Glow plugs

[WhistlinDiesel]

So I’ve tested and replaced glow plugs and relay. They still don’t work and WTS light doesn’t come on. Is there a fuse for them? I tried searching for one but couldn’t find one. Thank yall

 

[Jesus Freak]

By relay, do you mean the glow plug controller? Or the solenoid thing on the fender?

 

[Austin86250]

6.9 or 7.3 system?

 

[Big Bart]

So I am not a pro on 6.9’s but know some about the 7.3 gp system.

For the 7.3 gp system to work you need a few things to be in working order. (BTW some 87‘ 6.9’s got the 7.3 controller.)

1) At least 5-6 working glow plugs.
2) Good glow plug wiring harnesses.
3) Power from the battery to the relay/solenoid which is the top half of the gp controller system. It is two yellow wires, they should have +12v. Hook to the spot below in yellow. Test for power, no power you need to find out why.
4) Ground. The ground wire below is generally put under the bolt that holds the gp controller on the engine. This sometimes is overlooked during replacement. If there is no ground the system will not work.
5) Power from the ignition switch when it’s in the on position. Shown in red below.
6) The WTS wire is hooked to the special matching connector below.
7) The white wire with the red tag is the ground wire that the controller (Bottom half of the unit.) uses to activate the solenoid/relay (Top half of the unit.)

So the bottom half is a solid state computer of sorts. It senses resistance and heat and turns on the gp system and turns it off.

The top half is basically a starter solenoid. Some call it a relay. When one little posts get a ground and the other little post gets +12v (Positive) it energizes the solenoid and that makes a connection between the two big posts. (Just like a starter solenoid.)

One side of the two big posts is where 2 larger yellow wires bring +12v to the gp relay from the battery. The other side has a metal shunt on it. The one side of the shunt hooks to the other big post on the solenoid and the far side of the shunt is were the glow plug wiring harnesses hooks up.

When the ignition switch is set to “on/run” the red wire sends power to both the controller and the solenoid. That wakes up the controller and also supplies +12v to the solenoid. Then the controller tests for proper resistance (Good connections, good wiring harnesses, and good glow plugs.). If the resistance is within spec the controller sends ground to the white wire on the small post of the soleniod. The solenoid then activates sending power to the glow plugs. The controller decides how long to warm the glow plugs. Usually 8-20 seconds depending on resistance of the system and outside temps. When done warming the glow plugs it cuts the ground to the solenoid and shuts off the glow plugs And the WTS light goes off. Sometimes it cycles them on and off to help keep them warm.

So here are the challenges.

a) If the glow plug controller is not getting a good ground via the black wire, power from the ignition switch, or power from the batteries at the big post the controller does work and you will not see the WTS light come on.

b) If you have 3 or more dead glow plugs, the controller activates, the WTS light goes on for one second and the controller shuts off the power to the glow plugs. It’s a safety feature forcing owners to address the glow plug issue.

c) Now if your like most members your gp wiring harness is all brittle, cracking, and loose on the glow plugs, your resistance has changed. So the controller either will not come on (No WTS light.) on or come on sense a resistance issue and turn off. (WTS light flashes for one second and goes out.). Often followed by the controller cycling power to the glow plugs (On-off-on-off……) but not cycling the WTS light.

So check your connections are all there, tight, and right. Check for +12v on the big post with the yellow wires. Check for +12v on the red wire with the ignition in the on position. Check you have a good ground.

If all is good likely your glow plug harnesses are bad. (Classic Diesel Designs can help you.) Or your new controller/solenoid is defective.

Let us know what you find.

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[Jesus Freak]

If it's a a 6.9 or early '87, there's a thing that screws into the water jacket at the back of the engine on the top, drivers side. That's the GP controller. If you're a 7.3idi, big Bart is spot on, if it's 6.9 then that's the only other part.

 

[Austin86250]

i don't remember the 6.9 gp controller being that reliable that's why i bypassed it

 

[Big Bart]

i don't remember the 6.9 gp controller being that reliable that's why i bypassed it

I have read the 6.9 controller also had a nasty habit of getting old, failing, and thus not turning off the glow plugs and burning them all up at times.

Many bypass the 7.3 controller as it can be complicated to diagnose if it is not the glow plugs. But it is better to fix the system than mask a problem (Bad fusible links, wiring harness, glow plug wiring harnesses, controller, …) by by passing it. They were greatly improved in the 7.3 model and will help with glow plug longevity.

 

[Cubey]

have read the 6.9 controller also had a nasty habit of getting old, failing, and thus not turning off the glow plugs and burning them all up at times.

The problem is the original ones are electromagnetic (mechanical) and can stick. Modern ones (in the past 20+ years probably) are solid state, so they can't stick, just stop working.

Green background is the original style, it's fatter due to the mechanical bits inside. Red is new style, thinner, due to being solid state.

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[WhistlinDiesel]

Well I’ve replaced the whole controller and plugs and replaces the end pieces on the harness that connect to the plugs themselves. I’ve also tested the voltage and that jazz. I don’t wanna have to take it to a mechanic, but it may just be more simple at this point. It’s hard to start to sometimes, I think I may turn up the fuel pump 2 flats from stock and this winter replace injectors and etc.

 

[Jesus Freak]

Well I’ve replaced the whole controller and plugs and replaces the end pieces on the harness that connect to the plugs themselves. I’ve also tested the voltage and that jazz. I don’t wanna have to take it to a mechanic, but it may just be more simple at this point. It’s hard to start to sometimes, I think I may turn up the fuel pump 2 flats from stock and this winter replace injectors and etc.

What system do you have? We're here to help, but we need some basic information. On the original post, you said "relay"on the previous post you said "controller". On the older 6.9 trucks there's a solinoid/relay thingy on the fender and a controller like Cubey posted pictures of, if either one of those items are bad then you won't have WTS light. As far as the late 87 to 94 system, Big Bart's write up is the difinitive statement on that and I'm considering having my congressman codify it into my state's constitution.

 

[Jesus Freak]

I have read the 6.9 controller also had a nasty habit of getting old, failing, and thus not turning off the glow plugs and burning them all up at times.

Because I run almost straight WMO, I quit using my glow plugs more than a year ago because my GP system had quit working and instead of diagnosing it I just started using ether to crank it. But a while back, I was looking at something and noticed that my GP controller(red background of Cubeys pictures) had been on fire and was just a hollow shell. I'm not sure when that happened or why, but it'll sure make you glad most everything under our hood is steel and can't really catch on fire.

 

[WhistlinDiesel]

I replaced this

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[Jesus Freak]

That's the late 87 to 94 system. That's Big Bart's department.

 

[Nero]

There is a fusible link for the glow plug relay. On the engine side, it's the yellow wires (two big ones) and on the other side of the connector they are two black with an orange stripe wires. The fusible links are going to be on the black/orange wire side, towards the starting relay. Fusible link could have failed, or the connector got corroded and failed. Had both happen before, new fusible links and connector bypassed and all is well.

Just because the WTS light doesn't come on, doesn't mean the relays are working. My bulb burnt out finally, but I can hear the relay clicking at key on still.

 

[WhistlinDiesel]

I’ll check when I get home, but they definitely aren’t working. It takes a bit to start still. Also right before I replaced the plugs when I turned the key on the WTS would blink on then off and not come back on. It wouldn’t always do it when I turned the key either so idk