Electrical Question

I hooked up my trailer connection and all the basic running lights function, but the left side taillight is dimmer than the right. What could cause this to happen?


Senior Member
RE: Electrical Question

I'd say a contact needs cleaning or bulb replaced. Might be a different type bulb for running and "braking", like in a car (dual element). Did you check to see if brake lights work on both sides? You can also check with voltage meter to see if same power is going to both sides. just a couple ideas.... :)
Re: Electrical Question

Trailer lights are dual element type, all the functions work (braking, signalling etc) its just dimmer. Alot of the wiring for the various lights are all inside the walls of the trailer, so hard to really find where the ground connections are for any particular light is. Just tonight, the dim side went out...when first connected to trailer up to test out the brake controller/brakes, the lights worked on both sides. Took a drive and when we stopped the left hand rear light was out....the bulb is fine, so must be some loose wire somewhere that decided to let go...look forward to finding that baby!


Senior Member
Re: Electrical Question

If it is just one light then it probably is the light socket, or the ground to it. Have you attempted to clean the socket? Most dual element lamps have two buttons on the bottom of the bulb and the side is the ground where it has two pins that go into a slot and you turn the bulb. I would first clean the bulb, both buttons and the base with steel wool. Then with no power to the lights, use that same steel wool to clean the inside of the socket. While it could be a high resistance connection to both buttons, it isn't likely but a poor or dirty fit between the base and the socket. The other thing to check is to measure the resistance from the lamp socket (the part that contacts the bulb's base) to the frame of the trailer. It probably uses a chassis ground so that needs to be no more than 1 ohm. If it is, that can cause the dim light. If that checks good, install the bulb into the socket and with it not connected to the tow vehicle, measure the resistance from the edge of the bulb's base to the frame of the trailer. That will tell you if there is poor connection between the base of the bulb and the socket. You can also check that by supplying power to the lights to turn them on and then measure voltage from the base of the bulb to the trailer frame, or ground. If the connection is good you will get almost no voltage there but if the ground connection is poor it could easily be 4 or 5V and that would account for the dim lights.