Hello again everyone,
I'm having difficulty identifying a relay that is mounted on my 87 dually.
It is mounted with 1 screw, on the driver side fender, at the extreme rear corner.
I have looked at the wiring diagram for the truck, but can't seem to find it.

Any ideas ?
Peter A.

 

A relay someone added, it won't be in wiring diagrams so you'll have to trace the wires (if still there) to see what it is for. I saw one on a 1987 F250 IDI in a wrecking yard a few weeks ago, mounted on the firewall near the brake booster. Didn't appear to be a service truck or work truck. Not uncommon - could possibly be a modification for the lights (aka headlight mod) or something to do with trailer wiring/brake controller or anything electrical that was added to the truck in the past, maybe a winch that is no longer on the truck?

 

Thanks for the reply.
I don't think that this is an add on. I have a 90 F-250 that I will be removing the engine from, and it also has the same relay. It is not mounted on the firewall. It's mounted right against the inside of the fender lip, at the extreme rear of the fender. one of the wires is red with blue tracer. Something to do with the glow plug controller ?

 

OOPS, I forgot about factory installed and after-market installed turbochargers. IF that is a GP relay then it has to be from one or the other of those. I don't know how the controller and relay was configured in the trucks/vans with Factory Installed ATS turbo. I've seen GP controller relays mounted separate from the controller in different locations including the the passenger side and even in front near the timing cover (in photos of IDI engines with turbos) but they could be mounted anywhere.

For after-market turbo installations on a non-turbo engine & wiring harness, where the controller and relay are separated and relocated, extensions to TWO "red wires" one from the controller and one from the harness will have to be extended in length by splicing in some additional length of wires. One of the original wires will be a very small RED wire from the controller to the a small terminal on the GP Relay and the other will be a larger RED wire with LIGHT GREEN Tracer coming from the GP harness to the same small terminal on the GP relay. Someone could use any color of wire to do that, unless what you have is in fact original Factory Wiring.

There is also one RED wire with LIGHT BLUE tracer:

According to the drawings in my shop manual 87-91: FIRST, it's an early first edition manual printed in July 1986 and some of the drawings are not fully updated from the 84-86 design series. I don't trust this early edition manual for accurate wiring diagrams. Some are right and some are not, some combining both 1984-86 wiring with 87-91 wiring.
BTW, If you own a 1987 truck, don't buy this early edition manual, buy a manual printed in 1990 or 1991, or one advertised as being for those years and all of the wiring for the engine compartment should then be correct. The fact that the 87 is a 6.9L matters very little since the wiring and a few other features of the 87 6.9L should the same as 88-91 7.3L.

TWO drawings show a same small Red wire with Light Blue tracer which is the Starter Relay circuit going from the start position of the ignition switch, to the Starter Relay at the SMALL (+) terminal on the Starte Relay. Through that wire, contact is made inside the relay between positive and the large ground post only when the key is in 'Start' position, thus allowing current to flow through the LARGE (-) Negative Ground Post and the ground wire coming up to that post from the Starter Solenoid to energize the starter.

That is confusing, really. I have always thought that when the Starter Relay on the fender well was energized, that the internal contacts closed between the two large terminals, even in the 'RUN' position to continue supplying power for the afterglow cycles and the other components connected to that large positive terminal but that can't be true or the starter motor would continue to be engaged even in the 'RUN' position. I don't have the time or concentration right now to figure out exactly how that works and WHERE the ground is coming from in the 'START' position compared with the 'RUN' position. I'll be cutting a GP relay open next week so hopefully I can SEE exactly how it works. It should be close to the same internally.

Sorry to confuse anyone reading this!

What I described matches the original wiring on my truck and it should be the same on design series, 87-91 (8th generation) without turbochargers. Due to the changes in mounting the GP controller and GP Relay on turbocharged engines and I am only ASSUMING that the controller and relay were relocated on FACTORY INSTALLED turbo engines, the Factory wiring colors might have changed then. For after-market relocations of the controller and relay, the extended wire lengths could be any color.

BTW, where is the GP controller and relay located on engines with the original Factory Installed ATS turbochargers??

I don't know how to explain your relay at that location but it could be a GP relay moved to that location. How the Red/Blue wire described above might have been routed to the GP relay like splicing an extras wire run into the Starter Relay circuit wire, doesn't make sense. If it comes from the GP harness it would energize the GP Relay only in the ignition key 'START' position.
Was this some kind of GP controller bypass wiring???

If you are removing the engine to install into another vehicle I would just forget about that relay and how it's wired in the donor truck and locate/install your GP controller and relay however you want it. Unless you are planning to use the same harness the way it is now. I would still forget it, locate/wire the controller and relay wherever you decide you want it and correct the length of a couple of wires in the harness as necessary.

 

Sounds like trailer tow relay, Maybe for battery charge? I can't remember exactly but pretty sure it was mounted where you are describing.

 

Once again, thanks for the input.
I figured out that if I unplug the relay, I could find out what it is for. I just did that and it does not seem to make any difference in engine, starting or charging functions.
Still confused, I just got off of the phone with a friend who has an 87 non turbo F-250.
Same relay in the same location.

 

It's the trailer/aux tail light relay.
The trailer tail lights are run off it, along with (iirc) the cab marker lights(if you have them).

 

I ended up rewiring my trailer lights off a relay setup I mounted in the rear of the truck so wasn't sure exactly what it controlled even though I remember seeing it in my '88. I'm not sure about the cab marker lights as they still worked and I don't recall if I removed the relay or just noted it looked like crap and probably didn't work. For some reason I think it fell apart in my hands at one point.

 

Confirmed!
I unplugged the relay again and on the 87 Dually it does in fact control the overhead cab lights. I'm going to leave it at that, and assume that on the 90 F-250 it controls the aux tail light relay, since it does not have cab lights.
Thanks again!
Peter A.