We have a Ford 1996 F250 heavy duty pickup with a 7 wire trailer wiring connector. We have a Ford 2004 Expedition with a factory 4 pin flat connector. We have a boat trailer with a 4 pin flat connector that has a 7pin adaptor that can be used for connecting to the truck.
The problem is that the boat trailer is only 4 pin for both, and when connected to the truck, the marker lights all work, but when connected to the Expedition, the marker lights do not work. Previously we had an Explorer and the marker lights did not work on that either.

 

There could be a relay missing under the hood. Check your owner's manual.

 

Ford uses a separate fuses for trailer and vehicle lights. You might want to make sure that the trailer sidemarker fuse isn't blown. If memory serves, it will be a mini fuse located near the bottom of the fuse panel that's to the lower left of the steering wheel.

 

I'd take a meter to the plug on the expy. if all fuses check out, bad ground? corrosion in plug?

 

The trailer brake lights and back running lights and turn signal all work when connected to either vehicle. It's only the side marker light on the trailer that work when connected to the pickup and do not work when connected to the explorer or expedition.
Could it be that the 7 pin converter used when connected to the truck makes another connection or ground?

 

Your trailer should have the side marker lights, license plate lamp, and the running lamps all connected together on the trailer.

You only have four wires on the trailer: Ground, running/marker, left brake, and right brake.

I can't think of a single way that only one set of lights would work with one vehicle but they both work with the other.

Steve

 

Maybe one set is grounding through the hitch?

Weird problem.

 

I've had similar problems in the past and usually traced it to a loose or cut wire, corroded wires, or a bad ground.

I've actually had it to where the tow vehicle's lights worked fine, I hooked it up to the small trailer (4 pin), they worked fine, but if I hooked it up to the car hauler (7 pin) - all the lights on the trailer and the brake/rear markers on the truck wouldn't work. Ended up being a bad 4 way ground on the truck. Go figure.

I also had on my 79 when they did the Utility conversion (see my gallery), someone cut a wire under the dash and the rig had no trailer lights for about 10 years and the Fire Department never noticed. Took me 6 months to finally notice and 3 whole days to figure out what happened.

Have you checked to see if the vehicle side pins are getting signal? And I would check the ground on both the trailer and the tow vehicles, especially considering I've had some vehicles get better ground with some trailers than others.

 

Well, that rules my fuse idea out. If that were it, you'd have brake and turn but no tail or markers.

I'm leaning more towards a ground problem as well. It's going to take a bit more time to get to the bottom of that. I'm thinking that maybe your big lights at the back of the trailer are wired directly to the ground wire on that 4-pin harness and there might be no ground to the actual trailer frame at all. Either that or it might be damaged. There are several ways to check that.

One way is to plug the trailer connector into the truck but leave the ball unhitched. Do the little marker lights still work on the truck or not? If not, we've found it. That would mean that there isn't a sufficient ground to those lights and it's going through your trailer hitch. Then all you'll need to do is splice into the ground wire (standard convention is white) on the trailer's wiring harness and attach it to a clean metal spot on the trailer's frame. If you luck out, they'll have an obvious ground near the tongue and all you'll have to do is remove it, polish it up with a wire brush, tighten it back up, and paint over it.

You can also do it with an ohm meter, again with the trailer plugged in but not hitched up. All the lights should be functional without it being fully hitched up. It should be in the single digits ohmwise.

 

I am going to go with everyone else and say it is a grounding issue. 1) because it is a boat trailer that is in and out of the water, very prone to rust and corrosion. 2) from the situations described, I can only come up with a bad/intermittent ground.