I've got a '96 E350 with problems with some of the trailer tow circuts. We have left and right turn signals, but no tail lights or brake controller, and while we don't use it, it appears that the trailer battery charge circut has been compromised as well.

I have checked these circuts at the trailer plug, along the frame before the power distribution box and at the pdb. All of the fuses for these circuts are good. When put a test light on the test tab on top of the maxi fuses for the aux power, brake controller and running/backup lamps, I get a very dim light on the tester, indicating to me that something has shorted to ground before the pdb.

If anyone has any ideas on what (where) to check next, please let me know.

Given that all of the corresponding circuts on the van are working fine, I am guessing that my problem is somewhere in between the van's circuts and the pdb, but after they split those off for the trailering circuts. Where is that split off?

Any help would be appreciated

 

The fuse to feed the brake controller is fuse "D" 20amp, in the fuse box under the hood.

Fuse #1, 15amp in the underdash fuse box feeds the brake switch, which then feeds the brake controller box.

They also show the brown running light wire going to the brake controller, though I am not sure what this is for.

About the underhood fuse box.
Fuse "N" and "M" are the turn fuses and are fed by the turnsignal switch.

All the other fuses in the underhood fuse panel are fed from the battery with a large yellow wire. How this is hooked up depends on if you have dual batteries and a gasoline engine, and single battery with a gasoline engine, or dual batteries and a diesel engine. Whatever you have, I would try to find the large yellow wire feeding the fuse box, and follow it to one of the batteries, or the starter relay. The way it's looking, this wire(or multiple yellow wires) hook very near the battery, so it might be a corrosion problem were it hooks up.

 

I will verify fuse #1 under dash, but given that brake lights, turn signals and hazards work on the van, I expect that it is good. The mini fuses for the trailer turn signals in the PDB under the hood are good, and the turn signals work on the trailer. The larger maxi fuses in the PDB under the hood for the brake controller and trailer running lamps are good, but voltage is very weak. I took those fuses out and check them with a test light against the + terminal of the battery to be sure. So, something is impeading voltage to those circuts, prior to the PDB. I will look for the large yellow wire(s) going to the PDB from the battery or starter relay and check them back to their supply. I'm guessing that they might be protected by a fusable linK? (I hope) If anyone has a wiring diagram of this, can you tell me if there are any relays in this system that could be bad?

Any and all suggestions appreciated. Thanks Dave!

 

Figured it out. The trailer plug had melted on the backside, where the wires attach. It was not evident from the user side of the plug, nor from the appearance of the wires coming out of (actually in to) the plug. The aux power wire had melted down and shorted out the other circuts. I can't explain exactly why it presented itself as it did in the PDB, or why it didn't blow some of those fuses. It did finally blow the aux power fuse (it wasn't blown last night when we checked, but it was after driving the van today). When we replaced it, the new fuse was getting very hot.

Lesson learned...always check the obvious. Seems like those trailer plugs are always where the problem is...I should have opened it up to start with.

 

I was wrong. I thought we had it fixed, but the 60A Aux circut blew the fuse again. The running lights circut is still weak. I suspect a short somewhere in or near the PDB. I have isolated the running lights circut back as close to the PDB as I can get, and it is still shorted out. I need to get into the PDB, but I don't know how to open it up. Any help is appreciated.