Trailer Lights Work When Towed By Tahoe But Not When Towed By Ford Truck

Expert Reply:

Although it would seem that you have eliminated the trailer wiring as being the problem, taking a quick look at the trailer wiring would be a good first step. Start at the trailer connector, and work your way back the trailer until you reach the lamps. Look for pinched wires or wires with cracked or worn insulation that would allow a short to ground. You might have a minor short that is not affecting the Tahoe, but is causing problems with the Ford. Examine the lamps themselves for rusted bulb sockets. Repair or replace as necessary.

Then, go on to the trailer connector on the Ford. Use a circuit tester like part # 40376 on the pins fed by the yellow and green wires. Both pins should show voltage when the vehicle brakes are applied. Make sure that the connector on the truck is securely grounded to the truck frame. All sorts of silly things can be caused by a poor ground connection. Are the brake lights on the truck functioning? Is the third brake light working? Check the fuses that manage the towing circuits. These are usually located in a fuse block located under the hood. Look on the drivers side fender, up close to the firewall. You should see a small black box that once the cover is removed, will reveal a few fuses. Although I was not able to find out which fuse should be checked, there are only a few, so it would not be too big of a deal to check them all. From what I understand, the presence of this particular fuse box is not mentioned in the owners manual.

expert reply by: Mike L