Starting to trace a "short" in my running lights

Be sure to look through ETRAILER.com online... They have lots of info on how the wiring is done with lots of videos and drawings... Even wire colors but you never know what the manf ran...

Might give you some clues...

Also a great place to purchase safety lights and parts etc...

Etrailer was a great place for me going through my wiring a couple of times... I was always pulling my wiring out driving back off road through the high brush. Finally got smart and put everything under the trailer in PVC tubing haha... The PVC tubing hugs the trailer frames and comes through the flooring using PVC Electrical junction boxes with lid access on the bottom... Cut a large hole in the bottom of the PVC boxes to route up into the top side cabinets etc... All neat now and got to seal up all of the bug and insects hole to boot...

Kinda depicted here in this drawing for my POPUP... Under trailer wiring PVC tubing shown in BLUE...


Roy's drawings

Roy Ken

i would start by looking at the 7 pin connector from the trailer to the truck
open it up and look for frayed shorted wiring, or corrosion causing a short
seems more likely to me than a short in one of the trailer lights

Removing/inserting the bulbs may have dislodged the short in the socket or wiring. Remove each light socket and check the socket and wiring for chaffing, corrosion or any other issues. At the same time wiggle/pull/push the wires/connectors while watching the above mentioned fuse replaced light - if it lights well you know what to do.

Well all, I have good news and bad news.

Pulled all the lights, plugged into the truck, checked voltage at the rear... 13.3 volts... Plugged each light in starting from the rear. Went through all of them and they all work... That's the good news.

The bad news is I have no idea what I did. I did notice a slight bit of moisture inside a couple of the covers but????? Wondering if maybe putting some type of "grease" or something on the lip of all the cover. Heck, I don't know...

May go to O'Reilly's and look for that "fuse flasher" in the event that happens again. Or Harbor Freight...

Thanks all.

I went through this exact same thing, checked every light, drove me crazy. Pull the belly pan along the driver side all the way to the back corner,just enough you can follow the wiring harness. Mine was worn through where the wires went around a corner to the rear of the trailer, staring me in the face. Bet you have the same issue.

Put a test light in the fuse position. When the light goes out the short has been found.

ramyankee wrote:
Plug in the fifth wheel to the truck, turn on the lights and the fuse blows. The only lights that work are the turn signals and brake lights.

I will start by removing all the running lights, will then plug into the truck and begin plugging in each light one by one. Will this be a reasonable way to find where the short is?

Also, how would I find if it is maybe the wiring on the truck? I have two plugs, the bed mount for the 5th wheel and the bumper plug. Will add that I have no idea how long this has been an issue. When we would do the light check we would only really look for brake and signals and was always done during the day. Never have done any night driving..
BTW, IMHO, everybody that tows a RV should carry the flasher. If you need to unhook at the side of road you can bend the prongs a little to jump between "charge" and "Taillight" on the plug. This lets the trailer flash by itself...
Absolutely hate tracing shorts.

Thanks for any responses...

Get a 2 prong flasher, and a couple pieces of wire, use that to by-pass the fuse. This will make the lights flash and protect the wiring. (Cheap auto reset breaker) While lights are flashing look for the one or group that looks different. If all look same, problem is in TV, or between plug and where other lights branch off. If you have a AMP meter that reads when placed on wire, just follow the wire as the lights flash. When you pass the short, the needle will stop jumping, or start jumping the other direction. (Got to be careful. Sometimes the needle stops jumping because the meter is too far from wire...

Thanks ndrorder.....

Well now to plug in and start checking voltages...

You never know, you might get lucky. Often it seems that the short will occur from the a chaffed hot wire contacting ground. Hence shorting whether the bulb is present or not. Look for chaffing anywhere the wire goes through a hole in the frame.