More Trailer Problems

[Seabass77]

My buddy and I spent last night re-wiring his trailer and ran into some problems. We got his turn signals and stop lights to work but can’t get the running lights to come on. Plus, when we try to use the turn signals or stop lights while the running lights on the truck are on, they won’t work.

We do have power at the taillights when the truck running lights are on but can’t get any thing to work. We checked the truck pig tail and there is running light power as well.

Has anyone ever experience this situation? Any help would be appreciated.

[huskminn]

Seabass,

From my and others' experience, it's a problem with the ground. I am by no means an expert...not even a layman, but when things go wacky with trailer lights and you're not dealing with old, cracked wiring, then it is almost always related to the ground.

Check out the previous discussion of "Trailer Lights".

[hydro]

Here's a couple of tips.

1. Isolate the problem.

Separate the pigtail and using a VOM on Volts, check for voltage on the truck pigtail. You will see about 12V on the striped wires with the running lights on and an intermittent voltage on the solid color wires with the turn signals on, and a constant 12V when you step on the brakes. If you don't see this you have a problem with the truck wiring.

2. Diagnose the source of the problem.

If the truck checks out ok the problem is in the trailer. Lift the grounds at both trailer taillights, pull the bulbs, and do the same check at each light socket. The VOM will provide the ground path to the trailer frame. If the power checks out as above, the problem may be with the light bulbs themselves; replace them and check again. If the VOM readings do not follow the truck pigtail results, the problem is most likely with a ground. Run a long wire from the white wire on the trailer harness to the taillights and repeat the test using the wire instead of the trailer frame as the ground point. If the circuits check out correctly the problem is the trailer frame ground continuity and can be corrected by simply running the ground wire to each taillight assembly.

I have found that when trailers get a little old, there is almost always a grounding problem between the hitch and the back of the trailer and the wire eliminates all of the issues with the trailer frame ground.