Trailer wiring question...

Jeffncs

Jeffncs

Well-Known Member
Joined
Apr 4, 2017
Location
Wake Forest
I’m rewiring my car hauler and am wondering if something is right or wrong on my truck harness or if there’s something I need to do to have the truck work correctly...

Coming out of the truck trailer plug, I’m getting some odd multimeter readings and am struggling with the logic. 2012 f150 factory tow harness

The factory plug receptacle has a flip cover which has a “map” on it showing the 7 plug locations and their function. All of my lights and signals work as expected.

The “brake” location is showing hot at 16V.

The “+” location is cold (RESOLVED). Installed the relay and fuse - now showing constant 15v

Does this sound right to you? Logically, I would expect the brake circuit to only go hot when the brakes are depressed. I’m using the “-“ in the housing as the ground for all tests.

What am I missing here? Planning to wire the brakes and breakaway this week, but this is really question things.

Thanks!
 
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I think I found the issue - dealer never installed the trailer relay or fuse. That should resolve the charging power issue. Going to install and test.

Assuming that fixes it, should the “brake” circuit be full time hot?
 
The brake is progressive and the voltage holds the electro magnet off.
mad thenbokate approaches zero the springs take over gradually says the drunk swami
 
Jeffncs said:
That makes a lot of sense... I speak “Drunk Swami” very well! Thanks for the explanation.
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That doesn't make sense. The brake controller applies power, which activates the electro magnet, causing it to grab the drum, swing around, and activate the brakes.

No power, no brakes.
 
Jeffncs said:
Ok. Thanks @shawn

Does it have anything to do with the truck being in “park”? Would the brakes activate until out into gear?
Click to expand...

I don't know specifically how the Ford controller works. IME, the controllers only send a volt out two to the trailer when the vehicle is sitting still with your foot in the brake.... None of you're not on the brakes.
 
I have some research and diagnostic work to do...

just really odd as I know I’ve used the trailer brake controller. However, I’ve never tested it right at the socket. I wonder if it has to detect a trailer to work correctly. Maybe I’ll spend lunch out there messing around with it.

Thanks for the consult!
 
FWIW my ram (2012 2500) has a flat and round. It does nothing until it recognizes a trailer plugged in. The flat plug being just lights shows nothing on the dash but the round plug activated options on the dash for brake settings. Also it throws a warning when you unplug until you shut the truck off.
 
Thanks @shawn and @skyhighZJ I’m waiting on a new breakaway switch. Once I get it all wired up, I’ll plug it in and test it all properly.

I think I have too many variables in the equation to really diagnose an potential issue.

I’m hopefully Mr. Amazon delivers it today so I have the week to get things squared away before the weekend.
 
Just because your dmm is reading voltage doesn't mean there is any current. The truck controller is sending out power looking for a resistance range to tell it there is a trailer connected. If you're ever getting some phantom voltage or something you don't expect on a circuit, use a test light that will give you a real reading.
 
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