Glow Plug Operation

[Jagg82]

On a 6.9 diesel, how long do glow plugs stay on. When truck is cold, i turn key on and then off and back on to get them to turn on but then they go on and off 4 times for one second intervals. Is this how they are suppose to work or are they to stay on longer. Have read where some run for 10 sec then light goes off.

[Gary Lewis]

I've included two pages from the 1985 EVTM. As you'll see it says "zero to nine seconds, depending on engine coolant temperature." And the first page shows that the temp switch closes at 112 degrees, F. That would suggest that if that switch is bad you won't get proper glow plug heating.

HOWEVER, the schematic shows that the temp switch is only used to power the Cold Idle and Cold Advance solenoids. If that is true, and the 1986 EVTM shows the same thing, then I can't see how that switch would cause problems with the glow plugs. But, it does say the Glow Plug Controller is "threaded into the left-hand rear top of the engine block", so perhaps there's a temp sensor in it that is not shown?

I hope someone actually knows how that works. Obviously I don't.

[85lebaront2]

I've included two pages from the 1985 EVTM. As you'll see it says "zero to nine seconds, depending on engine coolant temperature." And the first page shows that the temp switch closes at 112 degrees, F. That would suggest that if that switch is bad you won't get proper glow plug heating.

HOWEVER, the schematic shows that the temp switch is only used to power the Cold Idle and Cold Advance solenoids. If that is true, and the 1986 EVTM shows the same thing, then I can't see how that switch would cause problems with the glow plugs. But, it does say the Glow Plug Controller is "threaded into the left-hand rear top of the engine block", so perhaps there's a temp sensor in it that is not shown?

I hope someone actually knows how that works. Obviously I don't.

Gary, I believe it is the same switch GM used on their 5.7L and 6.2/6.5L diesels. it has bimetal contacts for the switches so time on/off is temperature dependent and above a certain temperature off is the only operation. The glow plugs are probably 6 volt plugs like the later GM ones and they cycle on-off so as not to burn them out. If they are not staying on long enough that would be the controller.

Have you actually checked that the glow plugs are good? It can be done easily with a 12V circuit tester, battery positive and touch each glow plug in turn, it should be close to full brightness. Usually a bad one will show essentially an open circuit.

[grumpin]

Gary, I believe it is the same switch GM used on their 5.7L and 6.2/6.5L diesels. it has bimetal contacts for the switches so time on/off is temperature dependent and above a certain temperature off is the only operation. The glow plugs are probably 6 volt plugs like the later GM ones and they cycle on-off so as not to burn them out. If they are not staying on long enough that would be the controller.

Have you actually checked that the glow plugs are good? It can be done easily with a 12V circuit tester, battery positive and touch each glow plug in turn, it should be close to full brightness. Usually a bad one will show essentially an open circuit.

If I remember right, on my 1988 with the 7.3, which has the same glow plug system I believe, I would check each glow plug to ground with the wire off. No continuity to ground it’s a bad plug.

I also think that this could be indicative of a bad glow plug or several.

[Jagg82]

Gary, I believe it is the same switch GM used on their 5.7L and 6.2/6.5L diesels. it has bimetal contacts for the switches so time on/off is temperature dependent and above a certain temperature off is the only operation. The glow plugs are probably 6 volt plugs like the later GM ones and they cycle on-off so as not to burn them out. If they are not staying on long enough that would be the controller.

Have you actually checked that the glow plugs are good? It can be done easily with a 12V circuit tester, battery positive and touch each glow plug in turn, it should be close to full brightness. Usually a bad one will show essentially an open circuit.

