okay, long story short. '92 f250 7.3. the guy i bought the truck from had a push button installed to controll the glow plug relay because he kept burning up glow plugs.
thursday i had a glow plug burn out on me. i replaced it no problem. they're all either autolite or delco.
just for grins yesterday i took the switch out of the equasion. i hooked up the relay like it should be so it works off the key. it seemed to work fine. the wait to start light came on and i heard a click as it came on, light went off and relay clicked off. i started the truck, tapped the gas to start the fast idle. ammeter went up and down about once a second for 10 or 15 seconds i assume for the afterglow. i start to drive the truck, whenever i give it gas the ammeter drops as tho the glow plugs are on. i let off the gas and the ammeter goes back up as tho the glow plugs are off. i figure this is normal to help the fuel ignite. after the truck reaches operating temp and the fast idle shuts off the ammeter quits jumping i assume glowplugs are no longer being used. i figure yay the system is working as it should.
fast forward to this morning. it's 30 some degrees. i turn the key on and the wait to start light comes on and i hear a click from the relay. wait to start light goes off but no click from the relay. odd. turn the key and engine cranks but no start. grey/white smoke from the tail pipe when i turn the key i assume from un-burnt fuel. turn the key off and back on click when the wait to start light comes on but no click when it goes off. i notice if i listen carefully i can almost hear 2 clicks when i turn the key on, i assume the relay clicking on then off really quickly. i put the pushbutton back in the equasion and the same thing. i push the switch and i almost hear 2 clicks when the wait to start light comes on.
got a ride to work, went back this afternoon on lunch to investigate. tested the glow plugs with a test light. lead on the positive battery terminal, probe on the tip of each glow plug, no light. pulled two plugs out including the new one i replaced friday and tested with a fluke ohm meter. open. no resistance. i tested the new one friday before i put it in and got .5 ohms. all the others had 1.5 ohms. now all 8 are burnt up. no ohms. what the devil??
as my luck would have it oreilly wouldn't give me a firestone discount on the glow plugs and i can't afford 8 $20 motorcraft plugs right now so i opted for the *retch* $8 autolites just to get the thing running. i haven't put them in yet. i ran out of time and had to boogie back to work.
just wanted to know why my plugs, including the brand new one burnt out so quickly. is my relay bad??

 

First-off... PLEASE don't use Autolite GP's! They are well known, to be defective and can/will burn out VERY quickly. If you are looking for a better price for Beru/Motorcraft plugs, go HERE.

The solid state controllers are usually very dependable. They work by measuring the resistance in the GP circuit. They are known to be susceptible to corrosion on the terminals. You might want to clean all the terminals on both the relay and GP controller. I've heard that doing that will help, if corrosion is causing the controller to misread the resistance.

Bypassing the controller would make the GP's work around the corrosion, but they wouldn't work as well as they could.

It's worth a try anyway...

 

this resistance, is there a way to calculate and measure the total resistance of the glow plug circuit. what i mean is, they're all .5 ohms. are they wired in series or paralell? should i multiply .5 times 8 or are they wired in parallel, should i divide .5 by 8. also, which two points on the relay go to the glow plug harness. i should be able to measure the resistance at those two points, right? if there's too much reistance then there's corrosion or faulty wire harness right?
does the behavior i described earlier sound like a typical glow plug controller? the thing hasn't worked right since i've owned the truck so i wouldn't know if it was working properly.

 

I would think that the resistance the controller looks at,is the resistance in the GP's themselves. The slinky-looking thing that goes from the relay to the controller is a resistor. Someone with more electrical expertise will have to chime in about specific answers to your questions.

From personal experience... I have two trucks that have the same GP system. When one or more GP's go bad, the WTS light will come on and go off very quickly.. Not the typical 10-15 seconds. I do hear my relay cycling the GP's for at least 10-12 seconds. When all my GP's are good, it doesn't cycle them anywhere near that much.

Makes me wonder if the extra cycling is to help try to compensate for the bad GP's.

 

I've been through this before and there is no doubt in my mind that the controller is bad, just change it, put new GP's where needed and you truck will start.
The time and money you spend trying to cheap out on this just is'nt worth it.
A bad controller will act like it is working fine, but I promise you, it is just as likely to burn your plugs out as it is to not light them.
Been there, done that.

 

i went ahead and put the autolites in. i know they're bad news but i had to get my truck running and the only place in town wants $20 apiece for motorcraft plugs. i refuse to pay that when i can get them cheaper online. next summer i'll start replacing them but for now i just needed to get her running.

having said that. i had a realisation tonight after i got the truck running with 8 new glow plugs. i can't believe i didn't think of it before. my ignition switch is messed up. i have to consiously make sure i have the ignition off when i pull the key out. some times i'll pull the key out and the radio will stay on. sometimes the door just chimes like the key's in but it's not. i just turn it a hair untill it clicks.
i left the ignition on last night. after a few hours when the engine cooled down the glow plugs started cycling untill they burned out. i'm supprised it didn't run the battery down. i cranked the engine off and on for a good 15 minutes this morning trying to get it started and the batteries didn't start to run low. they must be pretty heafty batteries.
it's only a theory. i'm 75% sure that's what happened. maybe 80%. i hooked the glow plug relay up the way it should last night after work then drove the truck home. just for grins about a mile from my house i killed the engine, and started it back up to see what the glow plugs would do with the truck warmed up and they worked just fine. turned the key on, wait to start light came on, ammeter went down indicating the plugs were on ten seconds later light went off, ammeter went back to normal, started the engine. so it had to be that the ignition was left on.

 

If you get a week worth of starts out of the Autolites I will be surprised.

If the tips swell and break off when you go to change them, you may be doing a head gasket job to get the tips out.

160 for Motorcraft glow plugs, 150 worth of gaskets and 22 hours of wrenching to change head gaskets.

 

My 87 had a set of Autolites in when I bought it. I used the truck for over a year before I found out how bad they were from reading the forums. When I went to pull them I was expecting a nightmare. I guess I was just lucky but none of the tips were swollen, 3 of them were dead. I know they are junk but that is what I found on my truck.