I just picked up a 1985 f150 with an auxiliary battery on the driver side the battery is isolated from the main battery and feeds a relay on the fire wall that is triggered by the light switch. The cables are labeled motor motorcraft and has the ford style connector on the side of the relay that feeds the trailer harness. My question is has anyone else seen this?

 

Trailer tow option.

 

Here is the auxiliary battery wiring.

 

Here is the description of how the aux battery circuit works.

 

They say the factory trailer tow package has a wire over in that area and a solenoid is part of the package.
I dont know how many got the 2nd battery in them.

I wired my truck to pull a trailer with electric brakes and power to charge a battery on the trailer.
I dont have the solenoid / relay or 2nd battery and I have not had any issues and I drive with the head lights on when pulling.
Now I do run a head light relay kit that only uses the head light switch as a trigger and no power goes through it.
I also get brighter head lights as they are getting a full 12 volts from the battery and nothing lost thru old wiring.
Dave ----

 

That looks like the factory option for an auxiliary battery, typically for a trailer. Things get kinda confusing because the "trailer option" has three basic circuits, but not all trucks have all of them. Keep in mind a truck could have been optioned for carrying a slide-in camper in the bed, and so did not need provisions for electric trailer brakes, for example.

Start here with the wiring diagrams:

https://www.garysgaragemahal.com/1985-evtm.html

In no particular order:

See page 134 and 135 for the electronic brake control. Don't quote me on this, but I think most trucks have the wiring provisions for this system, even if no controller.

See page 72 for the fixed exterior lights (not the brake or backup lights) on a trailer or slide-in camper. That diagram is a bit confusing, because it shows the trailer/camper lights at the far left. The rest of the diagram shows the additional lights added on a factory dually, including a "marker lamp relay" at the far right. Your truck most likely has the "trailer light relay" shown at the far left. My truck has this one, too. You mentioned hearing a relay click when you turn on the lights. That would be this relay, and it is a small square unit attached to the lip of the driver's fender, near the hinge spring.

See page 20 for the aux battery and control. This is for the interior lights and other electrical loads in a trailer/camper. See that big round silver relay on the firewall? That is your aux battery relay. When energized, in Run or Accy, the aux battery is connected to the main battery for charging from the alternator. When the engine is shut off, such as when you arrive at your destination, the relay opens and the aux battery is now isolated from the main battery. You can run your trailer/camper interior lights to your heart's content with no worries about discharging the main battery for engine starting. My truck did not have this option, but I duplicated these provisions, same as the factory option. This big relay is NOT controlled by the light switch. You can hear it inside the cab when you turn the key to Run or Accy, it's that loud.

 

Sorry for taking so long to respond, things got a little busy, and I wasn’t able to check on hear for responses. After further diagnosis on the system Monday night I realized that the relay I was hearing was the parking lights, the aux battery solenoid on this truck is not tripped off of key power there’s a hot wire going into the fuse block from the battery constant side of the relay and then the trigger wire is above that and I don’t think it’s ever been plugged in. And I did see the brake controller wires. My other 85 f150 was optioned with trailer tow, I don’t know why on that truck have 3.00 axle gears, a TOD trans, with a step side and none of the trailer wires are there but I’m sure over time it’s been removed. This truck with the aux battery is going to give up the interior for my step side cause this new one has power everything.

 

3.00 gears and a TOD transmission are definitely not tow friendly options. They usually option the very low number gears with the overdrive 4 speed and the 300 six as a "gas mileage" option. Many of them were made in the f100 series of trucks.

 

I agree that’s why I have a 84 6.9 and a few other trucks for towing

Here’s my marti report for my step side

 

During the 80's trailer tow may or may not come with brake and light wires to the rear like you get with today's trucks.
Most of the time it was HD cooling and charging maybe they upped the GVWR out back?
Try towing with a 300 / NP435 granny first / 2.75 rear gear and then add over drive with a final drive of 1.97:1

I bought a new 86 K5 Blazer 6.2 with tow package and it did have the brake wires under dash to the rear along with lights but I had to wire the plug as the wires were tucked up in the frame.

Funny they say "regular pick up truck" 2 times and only list "flare side" with the seat?
One day I should get a report for my Flare Side but it was a pretty basic truck with only power steering.
BTW Ford is flare side not step side.
Dave ----

 

Ya my parts f150 has factory over loads and 5 leaf’s in the rear but the other one is light duty on the spring pack. I do marti reports on all my vehicles even the ones that we bought new and ordered cause it’s cool to see. And yes I know ford cdd as old them flare sides, I’ve just always called it a step side and everyone knows. The earliest I’ve seen the wires in the cab for the brake controller is 90 on a crew cab dually with a 460 and e4od