Intermittent tail & running lights problem.

[MrFire]

I think this is my first time posting, but have been on the forum for a couple of years. We have a 2018 337RLS, 2021 F350, 7.3 Godzilla, SW.
I have have a problem with the tail lights in that they must have an intermittent short that pops the 30amp fuse in the TV fuse block. I can't find it. Have wiggled every wire I could reach including the Ford wiring harness. Checked the electric box in the pin box, nothing. The stop & turn signals work just fine. It looks like all the running lights are individual running lights are fused. I have run the umbilical cord plugged into the bed and bumper connection, same failure in both positions. I am hoping somebody might have a different idea of where to look. And to top it off we plan to leave for Alaska on the 16th. Oh well, at least it stays light up almost 24 hours. lol.

[jkwilson]

Power the lights from the trailer battery with a fuse in the 7-pin plug like this

This will save changing the truck fuse.

Get a portable AM radio and tune it to a spot where there is no station. Volume up. Move it around as you gently touch the wiring for the marker lights. When you move the wire that is shorting, you’ll get a burst of loud static in the radio.

Does your rig have LED lights?

[Second Chance]

Moved to Electrical System and Wiring.

Rob

[powerscol]

I think this is my first time posting, but have been on the forum for a couple of years. We have a 2018 337RLS, 2021 F350, 7.3 Godzilla, SW.
I have have a problem with the tail lights in that they must have an intermittent short that pops the 30amp fuse in the TV fuse block. I can't find it. Have wiggled every wire I could reach including the Ford wiring harness. Checked the electric box in the pin box, nothing. The stop & turn signals work just fine. It looks like all the running lights are individual running lights are fused. I have run the umbilical cord plugged into the bed and bumper connection, same failure in both positions. I am hoping somebody might have a different idea of where to look. And to top it off we plan to leave for Alaska on the 16th. Oh well, at least it stays light up almost 24 hours. lol.

Take a look at all of the through frame connection wiring. Usually identified by a blob of foam and wires running out for the frame. I suspect you will find a badly chafed wire there.

Second area. Look under the back rear corners of the trailer. the rear tail light wiring will come through the frame, and run up into the back of the cabinets next to the rear sofa. First check outside, but inside the cabinets at the back are spices to the taillights, etc. The wire may be hidden by a panel that should be removable.

If you can find the splice there, carefully remove a taillight and softly pull the wiring out of the back wall. Look for a short along those wires.

The other place to look is the metal junction box on the back side of the kingpin. Hopefully you have a multi meter. Set to resistance check each set of wires against the trailer frame. Only the white wires should show this.

There are also numerous trailer cord test plugs for the truck and trailer that may help in finding the issue.

Good Luck

[llr2800]

Put a 12v incandescent light in place of the fuse. Then have someone watch the light while you wiggle, push and bump everything you can. If it turns on / gets brighter you are close. The light will limit the current so no damage will be done during testing

I doubt if an AM radio will help as there will probably be very little arcing on 12v.

[jkwilson]

Put a 12v incandescent light in place of the fuse. Then have someone watch the light while you wiggle, push and bump everything you can. If it turns on / gets brighter you are close. The light will limit the current so no damage will be done during testing

I doubt if an AM radio will help as there will probably be very little arcing on 12v.

There is plenty of arcing on 12V. You can weld the frame of a vehicle with a 12V battery.

[MrFire]

Power the lights from the trailer battery with a fuse in the 7-pin plug like this

This will save changing the truck fuse.

Get a portable AM radio and tune it to a spot where there is no station. Volume up. Move it around as you gently touch the wiring for the marker lights. When you move the wire that is shorting, you’ll get a burst of loud static in the radio.

Does your rig have LED lights?

Thank you for your input. I don't understand how the fuse in the female end will work as the power comes from the positive side. Yes, I do have LED lights, converted them incandescent two years ago. I'm going to start converting back to incandescent as I understand the LED's can heat up and short out until they cool off. I have shaken and wiggled every taillight and running light wire I could get my grimy hands on. Can't make it fail.

[FT4NOW]

Thank you for your input. I don't understand how the fuse in the female end will work as the power comes from the positive side. Yes, I do have LED lights, converted them incandescent two years ago. I'm going to start converting back to incandescent as I understand the LED's can heat up and short out until they cool off. I have shaken and wiggled every taillight and running light wire I could get my grimy hands on. Can't make it fail.

You need to have the RV converter on for the fuse in the 7 pin connector to work. The converter provides the +12v.

[jkwilson]

You need to have the RV converter on for the fuse in the 7 pin connector to work. The converter provides the +12v.

No, it comes from the trailer battery. You are connecting the 12V charging line, which goes directly to the battery, to the running light wire.

[FT4NOW]

No, it comes from the trailer battery. You are connecting the 12V charging line, which goes directly to the battery, to the running light wire.

Oh, oops, guess I had a brain fart. Made sense in my head when I wrote it.