Glow plugs randomly stopped working-tried everything

[Rob Ferreira]

I have a 1990 7.3 IDI with a valve cover mounted glow plug module. When I first bought the truck the relay would continuously click (bad glow plugs) but still start with no problem. I left it like that for a few months but now winter is here and needed it to start better. Replaced all 8 glow plugs with ZD9s and it started up like it was warm in 30 degree weather. PERFECT!

Then in the afternoon coming back to the truck the glow plug light flickered and shut off and now I have no glows with a clicking relay again. Could my glow plugs have gone bad within a couple of days??

I have replaced the relay three times and the module twice. With brand new one's (not motorcraft but.. Come on 250 dollars..?) Tried regrounding block to frame, battery to frame, box to battery, none made a difference.

I have 12V to the relay, switched 12V to the relay but no power to the glow plug side. Even trying to jump the relay lugs yields no sparking or indication of power going through. I have continuity from both front glow plugs to the relay so harness should be good to go.

What is there left to check????

 

[MtnHaul]

Have you tried a manual switch for the GPs? http://www.oilburners.net/articles/glowplugbypass.htm for more info.

As long as your relay is good and the wiring is intact your plugs should glow.
Have you checked for a melted connector anywhere?

My truck had issues with GPs when I purchased it and I ended up just making my own harness and using a momentary switch to run the plugs. All new wiring, new relay, always works.

 

[Thewespaul]

Welcome to the forum. Check the big harness connector on the passenger side of the truck near the dipstick. You will see two large yellow wires, these power your gpc. The plug is not rated for the amps the gpc pulls, and the plug often melts and will not allow significant current flow even though you may show 12v with no load. You can check to see if your glowplugs are shorted with a test light connected onto battery positive and touching each gp with the harness disconnected.

 

[Rob Ferreira]

Damn that was honestly the next thing I was going to try! I jumped it direct from battery and it worked like new, I'll take a look for that plug and take care of that
Thanks a ton for the idea! Thats why I posted here I knew you guys knew your stuff

 

[snicklas]

Here is my recommendation for the replacement of the high current lead to the glow plugs.

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https://www.orschelnfarmhome.com/4-gauge-switch-to-starter-cable-mas000612.html

Here is my 88 with the wiring connector that has not been swapped out. (Look at the insulation on the 2 larger yellow wires) (Not to worry the wires aren't connected to anything, and the batteries are disconnected)

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Here is my 89 with the replacement cable, and the yellow wires cut out.

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This setup has been working well since installation a couple weeks ago.

Another recommended upgrade that I do, I add and grounded push button to manually over ride the controller if it decides to not work, or if a plug or 3 burn out, you and fire the remaining plugs with push button and get the truck to start. When I do this on mine, I leave the controller connected like the factory, and add the grounded push button. This was they can run like normal, or they can be cycled manually.

Here is where I describe the push button override:

https://www.oilburners.net/threads/picture-request.82158/page-2#post-978087

https://www.oilburners.net/threads/glow-plug-controller-confusion.84098/#post-1004437

 

[sjwelds]

Just a note. The yellow wires do need to remain connected to power, at least on the later ('92+?) trucks. tried deleting them entirely on my 92 and had no ignition power and don't remember what else...hooked them back up and golden.

 

[franklin2]

That is a very large wire that electrically goes straight to the battery. Of course it needs to, it supplies power to the glowplug system, but having a large hot wire like that laying in the engine bay can cause problems if it rubs through somewhere. The factory used fusible links in each of their yellow wires. You could think about using a very large fuse, 175 amp should do. The parts stores sell these, GM used them on their trucks and SUV's for the main power wire in the late 90's.

At the least, I would put some split wire loom around it to give it a little extra protection.

 

[snicklas]

Just a note. The yellow wires do need to remain connected to power, at least on the later ('92+?) trucks. tried deleting them entirely on my 92 and had no ignition power and don't remember what else...hooked them back up and golden.

Thank you for that info. I've never removed the yellow wires on a 92+ only a Bricknose truck (87-91).

 

[snicklas]

That is a very large wire that electrically goes straight to the battery. Of course it needs to, it supplies power to the glowplug system, but having a large hot wire like that laying in the engine bay can cause problems if it rubs through somewhere. The factory used fusible links in each of their yellow wires. You could think about using a very large fuse, 175 amp should do. The parts stores sell these, GM used them on their trucks and SUV's for the main power wire in the late 90's.

