Trailer Running Lights Work But No Stop/Turn Lights After Installing New Trailer Wiring

Question:

4 way plug on vehicle seems to be fine because it works perfectly on another trailer. New trailer originally seemed to have a short somewhere since it was blowing the cars running light fuse, so I replaced all the trailers wiring and lights. Now running lights are fine, no longer blowing fuses, but no brake lights or turn signal. With a multimeter I tested the resistance on both sides from the ground at the plug to the bolts on the tail lights that act as a ground for them. Seems perfect same 0.2 ohm resistance as when I touch the two multimeter probes together, which leads me to think the ground to the tail lights is OK. Also tested plug ground to ground screw, and ground screw to tail light ground. All seem OK. Now Im at a loss. Id say there is a problem with the green right and yellow left wires that go to the tail lights, but it seems unlikely both sides would have that problem with brand new wiring. Similarly, I wouldnt think the brake filament in the bulbs is fried on both sides of the car.

asked by: Bob F

Expert Reply:

Thank you for detailing your troubleshooting efforts with the new wiring on your trailer.

First, check the ground wires on both the harness and the trailer. Make sure both are tightly affixed to a clean bare metal spot on the vehicle and trailer frames. You can sand off paint and secure the wires to a new grounding point if needed. A weak ground can cause all sorts of weird behavior with lighting.

Check all wiring for signs of cracks or worn spots in the insulation that could cause a short and for any loose connections.

You can also test the vehicle harness with the trailer disconnected by having a helper activate each light function while you apply a tester like # PTW2993. You can also test the vehicle wires leading into the wiring harness to make sure the correct input signals are reaching it. If you do not detect correct left and right stop/turn signals going into the converter you will need to check those connections where the vehicle's wiring and the harness splice together.

Sometimes a vehicle wiring harness will work fine with one trailer and not another; on occasion the simple act of plugging and unplugging the trailer connector can cause an intermittent problem to appear, but then disappear in other circumstances. If you find this to be the case you can try replacing the 4-pole with the item you referenced, part # 18004.

You can also make sure the two connectors are free or dirt and corrosion - even slight greenish or whitish discoloration can indicate corrosion that can interfere with signal transmission.