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School my dumb ass; F350 trailer tow wiring.

Saloon65

Saloon65

Red Skull Member
Joined
May 21, 2020
Member Number
872
Messages
506
Loc
Texas, by the Grace of God.
2012 F-350, started out with a bed but that got pulled off and replaced with a dog box. All the lights work on the truck, but we’re getting zero on the trailer connector with the exception of the turn signals. (maybe brake power too, haven’t checked that one. Running lights, backup and battery charge are dead. Fuses are good, but all of that goes through several relays. I’m trying to figure out just what the conditions are for those relays to be turned on… Any other ideas what would cause all these to fail like that? I can’t see any damage to the harness.

And yes, I hooked another truck to the trailer and everything works.
 
I assume all the trailer wiring worked before the bed swap? I'd be looking at any of the harness that was disturbed during that first. There's a ground for the trailer wiring at the rear of the left frame rail that might be worthing looking at. The charge circuit should be active any time the key is on, trailer running lamp relay is just triggered off the truck's running lights, reverse wiring is on the same relay as the truck reverse lights.
 
Any separate trailer fuses either under dash or under hood? My Rams have a separate set under the hood.
 
dave_dj1 said:
Any separate trailer fuses either under dash or under hood? My Rams have a separate set under the hood.
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Since at least the early 2000s Ford has had separate trailer fuses in the fuse box under the hood along the drivers side of the engine compartment.
Saloon65 you did check the fuses marked trailer lights under the hood, right?

Aaron Z
 
Saloon65 said:
yes, checked all the correct fuses, the power ones under the hood, and what i believe is the control circuits in the other box.
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Dumb question, but did you check the mini fuses under the dash in the passenger side right footwell? By the way, these are a giant pain in the dick to get to, but I was troubleshooting a trailer light issue with our 12' F250 and found one of the mini fuses wasn't pushed all the way into the connectors.
 
ridered3 said:
Dumb question, but did you check the mini fuses under the dash in the passenger side right footwell? By the way, these are a giant pain in the dick to get to, but I was troubleshooting a trailer light issue with our 12' F250 and found one of the mini fuses wasn't pushed all the way into the connectors.
Click to expand...
that would be the “control circuits in the other box”.
 
Saloon65 said:
that would be the “control circuits in the other box”.
Click to expand...

Just wanted to check since you referred to it as "what I believe is the control circuits in the other box", so I wasn't sure where that referred to.

Figured there was a chance you had found another fuse box I was unaware of or didn't make it to the passenger side footwell fuse box.

Any luck figuring out the wiring issue?
 
Having a similar problem with my 2013, no power to the trailer backup lights. Where was the spice at?
 
Basically most of the relays crapped out. Tbis truck rarely pulls a trailer, and apparently without something connected the contacts will build up crap on the contacts or something, without a load to cause an arc which will “blow them clean”. no idea if that’s accurate, but it sounds plausible, i guess. if it was just one i would have just gone through that relay as a matter of course, but with all of them failing I started looking for something more complicated. Live and learn. the bad splice was just a matter of finding it, but that was just one circuit.
 
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