Brake Lights not Working on a Trailer with Truck Headlights On

Published 04/02/2010

Question:

I have a 2005 Chevy Avalanche. It has seperate turn and brake lights. It has a 7 pin blade plugin. I bought a 7 to 4 pin adapter. The trailer brake lights dont work when the truck lights are on they seem fine when the truck lights are off. Otherwise the trailer lights work ok. It seems to me that the converter units I see for sale are for situations where one is starting from scratch with the wiring. The pin out on the chevy says both turn and brake. Any ideas?

asked by: scott k

Expert Reply:

A factory 7-Way on an Avalanche will have the brake and turn signals for each side combined, one circuit for left brake and turn, one circuit for right brake and turn. If the truck came with a 7-Way from the factory, everything should be wired correctly there.

I would check your ground connections. Most trailer light problems are the result of bad grounds. Check to make sure the taillight assemblies have good metal to metal contact on their ground connections. Make sure the ground wire from the trailer frame to the connector is clean and free of corrosion. Look for nicks or breaks on the wire as well.

If you suspect that the 7-Way is not a factory GM unit, you may want to use a circuit tester, part # PTW2993, to see that it was wired properly. Check our Wiring FAQ, linked below, page to see how the 7-Way should be wired and use your circuit tester to verify.

expert reply by: Patrick B