Trailer Light Issue
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Trailering
Joined: Jan 2012
Posts: 22
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Trailer Light Issue
Hi. I have a 2012 F250 Reg Cab Gas 8' truck. Recently the drivers side trailer brake and turn signal lights stopped working. The running lights and electric brakes work fine. I tested the truck plug with a test light and could not get power to the driver side turn signal or brake prongs. I took the housing off the plug and tested where the wires come up to the plug and still could not get power. What else could it be? all of the lights on the truck work fine. Should I run wires from the working truck wires to the trailer wires and be done with it? Any input would be appreciated.
Junior User
Joined: Feb 2016
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From: Hammond
Brake lights are not differentiated left to right in the plug wiring. If the passenger side brake lights are working I would start looking for a fault on the trailer first...in particular the ground path from the driver light assembly back to the trailer plug.
Junior User
Joined: Feb 2016
Posts: 66
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From: Hammond
I stand corrected...I was thinking taillights and brake controller.
Hotshot
Joined: Jan 2004
Posts: 19,760
Likes: 1,079
From: St Louis
The YELLOW wire is the LEFT TURN/STOP wire.
The GREEN wire is the RIGHT TURN/STOP wire.
There is no RED wire in the trailer plug.
What else could it be?
Cargo Master
Joined: Oct 2015
Posts: 2,745
Likes: 72
From: West Virginia
The brown wire at the trailer connector is the RUNNING (PARK) LAMPS.
The YELLOW wire is the LEFT TURN/STOP wire.
The GREEN wire is the RIGHT TURN/STOP wire.
There is no RED wire in the trailer plug.
Did you check the appropriate relay in the fuse box under the hood (see owner's manual)? Can you hear it cycle when the left turn signal is activated?
The YELLOW wire is the LEFT TURN/STOP wire.
The GREEN wire is the RIGHT TURN/STOP wire.
There is no RED wire in the trailer plug.
Did you check the appropriate relay in the fuse box under the hood (see owner's manual)? Can you hear it cycle when the left turn signal is activated?
Since all of the lights on the truck work fine, I doubt the problem is at the fuse or relay boxes.
I'd first look at the connectors where the trailer harness connects behind the bumper, then the 7 pin connector itself.
The 7 pin connector has a reputation for failure.
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Cargo Master
Joined: Oct 2015
Posts: 2,745
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From: West Virginia
I think we are all making this too hard. The driver's side brake and turn don't work on the trailer. Everything else works. Check the yellow wire. Keep following it back from the plug until you find the break in the wire.
New User
Joined: Feb 2016
Posts: 19
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If its a factory installed or dealer installed 7 pin plug, you can unplug the harness and pin it at the connector, it unplugs about 9 to 12 inches back., that will tell you if its in the plug or further up in the truck wiring
Fleet Mechanic
Joined: Dec 2007
Posts: 1,504
Likes: 339
From: Wildwood Pa
Trailer plug outputs are fused separately from the truck outputs. So, lights can work on the truck, but not at the trailer plug.
Get the owner's manual out & locate the fuses for the trailer plug functions that don't work, & see if they are bad.
Its very common for faulty trailer wiring to blow the fuses for trailer functions. I start at the fuse box first, before I start to trace wires.
Get the owner's manual out & locate the fuses for the trailer plug functions that don't work, & see if they are bad.
Its very common for faulty trailer wiring to blow the fuses for trailer functions. I start at the fuse box first, before I start to trace wires.
Hotshot
Joined: Jan 2004
Posts: 19,760
Likes: 1,079
From: St Louis
Your missing a ground somewhere...likely the trailer.
I got my schematic from etrailer.com which I see now is incorrect. However, there is a red wire which is for 12v charging.
Trailer plug outputs are fused separately from the truck outputs
There's a reason I told him to check the relays, they're a convenient spot for half-splitting the faulty circuit and they can be quickly checked for actuation with a touch of a finger tip. If the left relay clicks, swap it with the right one and see if the problem stays or moves. If it stays, it's probably a wiring problem and the yellow wire is going to need to be traced. If it moved, the relay is bad - replace it an you're done.
