Coach marker/running lights issue

The Ford chassis has a quirk to it, the lights that would be the marker lights on the dually fenders is tied to the trailer wiring which is what was used for the coach lights on a C class. The reason they do this is so the driver can tell by looking in the mirror that the trailer (which might be narrower than the duals) is dark.

You have either a funky ground, most likely because LED’s draw very little power, or a sketchy fuse for that circuit, or both.

LEDs that work with either polarity use a full wave bridge which adds 2 diodes in series which reduces the voltage to the LED diodes. Maybe related?

wrruehle wrote:
I did replace all the marker lights with LEDs. Would that make a difference? It’s not one bulb either, it’s all 8 marker lights and the taillights. All or none work or don’t depending on how long I drive.

Try putting several of the original bulbs back in and see what happens. Wondering if it is somehow related to the lower voltage after after starting until the alternator ramps the battery voltage back up. Are your LEDs rated for 12V?

Measure the voltage at the switch if not at one of the bulbs.

I had a issue with 4 house closet 120V non dimmable LEDs with the switch dimmer. Changed one bulb back and they dimmed just fine. Interesting I thought. Have since switched to dimmable LEDs for the closet. Or try a different type of LED bulb in your case.

Kinda grasping at straws...

But they do work after driving for 5 minutes or so?

Yes, reversed polarity will cause them not to work.

I did replace all the marker lights with LEDs. Would that make a difference? It’s not one bulb either, it’s all 8 marker lights and the taillights. All or none work or don’t depending on how long I drive.

With a voltmeter connected to frame ground (not a wire) check for 12V at a bulb and if so it's getting power. Then unless the bulb is bad there will be 12V on the ground side. Use this method to isolate the problem area and corrosion/loose connection are likely suspects. Or a critter had a tasty wire lunch. :B

Intermittens can be difficult to find. Yours is relative easy - turn on the lights and start isolating.

Behind the driver's seat on some Fleetwood Class Cs is a fuse block. Sounds like a corroded connection. Check the fuses and fuse holders for corrosion. Glass fuses corrode where metal cap connects to the glass sleeve.

You do not have led lamps so mostly a ground problem.