Trailer Brake Light Issue

[PhillyPhan]

Just hooked up a boat I picked up and for some reason I have no trailer lights. Last week when I towed a boat they worked fine. The difference between last weeks boat and this weeks was the one that worked used the 4 pin plug. The one that's not working now uses a 7 pin plug. I checked all my fuses that I know of and they were good. Voltage readings below.

Measurements are with the truck on:
4-Pin Connector
Pin 1 = Ground
Pin 2 = 0V (Tail/Run)
Pin 3 = 12V/0V as left blinker is activated or solid 12V when brake is pressed.
Pin 4 = 12V/0V as right blinker is activated or solid 12V when brake is pressed.

7-Pin Connector
Pin 1 = Ground
Pin 2 = 4.6V (Brakes)
Pin 3 = 0V (Tail/Run)
Pin 4 = 12V (Aux/Charging)
Pin 5 = 12V/0V as left blinker is activated or solid 12V when brake is pressed.
Pin 6 = 12V/0V as right blinker is activated or solid 12V when brake is pressed.
Pin 7 = 0V (Backup)

I'm wondering if I should have voltage at Pin 2 of the 4-pin connector and Pin 3 of the 7-pin connector. Any help is much appreciated.

 

[Richard320]

Check continuity to ground. On both vehicles.

A circuit is a circle, so if there is no path back to the negative side of the battery, it won't work. And often what happens --- assuming the boat trailer uses the old standard 1157 bulbs -- is that the ground is bad so the juice backfeeds the tail lamp circuit and finds a ground through a different path.

 

[PhillyPhan]

I forgot to mention although I get voltage for the left and right blinkers and brakes lights. When I plug in the trailer to the receptacle those don't work on the trailer. I'm suspecting I have to have voltage on pin 3 in order for the blinkers and brake lights to work as well.

 

[PhillyPhan]

Okay so I figured out why I wasn’t getting the 12V on Pin 3 and that was just because I had the headlights in auto, once I switched them on I got my voltage. Unfortunately still no lights. But I know all my voltages on the truck are good. Something wrong with the trailer wiring I believe.

 

[PhillyPhan]

Okay figured it out, so hopefully this helps someone in the future if they have a similar problem. It ended up being the trailer plug. I looked inside, cleaned the pins with alcohol, but what I think fixed it was bending the pins in closer to the center of the plug. All four pins were push to the outside of the connector housing so I don't think they were making good contact with the truck connector.

 

[pkman]

Co-worker with a 2018 HD had a similar problem - it turned out the 7-way had an internal problem with one of the pins being bent and not connecting. The back of the receptacle had to be removed to find this.