Some copy and paste action:
This is from the 87-94 glow plug sticky thread.

Post #13 by Dave: "Make sure they don't give you a starter relay, they are different electrically and have a small I and S, one by each of the two small terminals."



Post #25 by Dave: "
If your relay has I and S labels by the small terminals, you have a starter relay, not a glow plug relay.

You can make it work if you want to, by moving the white wire to one of the relay mounting screws.

Glow plug relays have no ground where they are mounted, so the two small screws are the opposite ends of the trigger coil.
Power on one, ignition label in the picture above and ground on the other labeled white wire in the picture.

Starter relays are mounted on the metal inner fender, so it's trigger coil is grounded through the mounting bracket.
On diesels, only the S terminal is used, which is where the wire from the key switch start position ends.

Been so long since I messed with a gas motor, I can't remember where the I terminal went on the gas engine trucks, which is what it was used on.

The controller and ignition terminal of the glow plug relay have constant power when the key is in the on position on the new style system.
The ground wire (black) must have a good ground, usually under one of the controller mounting bolts.
When you first turn the key to on, the controller checks the resistance of the glow plug circuit.
If resistance is low, indicating cold glow plugs, the white wire goes to ground, completing the circuit through the relay trigger coil causing the relay to close.
As the glow plugs get hot, resistance gets higher.
When it reaches the desired level, the relay opens to turn the glow plugs off and you start the engine.

Although I told you how to make the starter relay work, a better option would be get the correct relay, so if anyone else goes to work on the glow plugs they won't freak out. "



Post #38 by Dave: "The internal electrical connections are different.

Starter relay coil grounds through the mounting bracket, and I don't remember what the gasser motor used the I terminal for, diesels have nothing on that terminal.

The glow plug relay coil is grounded through the second small terminal, not the mounting bracket.

With a wire reconfigure you can make the starter relay work, but it will not work if you just swap them out."


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Ah, so the ground is different! As I suspected.

Also, just FYI. The "I" terminal of the starter solenoid was used on gassers for supplying a 12V while cranking only. It is wired internally to the post for the starter cable, starter side. Breaker point ignitions used this to send 12V to the coil, for hotter spark, while cranking. Later, it was used for powering electric fuel pumps during cranking, when they had an oil pressure cut-off switch.

For a homebrew GP circuit, you could actually use the "I" terminal to power a light for the GP's, to know when they are on, and when they are off. Like, for manual GP's......
That's just for those who choose to make their own GP circuit, and cheap out by using a starter solenoid, rather the actual GP relay..... You know, people like me........

 

Ahh, so you're a cheapo too!? Us cheapos gotta stick together!

 

I am a master at the cheapo dept. Not to be confused with someone that puts off needed maintaining or delays parts replacement.
Been planning to do a 6.9L swap in my F150. Not for power or anything like that, but the MPG that can be achieved. For "training", I have been driving it 13.5 years with a 300, C6 and 2.75 gears in the 9". Now that I am used to the right lane, I think I'm ready for a 6.9L, C6 and boost in the old truck.......
That *should* bump me from 14-16 MPG, to well into the lower 20's, without losing the ability to tow.