Ok gang, sorry to do it, but here is yet another glow plug thread...
Could be wrong here, but I feel this case is special...I have read so many threads about junk plugs, conversions, and pulling heads.. makes ya dizzy.


Here is the quick history... 85 6.9 factory glow plug system in tact..
received vehicle with a glow plug problem.. This particular one I believe the after glow circuit kept cycling ...doesn't matter really, the controller was believed to be junk.
No biggy , I replaced the plugs, start truck , run out and unhook controller from solenoid till I could change the controller...
Well that worked a few times then I found bad plugs again...ok
Read online about crappy plugs(even though I owned and ran a daily driver 86 6.9 that I never touched plugs in , and it had "Junk" glow plugs in it for like 3 years)
.... decided ..ok Ill buy, ill get me some ford, beru made in Germany glow plugs, and manually control the solenoid...
Now I know there are people on here talking about cycling for no more than 10 seconds..
I can honestly say I have not cycled this set any more than 6 seconds...
Guess what?? I have maybe 2-4 starts on this set and 4 brand new ford glow plugs are dead...
Now before anyone busts my chops about running only 4 let me just say, its my belief , its a simple circuit , all getting 12v in parallel , it shouldn't matter.. Oh ya , and when your instantly burning them out, I was in no rush to buy all 8...lol
The one good thing about the Motorcraft plugs I can so far positively report , is they did not swell up like the others and came out easy..
I have removed the glow plug harness for inspection...So far nothing crazy...
The only thing I could begin to guess is that somehow the controller back fed into the harness somehow, but I kinda second guess that thinking I would have cooked something in the harness if that was happening .
I know another guy running a pre 87 truck , probably early 80's ... running solenoid manually and who knows what plugs. Hand full of years on those plugs , haven't changed one yet.....
Something is definitely up here... Will I buy into some plugs are better made than others, sure? But to fail like this...???
I say there is something some of us are missing.. Could I convert and be done with it? sure... But I'm stubborn .. and so far I'm on a mission to figure it out...
So my next step is to eliminate the controller from the harness(up until now, I have had it still plugged in) that should eliminate the possibility of any back feeding...
The only other thing is that I am using the dash light (glow plug) circuit to ground my solenoid.. so when I hit my switch the dash light comes on , letting me know the solenoid is working...
After that... who knows

 

If you're going to due the GP's manually be sure to disconnect the controller. The problem with aftermarket controllers on the early system is when they fail they fail in the 'on' position thereby burning all your GPs out.

 

The wire from the controller to the solenoid is disconnected in this instance. .

 

I think the system is designed so you have a voltage drop in tbe wire to the glowplugs. Maybe it is able to deliver too much current to 4 plugs and overheat them.
Just an educated guess.

 

Hmm ..not sure . I thought that was true for the 87+ system where they had resistive wires?
Unless I am mistaken?

 

I've found the best option is to use the 87+ controller(with resistive zig-zag wire) with the 86-(spade-top) GPs, and a push-button. A lot lower chance of frying GPs.
Also, with them all in parallel, they /do/ create a voltage drop. 4 working will glow a lot quicker than 8.

In addition, I've been able to make /all/ GPs bulge, if you get them hot enough. If you only /slightly/ overheat them, /any/ GP will die without bulging. Get them hot enough to melt the casing before they die? They'll bulge.

 

Ok guys.. what your suggesting is the design of the system is dependent on the voltage drop across all 8 functioning plugs , and as I hypothetically unplug 1 glow plug at a time the voltage drop decreases thus 4 plugs vs 8 gets a higher voltage.. do you think even with two new batteries the voltage drop across 4 is to little that 10-11v (guessing here) is too much @ 6 seconds and would kill them after a few cycles?
I would love to believe this, but i have cooked all 8 as well... previous owner cooked all 8 twice i believe ...unfortunately I can't afford to keep throwing plugs at this thing without a fix... believe me I'd love to just to experiment. ..for the sake of knowledge and sharing with the board here..
And yes..I'm sure converting to the newer set up would Be the answer...but I haven't given up yet
What kills me is there are some trucks with major failure ...and some that are running flawlessly. ..and I owned one... I know for a fact it had what some consider to be "junk" glow plugs from napa most likely. .. I ran that thing for 3 or 4 years and never touched a plug...and that's on a factory controller. ..it was an 86
Not trying to be closed to the voltage drop thing...just need some more info....
In the mean time I will try to get a voltage drop measurement on my buddy's 6.9... I am pretty certain all 8 on his are functioning. .. I'll try it on the 92 as well .. just for some rough ideas

 

Buy from a different store, motorcraft glow plugs have a two year warranty at my local Oreileys then you're good for free replacements if 8 new good ones doesn't fix your issue.

 

are the plugs burning out, or burning up?

i remember reading here where incorrect timing will burn the tips off glow plugs rendering them useless.

 

hmmm now that is a good question. . Not sure how to describe ..they are carboned up..
not really broken or chunks missing from them...try to get a pic up

 

Unfortunately only have NAPA , Autozone and Advance... Rock auto still the cheapest as far as ZD1A
here is a pic of an autolite 7.3 plug(left)and the motorcraft in question(right)
the only good plug is the 7.3 plug which has never been energized. I stuck that in there way back trying to see if engine condition had something to do with it
I am wondering now...was reading on a Mercedes site where glow plugs were failing because of leaky injectors dripping cold fuel on hot glow plugs
not sure if that applies here..
on a different note....
Think I am going to have to look into an infared thermometer...
Oh ya....forgot ...measured voltage at the batt while energizing glow plugs.. voltage dropped 1.5v roughly on my 92 7.3..tried on my buddy's 6.9. ..barely dropped .80v we are guessing he has one or two out..