Trailer light problem

[Musky Mike]

Hello, on my trailer the brake lights don't come on, when the parking lights are on and you step on the brake the parking lights go out and the brake lights don't come on. Also, when I turn on either blinker, both blinker flash at alternate times. I checked all four pins coming out of the truck and they checked good (when I hook up my other trailer it works fine). I checked the hitch ground and the pigtail to the trailer ground and they were both good. I cleaned the pigtail to trailer ground and each ground coming out of the tail lights that were grounded to the frame. Is there more than 3 grounds on the trailer (ground from the pigtail to the trailer, grounds from the tail lights to the trailer)? I checked the voltage for parking light, blinker and brakes coming out the trailer hook up and it was good. Everything worked fine last fall. Any ideas? Thanks.

[billnorman1]

Classic open ground wire circuit. The parking lights are grounding out through the brake lights until you turn the brake lights on, then the additional current raises the potential (voltage) at the + side of the brake lights and there is no difference of potential so the parking lights go out. You're probably grounding with the ball on the hitch, you need to check the security of the white wire to a good ground.

[Musky Mike]

I did all the checks with the trailer not hooked to hitch. I cleaned and ohm checked the pigtail to the trailer and it was good and the tail lights to trailer and they were good. I also checked the ground coming out of the trailer hook up and it was good. I agree with you that it is a ground. Is there more grounds on the trailer?

[CatFan]

Get a set of jumper cables. With the trailer plugged in but not touching the truck, connect one clamp on the cable to bare metal under the truck. Connect the other clamp the same color to bare metal on the trailer.

If that works, you know the problem is the wire between the truck and tongue.

For everything to be acting up it's almost certainly the connection from the tongue to the truck. That is the feed to the trailer. The other grounds are just sources for individual lights.

[billnorman1]

An ohmmeter is a great tool, but you are fighting what is called a high resistance fault, a poor connection. Ohmmeters don't send any appreciable current through a circuit. The running lights are low current devices, the brake lights use about 7 to 10 times the current through the circuit. A brake light measures maybe 2 to 3 ohms, two in parallel half that. It will read short or good connection on most ohmmeters. Hook up the trailer ball and the electric wires, jumper between the trailer frame and a good ground on your truck, all SHOULD work.
I could be wrong, but the issue you're posting fits the solution perfectly.

[Musky Mike]

Get a set of jumper cables. With the trailer plugged in but not touching the truck, connect one clamp on the cable to bare metal under the truck. Connect the other clamp the same color to bare metal on the trailer.

If that works, you know the problem is the wire between the truck and tongue.

For everything to be acting up it's almost certainly the connection from the tongue to the truck. That is the feed to the trailer. The other grounds are just sources for individual lights.

I tried the jumper between the truck and the trailer frame and it didn't work. I tried jumper from the ground on the trailer pigtail to one of the grounds on the tail light and everything worked. So I cleaned all the grounds again, and it still didn't work. So I just ran a ground wire from the trailer pigtail to the tail light through inside of the frame. All the lights work good now. There must be a bad ground that I couldn't find.

[CatFan]

There is only one ground on the trailer that can cause problems with all the lights. Your tongue ground is bad.

[billnorman1]

Ground's ground the World around. You've added a ground in parallel with the ground from the tongue connection, and it's doing the job the one up front isn't doing. That's one way to fix the problem, you might think about removing the front ground, and abrading the place it bolts to, changing the terminal lug end, stretching the wire to see if it's open inside, and putting it back together.