Trailer Tail Lights Go Out With Headlights On And When Brake Pedal is Pressed On 1993 Chevy 3500

Question:

Taillights not working when lights are on, and brakes applied. 93 chevy 3500 with tekonsha brake controller. All other lights work, except brake lights, when headlights are on, then brakes applied. Thank you

asked by: Ted W

Expert Reply:

It sounds like you have a ground problem on the trailer. However, there also could be a problem on your tow vehicle.

We can isolate the problem to narrow the search a bit. Disconnect the trailer from the truck. Using a circuit tester, like part # 3808, test the 4-Way connector. Clip the tester to the truck frame as your ground, and test the brown, yellow and green wires to make sure they function properly. Brown is tail lights, yellow is left turn and brake, and green is right turn and brake. If the connector does not work as it should either there is a problem in the installation or some fuses or relays burned up in the fuse blocks on the truck. When head lights are on, you should always have power on the brown wire, but you should also get power on the yellow and green wire when the brake is depressed.

If all the functions at the vehicle connector work the problem is on the trailer. Check bulbs and wiring connections. Check to see if wires are frayed or nicked. Most trailer electrical problems are results of bad grounds, so keep an eye on these.

Please write back and let me know what you find.

I am sending a link to a FAQ page on trailer wiring that will also describe how to troubleshoot your trailer wiring.

expert reply by: Patrick B