While swapping out the 7-wire umbilical today on my 1994 Excella I made an interesting discovery. The cord and the connections look like they were OEM and done by Airstream, so I'm guessing that this problem has existed since day #1 for this trailer.
The first thing that I noticed is that at some point someone had sliced open the rubber jacket on the umbilical, tapped into the ground wire, and then made connection to the chassis via a screw-on copper lug. Seemed very odd to me, especially since the grounding block inside the 12v
panel where the end of this wire connects to is connected directly to the chassis via a solid copper wire.
Why the need for an extra ground wire, and why in a way which allowed water and dirt into the rubber jacket?
When I pulled the other end of the wire out of the grounding block I found my answer. If you look carefully at the place the set screw met the wire, you'll see that it was on the insulation and not the copper wire.
My guess is that on occasion the copper strands made contact with the inside of the grounding block, but not a good connection and not always. Someone realized they had a grounding problem and added the connection midpoint on the wire to the chassis.
Just goes to show how insidious a ground connection problem can be and how difficult they can be to find. Anyone putting a screwdriver to the set screw would have been convinced that it was good and tight, but they wouldn't have known no electrical connection was being made.
I think that making the problem even more serious is that they used the same port on the grounding block for the ground wire to the brakes. Having two wires in the same hole made it more difficult to get both properly seated.
My short term solution is to crimp both wires into a ferrule, and then to insert the ferrule into the grounding block where the set screw will be able to make solid contact with both. Eventually I'll add a grounding bus bar to provide space for each wire to have its own connection.