Running lights not working - how to troubleshoot?

Your tail lights and markers are powered by the truck through the towing umbilical cord. A quick check of the truck's socket pins for power is where I would start.

FWIW, my Ford F250 has a separate fuse for trailer light circuit. The owner's manual should help locate it's position in the main fuse panel.

Sometimes I have to wiggle my 7pin connector to get everything to work. When was the last time you cleaned the contacts in the plug and the receptacle?

Awhile back I had to `drill' out the sockets on my connector due to corrosion. Obviously using a small dia drill soas not to make the opening larger. But it's the only thing I could think of that would clean out the hole.
I now spray Corrosion X on the contacts when I think of it. Guess I better do that today!

I did take a small file and hit the trailer side connectors with it to get a clean(er) base. I did not do the truck side.

You know..... seems like I was having to wriggle it a bit a while back to get it work. Sounds like a good cleaning is in order.

Westend - are you saying your truck has a separate fuse for just the lights? and brakes and signals is on another? If it's a single fuse, then I don't think it's that as I had turn signals and brakes, just no running and tail lights.

Thanks guys.....

Check for a trailer connector fuse. It could be located in fuse block under the hood or under the dash. Check the owner's manual.

Fuse is bad or ground is not cleaned.

Since like me you have a Ford I would also check the High Current Fuse located under the hood. If you forgot to disconnect and the harness pulled then you probably blew the fuse. Happened to me twice. You'd think I would have learned the first time.

I got one of these 7-way testers from AMAZON... You plug this in place of the trailer plug and if all of the lights work good here then the problem is on the trailer side...


I also squirt wd40 into the connector at times and wiggle the 7-way plug around some to clear away some of the debris...

Roy Ken

THIS IS A RELIABLE TOW / TRAILER CONNECTOR




https://www.waytekwire.com/images/items/7way-SplitPin-Trailer-Connector-Socket-37668_f.gif

See the SPLIT PINS in the socket? That's because a tiny screwdriver can be inserted into the slit and the pin spread to maintain superb TWENTY FIVE AMPERE rated connection and transmission.

If I had anything to do with US DOT I would have outlawed junky trailer garbage connectors forty years ago. They are that bad in comparison.

The mounting holes were the same for many years. Without looking...

WHITE: Chassis negative
BROWN: Tail lamps
BLACK: Clearance
GREEN: Left turn
YELLOW: Right turn
RED: Brake (lights or electric brakes)
BLUE: Aux (could power charge lead line)

Starting in 1970 you could not pay me ten thousand dollars to run "R.V. Junk Connectors". I must have saved 500-hours in troubleshooting after I switched over. Show that RV connector to a trucker and he'll be rolling on the ground with fits of laughter.

You might try inspecting the connections in the junction box in the A-frame where the umbilical cord wires splice to the trailer's wiring. The connections are exposed to water, dirt and maybe salt from when it was originally delivered plus sometimes the factory does a really poor job of the connections. On our TT, the running lights have a ground and positive connection with a 2-wire cable in the junction box and there is a common ground for all circuits to a ground lug on the frame. Check that all the ground wires bundled and spliced together under a wire nut inside the box is good. If you don't have one, I'd get a voltmeter/multimeter to help in diagnosing.

You could always open up the splice to the marker lights and see if you can get them to come on by jumpering to a 12 volt power source.

A typical TT wiring diagram is below. They can vary between TT manufacturers so don't use this as a standard diagram. Ours does not use the center pin #7 for ex. Disconnect the TT battery before playing around with splices in the box.

If you do end up going into the junction box and find poor connections, you may want to think about redoing all of them. Some (like me) have moved the connections inside a watertight box and used better wire connectors. I had the main positive wire fall right out of a ring terminal when I touched it which could have had serious consequences.