1985 6.9 IDI. Took her out for a highway drive, 400 miles round trip. ran fine out to the destination, then driving home at night the glow plug light kept coming on intermittently. This had happened before but not constantly like this. It was staying lit for over 10 seconds and I could see the power draw when the headlights dimmed slightly every time the light came on. I pulled over and yanked the little switch wire off to stop the signal to the relay. That stopped the activation of the glow plugs.
I tried to start it today and no glow plug light, though I can hear the relay switch clicking on, it just isn't lighting the plugs anymore, there is no power drain when it clicks. Engine won't start without ether.
I have been using a starter relay on the glow plugs because it worked. When I put the engine back together in March I realized the GP relay was not working so I used a spare starter relay to run the glow plugs. There is no line to the "I" side of the relay, the wire that was connected to the old relay at that spot melted when it was energized for the first time I have a new GP relay coming but wonder what that melted line was supposed to do and if it has anything to do with the controller going nuts. Also wonder if this could be caused by having like a gallon of antifreeze missing from the system, it seems to happen more when I turn the heat on...
I wonder what to repair? Did I fry the GPs? Is the controller needing the signal from the melted wire? I will put the new relay on anyway. Is the controller fried? Any help so I can move in the right direction fixing this would be appreciated.

 

this a standard problem with the old style controller. it sounds like your controller is dying, engaging the glow plug relay.
if you continue to run it this way, you will smoke the glow plugs.
people usually fix this issue by bypassing the controller and making it a manual system.

 

Thanks for the reply. I want to restore the controller so the system runs automatically.
I am referring to the photo on Dave Spongaul's #2 post. The black wire has melted off of my GP relay, maybe because I used a starter relay in its place. The starter relay has worked perfectly energizing the glow plugs for 6 months until the controller started to go bonkers. I have another controller I will install soon, but wonder about replacing the starter relay with a GP relay and the necessity of restoring that black wire connection that Dave S. says goes to the wait to start light on the dash. I have a glow plug light on the dash but nothing that says wait to start. Is that what Dave means by the wait to start light? Anyway, is it important to restore that small black wire connection?

 

The glow plug light is the early version of the wait to start lamp, they figures we are stuiped and had to tell us to wait, the glow plug light wasn't enough, replace your controller, keep your coolant full, the controller uses coolant to sense the temp. and the black wire should go back on. the difference between the gp relay and starter relay is the gp relay is a continues use relay, meaning it can handle longer on times without welding or burning out. a starter solenoid is meant for short starts or on times. Get the proper solenoid in there, so you dont have to worry about it any longer.

 

I had a GP controller go belly up on my 7.3 one night. Suprisingly all but 2 of the motorcraft GPs survived the 10min trip home.

 

Thanks David, will do. I wonder why the wire melted on the relay when I first started the engine after the rebuild. There was all this smoke and the wire just burned off the connection. I will have to un-bundle the wires and find it to replace it. We will see how many of the GPs I burned up on my little trip, they were on for quite a while...

 

I traced this wire to ground. There is a pic of this wire on Dave Spongaul's VERY helpful entry about glow plugs at the top of this thread section. He says it goes to the glow plug light but it doesn't. I have had it disconnected for the past 6 months and my GP light has worked perfectly. When I first energized the GP relay after my rebuild that wire melted and almost caused a lot of damage to the other wires in the bundle. AND the GP relay is well grounded to the body. So, I have decided to leave it off when I reconnect everything, make sure it works, then connect it out of the bundle in case it burns up again. Any clues to what this wire is and why it would be frying now would be helpful.
Jonathan

 

I put a GP relay on and left the black wire off, relay will not work. If I put the little black wire on the relay it works. So, if you try to use a starter relay on the GP circuit (like an idiot) it will melt the wire because of the obvious (too much current for the wire). GP relays are designed for the connection, which is a ground for the switch part of the relay. All of the glow plugs were fine from a visual inspection, no melting, no expansion. I replaced them anyway since I had the new ones all set. Time for a test drive...
j

 

the relays are different in the internal wiring, the gp relay is a floating ground type, the start relay is grounded at the mounting bolts, So when you activated the start relay, the (I) post is made Positive, and the black wire is the ground, for the gp relay. so it shorted out.

 

Thanks Speedie, say hi to Racer. The big boy ran like a clock, a big noisy stinky clock, and I loved it. Coincidentally this is the first 35 degree day in these parts, and I have a brand new ignition system all ready to fire it up. Thanks for the answer.
J