Hi..I picked up a used clamshell aluminum trailer..seller had all lights working as verified when we met. My truck works fine with my other utility trailer (lights etc) but with this one the only way the lights work is if I run a jumper cable lead from truck frame to frame of trailer..any ideas?

 

4 pin or 7 pin?

 

Yes. There is a negative wire connection in your trailer plug (commonly called "Ground") that is not allowing the current that is flowing to your lights from the vehicle battery to return to it. Since your lights work with other trailers, the fault is in the trailer wiring. You need to fix the connection in the ground wire from the trailer plug to the trailer frame. I suspect the connection at the trailer frame is loose/not connected/broken.

- Jack

 

sorry..forgot to mention. Just the flat 4 pin

 

Thanks for your reply..I can see the point you make but confused as to when seller met me the lights were working..at least the brake lights were. Either way I will do some digging on the trailer grounds..thanks again

 

Could be that when you met the seller it was grounding through the hitch, and now it isn't.

 

There's only one reason a DC electrical component doesn't work. There is no complete path in the circuit from the (+) side of the battery through the component and back to the (-) terminal of the battery.. Since you get lights when you jump the trailer plug's ground connection, we know the positive circuitry is good, the lights are good, and since other trailer's lights work when plugged into your truck, we know the negative circuit in the truck is good. That leaves the trailer plug and the trailer (-) wiring, with it's connections.

The negative wiring in a trailer is usually colored white, if it is up to "standard". Thinking about this a bit more, I'm now inclined to think the wire connection to the trailer plug is bad, or the contact inside the trailer plug is dirty. If you have a multimeter, you can easily check for continuity between the trailer plug's (-) pin opening and the trailer's frame. Then, it's just a case of finding where the break is. You MAY have an intermittent connection inside the trailer plug. Try wiggling the wires at the back of that plug with it plugged into the truck. When I say "trailer plug", I';m talking about the one on the trailer, not the one on your truck.

What glc just posted is a possibility too. If you have good frame grounds on both the truck and the trailer, then the circuit can be completed through the hitch.

- Jack

 

So just to update..I took your advice and started looking closely at the trailer wiring...specifically the grounds. Near the coupler there was the white wire from trailer harness grounded on a screw into the neck of the trailer..also on the same screw was a single wire that ran to rear of the trailer..some sort of secondary ground ( I am not sure) but I traced it all the way to the back and found it hanging loose..grounded it to the frame and bang all lights working! Thanks again for the advice.

 

Great! Glad you got it working and thanks for getting back to us with the solution!

- Jack