The Turn Signal Wouldn’t Turn Off

I recently pulled my fifth wheel RV trailer home from a camping trip and unhooked the truck. A few days later, I left for work before the sun came up and happened to glance toward the trailer. I noticed a light reflecting off of the fence behind it. Further investigation revealed that the left turn signal/brake light was illuminated.

Knowing that no vehicle was attached to the camper to provide power for the light, I quickly realized that the light was being powered by the internal 12V system in the RV. Since the trailer was connected to 12V power, the light continued to be illuminated. I knew this had to be corrected before using it again.

Since I hadn't experienced this problem before, my investigation started at the wiring on the back of the trailer. I had recently added an extension for lights on a boat trailer that was towed behind the RV trailer. During this addition, I had noted that the RV trailer manufacturer had not followed the industry-standard color codes for trailer lights. I had connected the boat's wiring just prior to my camping trip. I first confirmed that the left turn signal was also powered for the boat trailer I was pulling.

Since that was the most recent change that had been made, I tore into the wiring at the rear of the trailer, only to find no reason for the left tail light to be on. My next step was to resort to the RV trailer manufacturer operating manual hoping to find a wiring diagram. After searching through the manual and finding nothing of use, I was baffled.

As I thought further about the problem, I remembered the homemade adapter I had made to convert the trailer lights from my truck to the trailer. It converted the round 6-wire plug on my farm truck to the 7-wire adapter on the RV trailer. I surmised that a wire had come loose and was contacting the wrong terminal thus providing an electrical path between the 12V RV system and the brightly lit turn signal.

After I removed the adapter, I walked to the rear of the trailer and the light was now off. After taking the 7-wire female plug apart, I found a loose wire that I knew was the problem. I replaced the wire under the proper terminal and put the plug back together, thinking this wasn't too bad and I was glad that I had found the problem. Now for the test. I plugged the trailer into the adapter and to my dismay, the light was back on.

I knew the problem was in the adapter and assumed I had connected the wire to the wrong terminal. So I took the adapter apart again. In the process of confirming the wires were connected to the proper terminals, I also took apart the male 6-wire plug. During my tracing of the wires I noticed the screw for the center pin on the 6-wire connector (the 12V power wire) extended too far and was touching the left turn signal terminal.

I tightened the screw slightly to eliminate the short and tried the adapter again. Apparently, the warmer temperatures we were experiencing had caused an expansion in the round 6-way male adapter, causing the two pins to short out and turn the left turn signal on.

This entry was submitted by Mark Knackstedt and edited by Rob Spiegel.

Mark Knackstedt honed his troubleshooting skills by working with his father who was a maintenance electrician. That was followed by a year at Graff Vo-Tech in electricity and finally a BSEE 1984 from the University of Missouri-Rolla. He received his PE in 1999. He has worked in manufacturing and mechanical testing, and is currently director of engineering for a consulting engineering firm.

Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.

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