores but for nearly $80. Nope.

 

[SilverSport]

thanks for getting back with the solution and sharing...

Bill

 

[rcecarbldr]

So I put resistors on all for corners of my Yukon and now no hyperflash and trailer brake lights work normal.

 

[Big Mama]

Thanks for closing the loop for the next guy.