1997 F150 trailer wiring has no voltage now

[MAW31]

gah.

Spent the day debugging a DOA (no power) to the trailer plug.

checked fuses, all good. (dash side and motor side)

truck trailer wiring was OEM ordered and installed when bought.

got all 4 trailer plug wires cut and spliced. ground wire checks out fine. (it grounds onto the frame rail)

never had a prob before. worked my way back to the gas tank (mid section of vehicle...

there's 2 connectors up under the rear bumper along the frame rail...disconnected, reconnected, contacts are real clean.

I am just mentally exhausted ATM, because all the truck brake lights, turn signals work, but not getting any power to the trailer connector.

I"m beginning to think the trailer package wiring is completely independant from the trucks rear turnsignal wiring. (after tracing the wires up through the midsection.

if you have any suggestion, i'd luv to hear something, anything.

I just dont know where to continue now.

thanks

 

[Silvertip]

Re: 1997 F150 trailer wiring has no voltage now

Better check the owners manual. Trailer wiring these days are generally on separate circuits and protected by their own fuses.

 

[pac city]

Re: 1997 F150 trailer wiring has no voltage now

i have same truck, check the fuse in the small rectangle shaped box , right upper corner of eng compartment , drivers side , sould be the 25 or 30a fuse, has a seperate syst for trailer lights,. hope that helps.

 

[MAW31]

Re: 1997 F150 trailer wiring has no voltage now

wow...i got myself a stumper...

spent yesterday (sunday) under the truck tracing wires. Learned alot, traced a ton of wires.

reset the white-ground wire to the frame-rail. (rear of truck)
under the hood, battery-side bulkhead, cleaned and reseated all grounds.

Here's what I observed but am dead-ending on what to try next.

with a stick holding the brake pedal down...

it appears that the positive green brake light wire at the rear frame-rail connection has a floating +12V.

That is, using a voltmeter and taking that to chassis-ground only shows 0.04v, but connect it to the connector ground its a +12V.

at the bumper-to-trailer (4 prong) connector:

white(ground) to chassis gets good continuity
but each of the +12V (green,yellow,brown) only ever see 0.04v. (when activated)

So my conclusion is I have a ground connection within the trailer wiring harness that is failing. problem is, i cannot begin to find it.

 

[kamby]

Re: 1997 F150 trailer wiring has no voltage now

so if i get this right, with the meter ground on the frame touching bare metal u get 0.04

Sounds like your ground connector for the 4 flat harness might be coroded or where its attached is rusted if you cant trace where it connects to on the truck from the wires then cut the white wire and ground it somewhere else
Rust and corosion dont ground very well causing low voltage numbers (been there done that).

If your is harness one of those multi tow connectors, my f250 had one, that has 7 way blad and 4 flat in the same unit that might have went screwy on you.

May want to try one of those kits that plug into your wire harness in the back hoppy makes them and t one. You unplug the harness and plug it inbetween and then you ground it to the frame. Try that and ground to the same spot the other harness is if known or some other frame spot. IF the new plug works then u know something is funky in the other connector. if it does the same thing as the connector on there now its grounding issue. Check for rust if the connection between the bumper and the frame rust ednought it might cause a bad ground depening if your grounded to the frame or bumper.

 

[MAW31]

Re: 1997 F150 trailer wiring has no voltage now

Thx Kamby, your post helped me 'have at it' again, and this time, I found it!

Not surprisingly, it was not what I thought it would be, and also, not suprisingly, it was something very simple...but then...most electrical quizzicals, once found are just that...simple.

I kept thinking in terms of a 'floating ground' when 'floating source' was the issue. My Clymers manual does not include any references to the THIRD breaker box which is installed for the trailer circuit.

anywho, once found, sure enough, the 20amp was blown. Replaced it and not suprisingly, voila!

I think what threw me off the most was my voltage tests to ground were returning 0.04 volts, instead of 0.0 as I would have expected on an open circuit. so mentally I had concluded the circuit was good with a poor ground, and all my time was searching for that poor ground.

I'll remember that one. lol.

 

[woosterken]

Re: 1997 F150 trailer wiring has no voltage now

wow glad to hear you got 'er figured out where is this third breaker box you found?

woosterken

 

[MAW31]

Re: 1997 F150 trailer wiring has no voltage now

Ahh yes, the infamous THIRD breaker box! PacCity had actually described it in this thread but the description he gave sounded too much like the 'normal' rectangular breaker box for me to make a distinction.

ok.

1st Breaker Box is inside cab drivers side.
2nd Under the hood Drivers Side 10" by 6" Rectangle
3rd Under the hood, Drivers Side, About Half-Inch width By 4 Inch Length.
(see image)

Mine had 2 fuses in it. and had black tape holding the cap on. (factory done)
You can see some of the tape in the accompanied photo.

It may seem obvious now, but didnt even look like a breaker box then.