Troubleshooting Haulmark Trailer Dome Light

Question:

My haulmark tail and running lights work when vehicle engine not running. When I start vehicle lights flash on and go out. If I turn on interior light with engine not running the lights go out. Brake lights and turn signals work all the time. I cleaned the ground but problem still persists.

asked by: David F

Expert Reply:

First I recommend you carefully check the condition of the vehicle and trailer connectors. There could be an internal corrosion issue that is causing adjacent circuits to short. If the trailer has been stored outside for any length of time moisture could have migrated into the plug and caused nearby circuits to short. Also check the condition of the dome light's wiring and bulb socket.

A trailer dome light will typically be powered either by a separate trailer-mounted battery or by the auxiliary 12V power circuit from a 7-way. It should not interact at all with the vehicle engine or other lighting circuits. If you refer to the linked photo you'll see how all circuits are arranged on the trailer 7-way connector.

If you have 4-pole wiring without a separate +12V power circuit then the dome light would have to be connected to one of the other circuits, most likely the running lights, but this is not ideal since it means your headlights have to be on in order for the dome light to work.

You can also test all the trailer lighting functions independent of the vehicle to help isolate the issue. You can use the Jeep battery or a fully-charged spare if you have one. A set of jumper cables and two length of wire make this easy enough. Connect the jumpers to the battery terminals and the pieces of wire and apply the ground wire to the 7-way or 4-pole ground pin. Then, one by one, apply the +12V lead to each circuit and observe what happens on the trailer. If any light behaves incorrectly then you know to inspect the wiring for that circuit.

My suspicion is that the problem is on the trailer in the dome light or its wiring and the battery test will tell you. Please let me know the results of the battery test.

If you do find the dome light or its wiring to be the problem I recommend adding a small trailer-mounted battery and new wiring to provide independent power for the dome light. To maximize the time the dome light will operate from that battery use a high-efficiency LED type like # RVILL34 which has its own power switch. You could use a small breakaway battery like # HM20008 to deliver power and a small solar charger panel like # PTW2997 to maintain the battery.