At the least, I would put some split wire loom around it to give it a little extra protection.

One or both are the plan. When was done in a limited amount of time, and we honestly didn't know if any of the glow plug system worked. It wasn't working when we bought it. Right now it's not parked because of a failing ball joint. I will protect it better.

 

[jwalterus]

You could think about using a very large fuse, 175 amp should do. The parts stores sell these, GM used them on their trucks and SUV's for the main power wire in the late 90's.

One or both are the plan. When was done in a limited amount of time, and we honestly didn't know if any of the glow plug system worked. It wasn't working when we bought it. Right now it's not parked because of a failing ball joint. I will protect it better.

Personally, I'd give a local stereo shop (not best buy or a national chain) a call, in my experience, you can get a higher quality AGU or ANL fuse holder with a fuse cheaper through them than you can an automotive use MAXI or MEGA fuse (MEGA fuses are what you are talking about BTW).
If you want to replace a fusible link with a like-functioning fuse, you use a JCASE fuse.

Personally, I install a positive side ANL fused 2-way splitter (to give me 2 new high-amp outputs) and a 4-way AGU fused splitter (to give me 4 medium amp outputs) on one output of that the first time I replace batteries on or install the new stereo in a new-to-me vehicle, but I run at least one amp in every vehicle (my truck has 3).

 

[chillman88]

That is a very large wire that electrically goes straight to the battery. Of course it needs to, it supplies power to the glowplug system, but having a large hot wire like that laying in the engine bay can cause problems if it rubs through somewhere. The factory used fusible links in each of their yellow wires. You could think about using a very large fuse, 175 amp should do. The parts stores sell these, GM used them on their trucks and SUV's for the main power wire in the late 90's.

At the least, I would put some split wire loom around it to give it a little extra protection.

For what it's worth... Mid to late 90s Chevy blazers have a pretty nice fuse holder inline with the alternator under the battery tray if you have a pic and pull.

Next best bet is this https://www.amazon.com/Littelfuse-2..._rd_t=40701&psc=1&refRID=1EWS2YE8XZ9WB54H1FKY

I blew out a 100amp. I can't remember if I'm running a 150 or 175 now with no issues for a couple years now.

 

[snicklas]

Another option I was considering is something like this:

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https://www.littelfuse.com/products...s/hi_amp-thermo-plastic-circuit-breakers.aspx

I actually have one of these unused, under the hood of my P71 (It was installed by IMPD Fleet when it was a cop car, all the wiring has been removed, but the breaker is still there). Figured I would try it since it is free. If it works, good, if not, go with a fuse..... I think the one I have is 200 Amp, but need to look at it. The P71 isn't here at the moment. The son who has the truck with the new cable has the car, the truck has a ball joint ready to come apart. So I will look at putting the breaker in when we are working on the front end. This cable is designed for almost this same application, it is the one that goes from the solenoid to the starter, but I understand it only has voltage applied while the solenoid is engaged.

 

[Jason1377]

I have a "93 where about on or under the dash n how to get in the without cutting a hole

 

[snicklas]

I have a "93 where about on or under the dash n how to get in the without cutting a hole

Get to what? I just passed a wire through an existing gromit in the firewall.

My wire/circuit path is"

One end of a short wire to a good ground, I think we used a "tek screw" (My son was installing overhead doors at the time) into one of the metal dash braces, or maybe even the pedal box. (Been a couple years now, and we don't have that truck anymore)

Drilled a hole in the very bottom of the dash to mount a momentary push button switch. (Ours was never going to be restored or a show truck, but a more "non intrusive" mount could used)

Connected one pole on the switch to the short grounded wire, mentioned above.

Connected the other pole to another wire that would be long enough to reach the relay.

Passed this longer wire through an existing pass through (I think we went through where the throttle cable passes through)

Put a ring terminal on the end of the wire and connected to the same post as the existing white wire.

The only permanent thing done to our truck was the hole in the bottom of the dash. If the button had been mounted in a bracket and used an existing fastener, there would have been no permanent changes to the truck

I used a switch like this:

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https://www.tractorsupply.com/tsc/p...on-sealed-switch-20a-12v-dc-720w?cm_vc=-10005

 

[Jason1377]

Thanks for the details @snicklas was trying to figure that thru the cab part right I dislke drulling holes unless its a toy or beater rig I'm using, I'll have to look closer for a spot like that since I believe one or two of my Gp's are dead