Troubleshooting Trailer Connection Getting HOT

Updated 05/29/2018 | Published 05/25/2018

Products Featured in This Question

Hook Probe Circuit Tester

(60)

$13.68

7- and 4-Pole Trailer Connector Socket w/ Mounting Bracket - Vehicle End

(2638)

$43.40

Question:

So, I have read the help articles that Im probably looking for a grounding or frayed wire as I have a similar issue of blowing the fuse on my cars fuse panel, but I have brakes and turn lights, but not the running lights. Additionally, the 4 flat connection at the vehicle is getting HOT! I mean melt the plastic around the pins HOT! The dealer says the car 2017 Ford Flex is good all the way to the connection so then I have to think its the trailer. Have you experienced the connection getting that hot that it melts the plastic housing around the pins? One last thing, the previous owner had switched the taillights to LED lighting leaving the side marker lights as the original bulb style. Would that have any bearing on this or am I probably stuck on a hot Friday night pulling wires through my boat trailer?

asked by: Dave M

Expert Reply:

It is very unusual for 4-pole trailer wiring to get hot enough to melt the 4-pole plug. This wiring format carries only basic lighting signals, no trailer brake power, no reverse lights and no auxiliary 12V power. It would be much more common for an auxiliary 12V circuit on a 7-way setup to have such a problem since it has higher current. I suspect the connector on your Flex is a combination 4-pole and 7-way like # HM40975.

The best way to proceed with troubleshooting is to disconnect the trailer from the Flex and test each one separately. This makes it easier to isolate the cause for the issue. Changing trailer lights to high-efficiency LEDs would tend to reduce this problem, not make it worse.

For reference I have linked an article concerning trailer wiring which shows you how each circuit/function is assigned to each of the pins in the connector.

You can directly apply 12V battery power (using the Flex's battery or a well-charged spare) to see if the trailer functions operate properly when directly fed this battery power. A set of jumper cables and two short lengths of wire make this easier. Apply the battery negative wire to the ground pin on the trailer's connector; then one circuit at a time, apply the battery positive wire to the contacts on the trailer connector. If the trailer light functions operate as they should then the issue is probably on the vehicle side.

If not then you will need to trace all the trailer wiring from the plug back to each light. Check for any wires with worn or cracked insulation, wires that may rub on the trailer frame or contact moving parts like leaf springs, or any other visible fault.

You can also test the vehicle connector using a circuit tester like # PTW2993. Apply the grounded tester to one circuit at a time while a helper activates that function on the Flex. For instance, with the headlights on test the contact for that circuit to see if the appropriate signals are present. Please refer to the linked video to see how to go about this.

Often the cause for a whole range of wiring issues is a weak ground, either on the vehicle wiring harness or on the trailer wiring harness. Since this is simple to check I recommend starting here. The ground wires on the car and trailer (usually but not necessarily white wires) MUST make solid contact to clean bare metal on the vehicle and trailer frames.

Please let me know the results of your testing and I will be glad to offer next steps.