Finally! I solved the case of the Intermittent Trailer Running Lights

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I have battled a problem of intermittent trailer running lights for a few months now. Every once in a while the marker/running lights would go out and I would find the fuse powering that circuit in the truck blown. I would replace the fuse and things would be good for a few trips then bam it would happen again. Every time I would check the circuit at the next stop everything would be fine. Arrrrrgh!  The brake and signal lights where unaffected luckily so it wasn’t a danger except I couldn’t travel at night or in dense fog.

Since the fault was so intermittent and happened infrequently I let it go and lived with it. Being a Fifth Wheel it’s not like you can look at all the wiring, most is behind fiberglass walls with the lights sealed with silicone. Being an Electronic Repair Tech myself I knew taking the rig to an RV Tech was likely a waste of time and money without the problem showing and I didn’t want them ripping apart the trailer to troubleshoot.


trailer wiring
Photos from This is a great site for trailer information

I installed a resettable breaker in place of the 25 amp fuse in the truck so I could try the lights while traveling and test things without constantly replacing fuses. About a month or so ago the problem started to get really bad and it was happening far more often. I was able to determine it was definitely a trailer wiring issue and not a problem in the truck by putting a fuse across pin 3 and 4 on the 7 way plug. Usually this lights up the marker lights but this time it blew the fuse instantly. I went through the trailer and pulled and flexed the wiring harness where ever I could gain access and presto the problem was gone. Yay!

7 way plug

Sad smile Not so fast. The lights worked flawless for a month during our trip in the Nevada/California areas but as soon as I hit the Oregon coast and rougher roads and fog, where I really needed the marker lights they quit again. Double ARRRGG! This time though they weren’t intermittent. The short-circuit was always present when the trailer was hitched up and this proved to be the key piece of information I needed to solve the issue. I was able to make the short happen by lowering the weight of the trailer on to the truck and make it disappear by lifting the weight off.  Ah HA! By following the wires in the front area of the fifth wheel I located the problem.

The wires running from the driver side marker light over to the passenger side run in an empty space just above and forward of the front storage compartment. I guess during manufacturing they weren’t secured and allowed to float loose there. Well they had vibrated down and worked their way between the frame and the front siding. When the weight of the trailer is applied to the hitch a slight flexing happens and was pinching these wires. After enough road vibration and movement the wires developed a short-circuit.

marker lights

marker light wiring

The mysterious case of the intermittent trailer lights is now solved. Well that was a weird one. I’m so glad to have my running lights back and fully functioning.

My video I shot showing the problem


Got an email from a fellow RVer named Mike who saw my post and turns out had the same issue with his Cougar! Glad I was able to help out.

OMG!  I feel as if I owe you for the repair bill I’m not going to have to pay now.  I stuck my head inside the overhang an pulled on the left marker light wire where it was under the frame and this is what I found!  Thank you thank you thank you for that video you posted!!  My next step other than to repair the wire will be to the rv dealer who charged me $135 to diagnosis the problem and tell me I had a bad 7 way and replaced it.  I think they owe me a refund..

Again thanks

Here is his photo of the fault.

cougar marker light wire

If you enjoyed this post please like, tweet, +1 or share it, thanks Ray

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