Loose connection at truck receptical.
Loose connection at trailer plug.
Loose connection at junction box under trailer if its a tt. In the pin box if it a 5th wheel.
Bad ground on running light circuit.
Chances are slim it will be a grounding issue if all the running lights stop working at the same time. Many of the lights will be grounded to the frame, so if there was a ground issue chances are ALL the lights wouldn't work at the same time... (brakes and all)
Just make sure the pins on the truck are making good contact with the receptacles in the plug for the wiring on the 5er. Chances are the pin and recep are loose. Or as turbs said the wiring in the junction box on the back of the pin box for the pig tail to plug into the truck may have a loose wire... Those would be my two best guesses.
Are All the running lights intermittent at the same time or are the lights individually intermittent? I have the latter problem and it appears to be leaking lense covers. Some of the lamp sockets are corroded. As the trailer goes down the road vibration makes them go on and off.
If it is all the running lights check the wiring around the front of the trailer for an open or frayed wire. If the frame is used for ground, it is most likely an issue with the power side for these lights. The 7 pin connector and the junction for the first light are where I would check.
Here is an update. The trailer lights checked out fine using the technique described in this thread. The truck's seven pin was checked at a dealer under warranty and also was fine. It seems like the female end of the running light plug on the trailer is spread open too far. I used a flat blade screwdriver to pry the two ends closed together. This solved the issue for the moment.
Since the trailer plug is molded there is no way to open it.
Does anyone have any suggestions on how to make the connection better long term besides replacing the entire plug?
Thanks for all of your suggestions!
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Jim & Nicky
2012 Forest River XLR 29 MBV
2010 Dodge 2500 Cummings
Honda ST1100
BMW R1100R