Turn Signals Not Working in 7-Way Trailer Connector on a 2002 Ford F-350

Question:

I have a 2002 F350 diesel crew cab with a factory wiring harness for 7 point towing hook up. I have power in the running lights, electric brake, and the hot wire. The ground is functional. Neither the left or right turn signal wires have power. I have traced them back to the point where the harness traverses up to the drivers side engine compartment. Is there a relay or something that could account for loss of both left and right turn signals? My truck turn signals work and the fuses for the towing turn signals are good and tested. Is there an actual towing turn signal relay that could be bad? where is it located.

asked by: Rob H

Expert Reply:

On a trailer, the turn signals are on the same circuits as the brake lights. On a 7-Way trailer connector they would be the pins in the 9 oclock and 3 oclock positions. If you are not getting turn signal functions on those two pins, but you are getting brake lights, then that does indicate a possible blown fuse. But since you said the fuse is fine there could be a relay somewhere.

The easiest way to find the locations of fuses/relays is to use the truck owners manual. On many trucks there are more than 1 fuse or relay for a given function. That could be the issue here.

Also make sure that the trailer and truck side connectors are clean and free of corrosion.

It could also be that there is something wrong in the wiring between the trailer connector and where you found the signals for the blinkers further forward on the truck. You may have a broken wire or pin inside a harness. It might be easiest to tap into the wires where the signals are working and run wiring back to the trailer connector manually.

expert reply by: Michael H