Trailers Signals Only Work When Trucks Headlights Are in Auto Mode

Published 11/26/2019

Question:

As long as truck headlights are on auto mode trailer turn signals work ok but not when vehicle headlights are switched to on or manual mode Is this also a potential ground problem or just normal?

asked by: Mark B

Expert Reply:

I have not ran into anything like this before. But it sounds like there is an issue with the truck side of the wiring. I recommend that you isolate the truck and trailer and inspect the grounds on both. You can also check the contacts inside the connector of the truck and trailer for broken terminals, or corrosion build up.

If all the grounds look good and have solid contact with a bare metal surface, I would test each function of the truck, verifying each signal when not hooked to the trailer using part # PTW2993. Also you can check the operation of the trailer lights with a different vehicle. If you don't have another vehicle, you can use a car battery. Attach the negative terminal to the ground on the trailer's frame, and the positive terminal to each pin on the trailer's connector 1 at a time.

This will show you what is working separately on the truck and trailer, and what functions are not. If it still doesn't resolve the issue, please follow up with me so I can continue troubleshooting. I have attached an article on troubleshooting trailer wiring for your reference.

expert reply by: Robert C