I hooked my little 5X10 utility trailer to check the hook up as I need to move my mower. While checking the lights, I had turn signals but no tail lights on the 4 wire plug under the receiver. I dug my 7 to 4 prong adapter out and plugged it in and the tail lights were working. .I walked up to turn the turn signals on and went back and they weren't. Heading up to check the right turn and then they were. As I'm standing there, they came on. The adapter was seated fully, I had put dielectric grease on the contacts and made sure the connections were good. While I'm watching, the lights went off. Later they came on.

The Expedition (2004) was running and the lights were on and didn't go on or off except when I used the switch. I'm going to climb under and try to trouble shoot the wires when I get a chance. On my '97 Dodge Ram and 2016 Mazda CX-5 the lights work properly. Anybody got any ideas? I didn't recheck the 4 prong to see if the lights went on and off.

 

99% of the time, all you have to do is take the trailer plug and plug it in the vehicle, then take it out and then plug it in again about 5 or 6 times, and it will straighten out. The grease helps but it's hard to stop corrosion when the trailer sits around unhooked for weeks and the vehicle is driving around with nothing in the trailer receptacle for weeks and driving through the water and the muck.

The next thing to check if that doesn't work is the plug on the vehicle itself. It's difficult to seal the back of the plug that faces forward, and dirt and water get up inside the plug wiring connections. Sometimes it's so bad and corroded you have to just get a new connector.

 

I'm suspecting a problem with the wires. Both plugs did the same thing so I'm thinking a source wire might be the issue. I keep the plug holes sealed with grease when the trailer is sitting. I worked in Logistics for 30+ years so I maintain my equipment. The trailer lights work fine on two other vehicles. I will probably find a small spot somewhere on the harness.

 

Crawled under today and found a piece of gravel jammed into the wiring harness. It was really hammered in there. Popped it loose with a screwdriver and the lights work now.