Hello,

I have a 2015 F150 Lariat FX4 with factory towing package. The truck is low mileage and low use as it was garage stored for about 5 years while lived in Australia. I've been troubleshooting a lighting issue with the 7pin connection controlling LED lights on a 2020 Patriot X1 camper trailer. Before I get too deep into the issue, please note that the camper trailer 7pin connection tests correctly, has a very solid ground and all light functions work perfectly when plugged into the 7pin connector on my 2021 Jeep Wrangler and other vehicles. If I put a 7pin tester on the truck, all functions test correctly also.

The issue with my F150.

• When the camper is plugged in to the 7pin on my F150, all functions work correctly. The truck sees the trailer connected with no errors, the brake controller works, the turn signals work, the brake lights work and the ground tests solid on truck side and the trailer side of the 7pin connection. All good....
• Now, when I turn on the F150 headlights, all the trailer marker lights illuminate correctly, but I lose the turn signals and the brake lights on the trailer. All lights on the truck working correctly, but not on the trailer. Again, when the trailer is connected to my Jeep or another vehicle, all lights work correctly with the headlights on or off.
• All connections in the 7pin wiring harness, under the truck and under the dash are fully seated, are clean and look new. There is no corrosion, and all pins/connectors are fully seated.

I did some side-by-side comparison testing with the 7pin on the truck and the 7pin on the jeep. The only difference I could find between the jeep and the truck was the turn signal/brake light connectors on the 7pin would read just over 11v on the truck and the same pin on the jeep would read just over 12v. For some reason, there is about a 1v difference. I assume this is causing the issue. I also tested on the F150 turn signal in the driver's side brake light assembly and am getting a little over 12v.

1. Could the 1v drop in power coming out of the 7pin on the truck be causing the camper turn/brake lights to not function when the truck headlights are turned on?
2. If so, what in the 7pin trailer wiring circuit could be causing a 1v drop in power?

I am out of ideas and any help would be appreciated.

Thank you!

 

Looking at your user manual fuse list, #35 is listed as the stop/turn signal relay fuse.
I see no relay for that function, listed as such.
Check fuse seating in that area.
A drop is not an open or a short but excess resistance or excess current being drawn or the #7 contact is faulty for connection resistance.
A simple voltage check is not good enough unless there is a load to cause that drop or the series resistance is so high the meter acts as enough load. Open circuit testing will not pick up a drop unless under intended load.
Good luck.

 

A check for DTCs using FORScan or diagnostic scan tool might shed some light. When using the 7-way trailer plug, the Stop/Turn trailer lamps are controlled by the TRM. I don't think there is a fuse problem as the lights all work until the brake pedal is actuated. Here is the schematic.

From the WSM:

The SCCM monitors the LH steering column multifunction switch position. When the LH steering column multifunction switch is in the left or right turn position, the SCCM sends a message over the HS-CAN2 to the GWM then the GWM sends the message to the BCM over the HS-CAN1 indicating a request for the LH or RH turn signal.

When the BCM receives a request for a turn signal, the BCM sends a turn indicator command message over the MS-CAN to the TRM to activate the requested stop/turn indicator output to the trailer tow connector.

When the BCM receives input from the stoplamp switch indicating that the brake pedal is being pressed, the TRM receives a stoplamp activation message over the MS-CAN to activate the requested stop/turn indicator output to the trailer tow connector.

 

Notice on the schematic the green and yellow lines for left and right turn and stop and would also be 4-way flashers.
They have two junctions in the harness.
Good luck.

 

Update:

I decided to test with the truck drivers' side rear turn signal wire. I measured the voltage and am getting over 12v to the turn/brake signal wire. I then ran a jumper wire from this connection to the trailer side 7pin drivers side brake/turn signal connection. Just like the jeep, all drivers side trailer turn/brake function correctly with the headlights on or off. That leads me to believe the issue is definitely somewhere in the trailer wiring harness. I took apart and tested each connection working backwards from the 7Pin towards the front of the truck. So far, I am getting the same results of just over 11v. The wiring harness goes up over the gas tank and I am not sure where it goes from there. I'll keep looking to find where it connects at the front of the truck and continue testing. I want to find where the voltage drops from 12v to 11v.

This may be a dumb question, but is there any way the computer can adjust the voltage in the trailer harness circuit? I assume not, but it can't hurt to ask.

Thank you to all who responded.

 

From the fuel tank the harness runs up to the top of the driver side inner wheelwell. There are several connectors there. After that it goes through the firewall to the TRM. Are you getting +12 V out of the TRM?

Have you performed a scan for DTCs? It might be helpful.

You can't adjust that voltage. If you are getting a low voltage there is something wrong.

 

I tested both turn/brake connections coming out of the TRM and both measures just over 11V. With the information you provided about how the turn signal, (When the BCM receives a request for a turn signal, the BCM sends a turn indicator command message over the MS-CAN to the TRM to activate the requested stop/turn indicator output to the trailer tow connector.), I assume the TRM is faulty. That would not surprise me too much. The brake controller was faulty from the factory also. I replaced it a few months ago. I'll check into the DTC's next and look into pricing/ordering a new TRM.

Thank you!

Mark

 

I've not seen many posts with faulty TRMs so I don't think it is a common as problems with the TBM. I would definitely perform the DTC scan first just to confirm that it is the issue.

 

Similar issue?
Depending on cab style maybe connector C316 or C327 is compromised?
I think both L and R side turn and stop are powered by the TRM power source circuit SBB35 - fuse 35, 20A. Is this power source circuit compromised?

 

Great video, thank you!