glow plug controler

[beaver55]

I have a 1994 f250 idi, I just bought new glow plugs for it and I was wondering if the controler has a senser in it that tells when the engine is warm that the plugs do not have to heat up.Or do they heat every time the key is turned on.If they heat every time is it a good choice to put a manual hold down switch in side the cab, all info well help:

 

[argve]

yes it does sense the temp of the glow plugs. It does so by how much current they flow.

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.

 

[argve]

Now if you want to convert your system over to a manual system here is how...

Run a power wire up to the dash through a momentary push button to the positive side of the coil on the relay that is mounted on top of the glow plug controller. Remove all other wires off this terminal then ground the solenoid.

I say use a momentary push button because you don't want accidentally leave the glow plugs burning and burn them out the first time you start the truck because you forgot or bumped the switch back on....

I will tell you I'm not a fan of manual control on the glow plugs because the factory system works just fine - you just need to keep the glow plugs replaced as they burn out.

 

[HammerDown]

That Mel is just soooo dreamy in how he explains things

 

[EMD_DRIVER]

That Mel is just soooo dreamy in how he explains things

Mel?!?!?!

While I agree the avatar picture does bear some resemblance to Mr. Agne, I honestly believe that the explanation credit should go to Travis...

 

[franklin2]

He has a 94, so to go to manual, all he needs to do is take the white wire off the controller and tape it, and then run a wire from the white wire terminal on the controller inside to the cab to a momentary pushbutton, and then run a ground wire to the other side of the pushbutton. Grounding the white wire terminal will manually activate the glowplug controller on the later models.

 

[argve]

He has a 94, so to go to manual, all he needs to do is take the white wire off the controller and tape it, and then run a wire from the white wire terminal on the controller inside to the cab to a momentary pushbutton, and then run a ground wire to the other side of the pushbutton. Grounding the white wire terminal will manually activate the glowplug controller on the later models.

I was thinking that was what it was but couldn't remember - it's been a few years since I have owned an IDI... but I knew that if you just disconnect the relay and run new wires it would work....

Oh yes and I'm not MEL!!!!!

Yes I may run around a campground in skimpy clothing but I haven't had my picture taken while doing so....

 

[franklin2]

I was thinking that was what it was but couldn't remember - it's been a few years since I have owned an IDI... but I knew that if you just disconnect the relay and run new wires it would work....

I believe the method you described is correct for the older 6.9 controller.

 

[argve]

actually it will work for the newer styles because if you remove the control from the controller to the relay then the relay will only be controlled by the user and have no input from the controller. So even if the controller is bad the relay will work because you have established power and ground for the relay and the power to the glow plugs comes from off board the controller. But in my travels the controller really doesn't go bad very often - most times it's bad connections because the controller is looking at voltage drops and bad connections will fool the controller into thinking that everything is heated up fully. I had a couple of guys send me their old controllers years ago and when I got them I installed them on the Enterprise to test them and they worked great after I cleaned up the electrical connections which were nasty.

Now by just grounding the wire control wire will work - like I said it's been a while since I have had my head under an IDI hood working on one and was going off memory and didn't want to quote totally from memory after all these years for fear that my memory had shifted some... but you reconfirmed that my memory was correct - which surprises me. Thanks...

 

[beaver55]

Thanks for all the info, I think I will replace all my plugs and then make sure all conectors are clean,thanks again. It won't happen today since we are getting a dumping of snow.

 

[franklin2]

But in my travels the controller really doesn't go bad very often - most times it's bad connections because the controller is looking at voltage drops and bad connections will fool the controller into thinking that everything is heated up fully.

I am finding that out with mine, even though I have converted to a manual switch. When I had the controller, it was always the connections, either at the main harness plug or I found out the connectors to the glowplugs themselves. After bypassing the main harness plug, I still couldn't get it to work sometimes, and I found out my bad connections were on the glowplugs, since after I unplugged them to test them, the controller would work ok for a couple of weeks.

What I can tell now is my connections on the glowplugs are still sometimes giving me problems, because after using the manual switch, sometimes the engine is missing a few cylinders even though all the glowplugs are good. So I am figuring some of the bad connections on the glowplugs are keeping some of them from heating up properly. But it still starts with the manual switch, even with 3 bad glowplugs, and that's my priority, for it to start when I need it to start.

I am thinking of making my own connectors for the glowplugs. Right now I am thinking of using some small diameter brass rod(maybe 3/8") cut in short 1 inch pieces. Put one end in the lathe and drill it to accept the diameter of the glowplug, and have enough meat on the outside to drill and tap for a setscrew to hold it to the glowplug. Then drill the other side a smaller diameter to accept the wire, and solder it in place.

 

[Agnem]

Oh yes and I'm not MEL!!!!!

Yes I may run around a campground in skimpy clothing but I haven't had my picture taken while doing so....

Yet.....

 

[HammerDown]

Mel?!?!?!

While I agree the avatar picture does bear some resemblance to Mr. Agne, I honestly believe that the explanation credit should go to Travis...

Yep, my-bad...I just get so worked up when these two guys go into detail