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I have a 99 f-250. I hooked up a 7 pin for a trailer everything works except the 12 volt constant. I looked under the hood for the relays and can't find them. Does anyone know where their at?
 

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Its right next to the fuse box under the hood on the driverside, not the main box but,1 of the 2 little ones next to it.I think there is also a relay for trailer back up lights there too.
 

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what mnpowerstroke99.5 said.

The "power distribution" fuse and relay box is under the hood near the firewall on the driver's side. But the trailer tow pkg relays are in a seperate relay box, about 1"x2" rectangle near the bigger power distribution box. And if you have ESOF, there will be two small relay boxs near the big one, one for trailer tow and the other for ESOF.
 

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Discussion Starter · #4 ·
Yea that's where i thought it was too but their isn't any other box beside the main fuse box. My brother was even over and has a 02 f-250 and thats where his relays are. He even looked at my truck and we couldn't find them under the hood. I read one place that mentioned they were under the dash above the 4x4 shifter. I haven't got to really look under their though.
 

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Interesting that you don't have that relay box. Maybe your truck is even more plain-jane than mine.

I had the trailer tow relay box, which BTW is the one inboard of the main fuse/relay box, but only a bracket for the ESOF relay box (mounts in front of the main box). I cut the that bracket off to use for my headlight relay box that I mounted over on the right side fender.

My truck came with the Trailer Tow wiring kit, which included the two relays (trailer battery charge and trailer backup lights), and the appropriate fuses to go along with them.

The relay under the Instrument Panel is the trailer running (tail/marker) light relay, which should be in place on all trucks. in the IP relay block with the turn signal flasher and the fuel pump relay.
 

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My truck came with the Trailer Tow wiring kit, which included the two relays (trailer battery charge and trailer backup lights), and the appropriate fuses to go along with them.
Mine did to, but back then it was a option called the Trailer Tow Pkg with a list price of $190. The Trailer Tow Pkg was nothing more than wiring and fuses and relays to provide a 7-wire trailer plug at the back of the truck. Doesn't sound like much for $190. The pkg didn't even include the 7-pin plug. Just naked wires at the rear bumper. But it did include the plug under the dash where you plug in the trailer brake controller. It did not include the receiver, which was another $150 option, or the trailer tow mirrors which cost me another $155.

As Kevin mentioned, the relays and fuses that were part of the trailer tow pkg were in the glove box, but the relay box to plug in the two relays was installed at the assembly plant. Maybe if you didn't get the Trailer Tow pkg you don't even have the place to plug them in? So maybe that $190 option wasn't so expensive after all. :wink2:
 

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I have a 99 f-350 cab & chassis and have all the wires at the back but also can't find relay #38 & #39. Did you find them somewhere else?
Look for a sheet metal bracket inboard of the underhood fuse box. It's about 1" wide with a square hole that the relay block snaps on to. If you have the bracket, but not the relay block, then maybe your truck didn't come with them. There's a similar bracket in front of the fuse box that would be for the ESOF relays.
 

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I found 4 bare wire to the right of the master cylinder but it would need more than 4 wires for the 12v charge & backup circuit. Unless power in & ground were somewhere else. I will try to find out what the 4 wires do tomorrow. If that is what they are, I will wire in two relays. Still waiting on Amazon to ship my manual with wiring diagram.
 

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2000 F350 CC DRW HD Edition, 4r100, ESOF, AIS, 4”, Gear Vendor, hpx, frx Mishimoto trans cooler
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I believe these trucks have four firewall pass-through wires in the location you’re describing for up-fitting
 

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I believe these trucks have four firewall pass-through wires in the location you’re describing for up-fitting
Correct. Those go into the cab, but you wouldn't need to go into the cab for trailer wiring. If you don't have factory relays, mount aftermarket ones under the hood and run your wiring under the truck. The trailer battery charge relay just gets activated by a key-on powered circuit (isolates the trailer battery if the engine isn't running), and the backup light relay gets activated by the truck's backup light circuit. You could actually install that back by the rear and tap into the backup light circuit to activate it.

[on edit] You could get a two-relay block from a wrecking yard. I got one from an explorer to use for my headlight relay setup. PICTURE
You could also use the relays with a mounting tab. another pic (you can also see the two-relay box mounted by the fuse box in the lower part of the picture).
 

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I found the plug under the master cylinder & the 2 female terminals are not there. Is there anyone who sells the crimp on terminals? If it were a weatherpack connector, I would have those terminals.

The big one on the top left of the picture is 12v all the time, I could move one of the pins so they align but I am assuming that is not advised.

Or I may be able to remove the two I need from - https://www.amazon.com/gp/product/B001GBJQCA/ref=ox_sc_saved_title_1.


Helmet Automotive lighting Sports equipment Bumper Motor vehicle
 

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IF you can ID them, you could try Terminal Supply: Link
Or you can get wiring pigtails from Ford. Again, you need to ID what you have. Link I believe you can also buy individual terminals from Ford (probably in minimum quantities of 10 or more)
Or your local wrecking yard to harvest what you need.
 

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