Question 95 7.3 glow plug circuit problem

[mbutts]

Hello,
We just acquired a 95 F-350 at work. Part of my responsibilities is to do maintenance on all of the equipment. From heavy equipment to the boss's car. I am familiar with some of the basics of the Ford 7.3's. We have a couple of them already. I have been googling on several of the problems I have found so far. One that I have not found any info on is this.

The Glow plug relay stays on all the time. On the dash, the "wait to start" light comes on and goes off after a few seconds like it should. However the relay stays energized. The control wire is always hot. (We checked the relay and it isn't welded closed.) I have pulled the relays out on the fuse block on the drivers side and the relay turns off as it should. I don't have a wiring diagram of the motor so I don't know where the light in the dash and the relay get their power from. Has anyone run into this before? Is there a diagram of the wiring that is available on the internet? If we have to, I will put a manual push button in the cab for the glow plugs, but if its not too expensive, would prefer to fix it right. Tnx in advance, martin

 

[BIG JOE]

Are the GP wires >after< the relay energized (hot).... Even after it warms to Op temp ?

Depending on outside air temps, the GPs will cycle on & off 'til the Oil and Coolant temps reach a certain temp value.


OBTW.. WELCOME tp SDD !!!

 

[Patrick Feeley]

The only time I ever hear of this problem is when someone puts in a case-grounded relay as replacement for the original. Any relay used for the glow plug relay must of the case-isolated type otherwise the relay will stay energized. The circuit is fairly simple. One big wire is hot all the time through fusible links. The other big wire goes to the glowplugs. One small wire goes to the engine compartment fuse box (#22) via a splice into the fuel heater wiire. The other small wire will go to the PCM where it will be grounded by the PCM to complete the circuit when "waiting-to-start". The WTS light really has nothing to do with the glowplugs or relay. Hopefully that provides some insight. Cheers!

 

[BIG JOE]

Thanks for posting that more indepth info Patrick.

 

[mbutts]

Thanks for the quick replies. I went in yesterday (sunday) for a couple hours to do some more work on the rig. I pulled the valve cover off to get ready to replace the glow plugs. I pulled one, and it looked to be in very good condition. I checked it and it was ok. This threw up a red flag for me because I expected it to be bad. All 4 on the drivers side were good. So I decided to go outside and see what the 94 F-350 we retired would do. Unfortunately, the batteries were run down so I couldn't see how long the relay stays on. (apparently someone decided to turn the ign key to acc to listen to the radio) Ran a battery charger to it but never got back to it to try it. Might try it Monday.
Anyway, I went back inside and checked to see if power was going to the glow plugs. I does. While I was checking the glow plug leads I heard the relay click off. I recycled the ignition key and after a minute ( i timed it) the relay clicked off.
Before I brought it into the shop, I plugged it in for 2 hours. I was told that possibly some of the glow plugs were burnt out because it didn't start good cold. It was about 35 deg outside and it started like crap. It also had a rough idle. I let it run for 10-20 minutes to warm the oil so i could change that too.
It sat in the shop overnight at 60 degrees so I'm wondering if that made a difference on the relay cycling. I would of thought that the low midrange on the temp guage would be close to "at operating" temp when I brought it into the shop.
Am I wrong? Can these glow plugs run for in excess of a minute continuously without burning out? I am familiar with glow plugs cycling til the engine is warmed up.
I don"t know what condition the vehicle actually is in yet but I'm a bit concerned. On my initial quick check I found one u joint completely shot and the real kicker was a grease zerk put in the exhaust pipe just ahead of where the cat coverter used to be. That coupled with fancy graphics on the side and aluminum rims makes me think it was some kids ride. I don't know if it has a chip in the computer but i was told it will squawk the tires. Monday ill pull the other valve cover and check the rest of the glow plugs. Once again tnx in advance. martin

 

[94f450sd]

the psds glow plugs can stay on for up to 2 minutes depending on intake air temp,coolant temp and oil temp.they are controlled by the pcm as been mentioned.if your worried about it a newer pcm code will fix it.the tde1 pcm only turns the glow plugs on when needed,so ive been told.my chip was burnt useing the tde1 code to override the tee4 pcm in mine.my glow plugs still come on with every start up but they dont stay on long when the motor is still warm.

 

[Crumm]

Can these glow plugs run for in excess of a minute continuously without burning out?

Yes like Rich said they can stay on for several minutes. Like Patrick said the dash light and the Glow Plugs are not connected together. The glow plugs stay on much longer than the light. If your having starting issues and the plugs are OK check the Voltage drop across the two big terminals on the GPR. The relay getting weak seems to be a regular problem. If you find that the relay needs replaced and you plan on keeping the truck for a while you might replace the stock type relay with a Stancor 586-902. You can find the relay at Mouser or Allied. More money but twice as tough.

 

[mbutts]

Thanks for all the quick replies, I'll finish up with the last bank of glow plugs and see how things go from there. I won't worry about burning out the new set of plugs. This takes a load off my mind since I have to get this thing going asap to temporarily replace our other crew cab that was broadsided and will be down for a minimum of 2-3 weeks for doors and utility box replacement.