Low Voltage at Trailer Plug
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Junior Member
Joined: Sep 2015
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Low Voltage at Trailer Plug
I have a 2006 F-150 with a low voltage issue to my trailer plug. I am only seeing it on my right turn signal and brake light. I am getting anywhere from 9.5 to 11 volts to the pin going to the right turn and brake with a steady 13 volts to the other side. This low voltage is causing a dim turn/brake to the right with no running lights on and no turn/brake to the right with running lights on. The running lights to both sides are working fine. This has replicated itself on 3 different trailers and is on both mt 4 pin and 7 pin plug. Any ideas on where to start?
Senior Member
Joined: Oct 2002
Posts: 7,295
Likes: 125
From: St. Louis (Out in the woods)
Go to the CJB and check the voltage at both test points of fuse F36 with the brake lights applied. If one is good but the other isn't the fuse is bad although not blown.
If the voltages there is normal, there's a corroded connection, probably in one of the connectors between the CJB and the rear bumper. There are two connectors to pull apart and check.
If both are good and the Rt rear stop lamp of the truck is normal, there's an internal problem with the CJB.
Running lamps are a complete separate circuit.
Most likely, it's still there but is too dim to see next to the running lamp filament.
If the voltages there is normal, there's a corroded connection, probably in one of the connectors between the CJB and the rear bumper. There are two connectors to pull apart and check.
If both are good and the Rt rear stop lamp of the truck is normal, there's an internal problem with the CJB.
Running lamps are a complete separate circuit.
This low voltage is causing a dim turn/brake to the right with no running lights on and no turn/brake to the right with running lights on.
Technical Article Contributor
Joined: Jun 2002
Posts: 10,511
Likes: 10
From: Under the flightpath of old ORD 22R
dumb question time for me.
Are you testing it only with the trailer plugged in or is the test case of " on both mt 4 pin and 7 pin plug" with the trailer unplugged ?
As a SWAG, this sounds like a high resistance ground issue
- If it is the test side of a high resistance bleed to ground is another question all together.
Maybe start with what are you using as reference ground ?
- ground lug in the trailer tow adapter or frame.
Are you testing it only with the trailer plugged in or is the test case of " on both mt 4 pin and 7 pin plug" with the trailer unplugged ?
As a SWAG, this sounds like a high resistance ground issue
- If it is the test side of a high resistance bleed to ground is another question all together.
Maybe start with what are you using as reference ground ?
- ground lug in the trailer tow adapter or frame.
Senior Member
Joined: Oct 2002
Posts: 7,295
Likes: 125
From: St. Louis (Out in the woods)
If the trailer were unhooked for the voltage measurements, they'd probably "look" normal as you'd be measuring the no-load open-circuit voltage.
If it were a ground problem, it wouldn't restrict the symptom to only one of the three circuits that go to the trailer's lighting as all three share the same ground.
A "high resistance" in that faulty circuit's supply is the only thing that makes sense to this old electronics guy.
If it were a ground problem, it wouldn't restrict the symptom to only one of the three circuits that go to the trailer's lighting as all three share the same ground.
A "high resistance" in that faulty circuit's supply is the only thing that makes sense to this old electronics guy.
