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Troubleshooting Pop-Up Camper Trailer Running Lights

Question:

This has me stumped. 4 prong connector on truck working correctly. Next is short connector with 4 prong to connect with truck and 6 prong to connect with trailer. the brake and battery wires are abandoned and cut in between . With this connected to truck, turn signals and running lights test fine at 6 prong end that plugs into trailer. To test trailer at 6 prong trailer connector I set meter on ohms. I can send a current thru all 3 functions rt. turn, left turn, running. Next I removed all bulbs and rechecked. No current thru all functions, which tells me there no open grounds. Good. But when I connect trailer to truck at 6 prong connector I only get turn signals and brake lights, no running lights. Well I thought I am not getting a running light connection at 6 prong connector. But if I disconnect the 4 prong connector at truck I can send a current for the running lights thru 6 prong connector. To me everything test like it should work but running lights dont. I am thinking I will redo running light grounds and bypass 6 prong connector, but running lights test OK for ground and there is connectivity across 6 prong. I dont like to do the work until I can locate the problem. Any ideas will be greatly appreciated.

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Expert Reply:

The additional electrical connection points created when a wiring adapter is used, something like # 30637 that connects a 4-pole flat to a 6-way round, can cause connection issues, but based on your testing and troubleshooting it sounds like your trailer running light issue is a weak ground on the trailer or on the vehicle.

Often running lights reveal a problem with the trailer ground since these lights remain on. Brake and turn signal lights, which are typically on only briefly, draw less current and this can mean those other function may still work UNTIL the running lights are turned on. This will mean the trailer wants to draw more overall current from the vehicle connector than it can get due to the weak ground.

The fix is usually as simple as moving the trailer's main ground wire to a new clean bare metal spot, which can be created by sanding off rust, primer, paint or anything else that prevents metal-to-metal contact. Dielectric grease like # 11755 is great for protecting that new clean grounding point.

You can repeat this same process on the vehicle's main wiring harness ground if the trailer ground turns out not to be the issue.

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