Troubleshooting a Trailer with Brake and Turn Signals but No Tail or Running Lights

Updated 03/29/2010 | Published 03/28/2010

Question:

Turn signals and brake lights work but the running lights do not. Its a standard 4 wire boat trailer.

asked by: Jayson

Expert Reply:

The first thing I would check is the trailer connector on the vehicle. This can eliminate the vehicle as the source of the problem. Turn on the vehicle head lights and use a circuit tester, like part # PTW2993, to check the trailer connector. Clip the circuit tester to a good ground on the vehicle and use the probe of the tester and touch the brown or running light circuit pin. If the circuit tester lights up, you have a good connection, and the vehicle is not the problem. If it does not light up, trace the wire and look for breaks or frays. You can also check the fuse panel and see if a fuse has blown.

If the vehicle checks out, then the problem is on the trailer. First thing to do on the trailer is check grounds. A bad ground connection is the usual culprit with most trailer wiring problems. Check the ground connections at the trailer lights. Depending on the lights, the taillights probably have their own ground, so make sure the connections have good metal to metal contact and are not loose or frayed.

Check the bulbs themselves. The brake and turn lights either use a different bulb or a different filament within the bulb which is why it is possible to have brake and turn, but not running lights or vice versa.

You should find the problem by doing the above. If you try them and do not find them, write back and let me know and we can go on from there.

expert reply by: Patrick B