12V Electrical puzzler (trailer lights)

It's become commonplace for the chassis to have separate circuits, therefore fuses, for its own stop/tail/turn lighting and the trailer's stop/tail/turn. It's possible a fuse was blown in previous damage, something shorted plugged in, etc. Your dolly and your coach could both be OK now but the fuse blown because something happened in the past.

Sometimes, possibly for convenience, a coach builder connects the rear coach lighting to the trailer circuit. Doesn't seem that's the case here, but we've seen that on the forum recently.

Some vehicles do have separate towing circuits for the trailer vs the vehicle.

But I would think the brake lights both work off the same circuit.

It would not hurt to check the fuse box to see if there are separate circuits.

Edit: You said it's not the dolly, but there should be only one brake wire in a 7 pin plug. Ohm out the dolly right side wiring and bulb.

I had a similar issue with my last camper. Turned out to be corroded connection in my 7pin connector. Cleaned and never had the issue again.

DrewE wrote:
Do you have separate turn and brake bulbs/filaments on the coach itself? If so, possibly even if not, there is some circuit or box that combines the brake and turn signals for the trailer. That would seem to be the point of failure.

I suppose it may also be a bad ground somewhere, though I can't really think of precisely how that would actually work out. Still, many mysterious truck and trailer wiring problems end up being bad grounds.

X2 It's always the ground.

Turn and brake are combined on the coach. They work properly.

The brake and turn functions get combined in the multi-function (turn signal) switch. Since the ones on the coach are fine, the only thing I can think of is that the circuit for the trailer connector is separate from the circuit for the coach lights.

Dolly is fairly new. LED lights. Right side lights up as expected when I use the turn signal or 4-ways. Checked at the connector and no voltage when brakes applied on that pin.

Will be seeing my favorite Ford man on Saturday so I guess I will have to ask him. Was hoping that someone here knew for sure.

dual filament bulbs on the dolly lights?
check the dolly wiring

If you have no power to right side brake or running lights, the multi function switch would be a suspect. You may also have a poor connection somewhere as partial function may indicate a poor connection.

Do you have separate turn and brake bulbs/filaments on the coach itself? If so, possibly even if not, there is some circuit or box that combines the brake and turn signals for the trailer. That would seem to be the point of failure.

I suppose it may also be a bad ground somewhere, though I can't really think of precisely how that would actually work out. Still, many mysterious truck and trailer wiring problems end up being bad grounds.