91 IDI glow plug issues help please

[Whit]

I volunteered to work on an old IDI....first time ever on one of these cats, anyway it runs just fine but he says it doesnt start without starting fluid, its in my shop now so I pulled the aircleaner to get good view of things, I turned the key on to activate the glow plugs and the module behind the intake ontop the motor is clicking in and out terrible, this is the realay that feeds power to the glowplugs maybe??????? So I cleaned up all the battery terms as they were all corroded but the module still clicks like crazy

guide me straight here men

Cheers,Kevin

 

[bigoilburner19]

ok well the solenoid could be bad on that module I had the same prob it was jsut a basic starter solenoid that I bought for it and make sure its grounded properly or else it will do that. I had a 1987 ford f350 dually with a 6.9 banks turbo and i pulled the aircleaner assembly and other things four times before I actually had a brain. lol it was the solenoid and then the ground wire was cut too so just make sure its gettin good power and grounded.

 

[argve]

Kevin,

Here is how the system works.

When you turn on the key the controller is activated and it closes the relay that is mounted on top of the controller then it reads how much voltage is dropped across the Glow Plugs. As current flows through the system it has a fixed resistance in which to use (the big zig zag piece of metal on the controller) so it waits until the voltage on the glow plugs comes up to a set point. We don't know what this voltage set point is and really we don't need to know.

What happens is as the glow plugs heat they change resistance value - they actually increase in ohmic value so as they increase in ohms they become more of a restriction to current flow and so they will increase their voltage drop. Once the voltage drop gets high enough the controller knows that they are HOT HOT HOT and so it switches to a pulsing action so to keep them at that temp and not burn out. Once it switches to the pulsing it will turn off the WAIT TO START LAMP in the cab to let you know to start the truck. Once it has pulsed the glow plugs with power for I think it's 10 seconds or so it will shut down and wait for another key cycle before they will work.

By key cycle I mean OFF and then back ON.

In 70 degree weather the glow plugs are sitting at a higher temp from the get go than in 30 degree weather so the controller will not run the glow plugs for as long because remember as the glow plugs heat they gain resistance and since they are already warmer (it's summer time now) the controller will see it's magical set point voltage sooner because the plugs don't have to heat as long as say they do when the outside temp is at 30 degrees.

Now if you have at least one glow plug that is not working this will fool the controller in that it thinks the plugs are warmer than they actually are.

This is because the glow plugs are in parallel so we know from OHM's law that you as remove resistors in a parallel circuit that the overall resistance increases for that section of the circuit. Well because the resistance increased then so will our voltage drop across that section of the circuit, so the controller is looking at the voltage from backside of the zig zag resistor to ground and it will see that magical voltage sooner so the plugs did not get enough time to heat fully.

This is what causes short cycling as some call it. Where the controller only ran the wait to start lamp for a very short period of time and will go right into pulsating the plugs. So then you have a hard start condition.

Typically you can get away with one bad glow plug and not really see too much of difference unless you really pay close attention - with two glow plugs not working your gonna have a hard time getting her to start and you'll know there is a problem. With 3 or more bad glow plugs if you get it started in the cold you got dang lucky.

 

[bigoilburner19]

well yeah but im not that smart to explain it thanks argve you really showed my fat a** up lol just kidding but seriously.argve is the man he knows alot about these truck and so does agnem but good luck dude

 

[argve]

That said I would start by checking the glow plugs by either a Ohm meter or test light. as I posted above one bad glow plug can cause you problems but in my experience you can still get buy with one bad she just complains a little on start up, but 2 or more = Good Luck buddy...

Bad connection on these things can cause problems as well. I like to keep those terminal connections bright and shiney on the glow plug controller as I have seen this be a problem before.

 

[argve]

well yeah but im not that smart to explain it thanks argve you really showed my fat a** up lol just kidding but seriously.argve is the man he knows alot about these truck and so does agnem but good luck dude

Wasn't meaning to show you up. If you think this is a good write up you should see some of the write up's that Kevin does for the Cummins stuff - wickedly bad and I have copied him more times than I can count....

 

[Whit]

great info men.....................I will mess with it and see what I can come up with

 

[Agnem]

As has been said, disconnect the plug from every glow plug, and use a test light on the glow plug. You should get a nice bright test light reading. Just hook the aligator clip from the test light to the battery + and then touch each glow plug. If you get any that are dim or not lit at all, then those plugs have to be changed. Replace the whole set, and don't fool around with just replacing the bad ones. Beru (Motorcraft) is the only brand to get. About $10 a plug is a good price these days.

 

[bigoilburner19]

lol its alright argve how much u wanna bet that guy has nolights in that truck lol hahahah but agnem and argve you guys are freakin awesome man lol plus ive seen that kevin dude he helped me with my bosses cummins he is sick as hell man he knows what he is talkin about I wish I could know even half of what he knows

 

[Whit]

thanks men its running and starting good now, all 8 glowplugs showed bad so I went to town and got 8 and also a solonoid, turned out to be just the plugs but I replaced the solinoid since I was there

not bad once ya get guided straight, it is amazing I know the Cummins quite well and have a ton of experience with them but still I open the hood on an old IDI for the first time and learn as I go, I knew from the begining it wasnt getting glow plug action but still needed help.....thanks again men

 

[plywood]

And the GPs were Autolites right?

 

[plywood]

Kevin,

Here is how the system works.

I wish you'd of told me that a couple months ago and saved me a lot of hair. Oh well, prevents Alzeimers right.

I kept reading the controller senses the resistance of the plugs which I knew wasn't quite right since that would be impossible with voltage present.

I kept wondering why that wavy metal band was there and ended up figuring out that it's used it as an inline resistor to read voltage drop and therefore indirectly read the resistance of the GPs.

Finding the "click off" voltage would be interesting and not too hard. I was working on it last week with a controller that was handed to me when I bought the truck, but, turns out the controller is bad.

Not sure if it is reading voltage drop across the GPs or the metal strip, based on where the wires are it could be doing either. I think for the initial burn it is reading one and for the afterburn the other.

If someoned has a good working system with Motorcraft/Berus and hooked up a voltmeter across either until the controller clicked off, that would be it. From what I can tell it is about 10.5v across one and about 1.1v across the other.

 

[Compu Doc]

Fast clicking on the glow plug relay simply means that you have some bad glow plugs. Instead of narrowing down which ones are bad just replace them all with Motorcraft and be done with it.

Mine had 3 bad and as they were being pulled out I was checking them. The time was not wasted figuring out which ones were bad beforehand because it took about 2 hours to replace all 8. We took our time and as they were bing loosened we sprayed PB blaster around them as they were coming out. Once resistance was felt from them we would spary a little more blaster and then screw it back in. This would work the PB Blaster into where the plug sites and dissolve the carbon off the plugs. Doing it this way we did not break one tip and we pulled out Champion plugs that were replaced with Motorcraft.