1999 F250 Extended Cab 7.3
Need help identifying these relays. Just got this truck recently and it has a lot of electrical issues. I can't identify these relays (circled in red) in the service manual or the owner's manual. Hoping someone can tell me their purpose. They've both been replaced by the previous owner; I don't know what they are for or why they were replaced.

 

Towing package. The front relay is for supplying 12v power to the seven pin connector. It turns off when you shut off the truck so the trailer can be used without running the truck batteries dead if you forget to unplug the trailer from the truck. That circuit is protected with a 30 amp fuse in the main fusebox. Cannot remember which fuse it is now. I'm not sure what the rear one is for.

 

I believe they are the Low-To-High and High-To-Low relays.

 

(Red cross added)

Just one problem with the above theory.

The OP doesn't have ESOF.

In fact, the OP doesn't even have a transfer case.

His new to him 1999 F-250 is 2WD.

Kwikkordead nailed half of it. (Trailer battery charge relay).

The other relay in that little mini relay box is for the Trailer Brake Controller circuit.

Notice I said "circuit", because the 1999 was not equipped with a built in trailer brake controller.

But it did have a circuit available for an aftermarket trailer brake controller. Hence the second relay.

 

I don't think that's right, Y2K. At lease on my '00 and '01, the rear relay is not for trailer braking, but for trailer reverse lights. Not many RVs use them, so it's often not loaded. Did '99's do it differently?

I do have an aftermarket brake controller wired into the factory pigtail, but no trailer specific relays are involved, just fuses.

 

Yes @BWST , you are correct. Thanks for speaking up.

I got my "wires" crossed. Or rather, my truck brands. I've been in GMC mode recently.

With GM in 1999, one had to add something to a post in the underhood electrical center for the aftermarket trailer brake controller to work.

But with Ford, the additional relay is indeed for the Trailer Reversing Lamp circuit.

This can be confirmed on page 95-5 of the Ford Super Duty wiring diagrams published in December 1999.

 

Agreed that RV’s don’t use the back up feature.

However this signal becomes very important when backing a trailer with hydraulic surge brakes. Reverse signal triggers that they don’t have to apply when feeling pressure.

 

Thank you all for your input! Really appreciate it.

 

For the record, my 1999.5 F550 has two relays there. I am positive on the front relay being 12v supply to the 7-pin plug because that circuit is now disabled in my truck due to the wiring got tired and I had to run a new one along with an extra heavy duty relay to power it. Used the trigger wires for that relay to control the new one. I installed a self resetting 30 amp circuit breaker for that circuit because I found the fuse like to blow out if my trailer battery was too low. I wouldn't at all be surprised if the rear relay is the one that controls the brake controller circuit because mine has an aftermarket brake controller in it, not a factory installed unit.

 

My bad. I tried looking up the component locations diagrams in the shop manuals and the closest it came to telling me was that these relays are "Close to the junction box on the LH side of the engine bay".

 

Agree FTN, it’s strange that there is not a clear diagram in the FSM for these relays. I’ve had to piece together the information from schematics and forum posts and my own trucks.