I have not checked them as wasn’t sure how too. If motor is warm they do not come on but it doesn’t matter how cold the motor is, they operate at same interval. They never come on when key is first turned on. You have to turn key ofF and on one time when cold to get them to cycle and then it’s just 4 cycles For 1 sec each time u turn key off and on. Will start by testing them. Then maybe replace controller? Then guy I bought it from did say they were 6 volt and they should cycle short but never said how short. 😁

[Ford F834]

I have not checked them as wasn’t sure how too. If motor is warm they do not come on but it doesn’t matter how cold the motor is, they operate at same interval. They never come on when key is first turned on. You have to turn key ofF and on one time when cold to get them to cycle and then it’s just 4 cycles For 1 sec each time u turn key off and on. Will start by testing them. Then maybe replace controller? Then guy I bought it from did say they were 6 volt and they should cycle short but never said how short. 😁

To check the glow plugs, use a multi meter and test the resistance between the wire terminal and the threaded collar/hex collar. The resistance needs to be less than 1 ohm. If it’s more, the glow plug is burnt out. They are pretty much like little light bulbs with no glass.

While testing the plugs is a good idea, you need to double check what controller and system you have. I say this because short glow cycle is more common on the later glow plug system. The 84-86 system was so bad that most are converted to manual push button or have been rewired to the 87-94 control system. The two are very different. Here is how you can tell them apart: The early system is 12v with flat spade connectors and the later system is 6v with round bullet connectors. The early system has a large round thread in piece threaded into the cylinder head back by the firewall. It has a multi pin connector to the wire harness on top. The late style controller is a solid state block with the relay mounted right on top of it instead of on the fender. This is mounted to the aluminum intake on 87+ trucks, but is often fender mounted on conversions since the earlier intakes lack the threaded holes to mount it. You will see a metal “Z” strap coming from one of the relay terminals. This is what limits the voltage to the 6v glow plugs.

It is important to understand how these systems work differently. The early 6.9 system uses a temperature switch in the cylinder head coolant jacket (in the controller itself, separate from the cold advance temperature switch). This switch was prone to failure in the “on” position, which would burn out all of the glow plugs. Often the tips would swell when this happened. They wouldn’t fit out of the glow plug hole. The tips would break off and fall into the combustion chamber. This means the engine needs removed and the heads pulled. Or many owners just ran it and hoped the piston would pulverize the glow plug tip and spit it out the exhaust valve 😖.

The 87-94 solid state system actually uses the glow plug resistance to determine temperature (each one is essentially a thermocouple). As they heat up, the resistance increases, and the controller shuts them off. If you have one, or more than one dead glow plug, the controller “thinks” they are hot and you get a very short glow cycle.

You have probably read this, but avoid aftermarket glow plugs. Especially Autolites... they are notorious for swollen tips even without controller failure. Motorcraft (Beru) is OEM, the spade 12v plugs are ZD1’s and the bullet connector 6v plugs are ZD9’s. I personally won’t run an early 84-86 controller. It’s not worth the risk. The later style controller can be twitchy but is an acceptable system if you want automatic operation. Both controllers cycle on and off after the engine starts to clean up smoke, and this shortens glow plug life. A manual momentary switch can be a good solution, but is only as good as the person pushing the button! If someone other that you needs to drive the truck keep that in mind. If you choose to convert from 12v system to 6v system this will help:

[grumpin]

To check the glow plugs, use a multi meter and test the resistance between the wire terminal and the threaded collar/hex collar. The resistance needs to be less than 1 ohm. If it’s more, the glow plug is burnt out. They are pretty much like little light bulbs with no glass.

While testing the plugs is a good idea, you need to double check what controller and system you have. I say this because short glow cycle is more common on the later glow plug system. The 84-86 system was so bad that most are converted to manual push button or have been rewired to the 87-94 control system. The two are very different. Here is how you can tell them apart: The early system is 12v with flat spade connectors and the later system is 6v with round bullet connectors. The early system has a large round thread in piece threaded into the cylinder head back by the firewall. It has a multi pin connector to the wire harness on top. The late style controller is a solid state block with the relay mounted right on top of it instead of on the fender. This is mounted to the aluminum intake on 87+ trucks, but is often fender mounted on conversions since the earlier intakes lack the threaded holes to mount it. You will see a metal “Z” strap coming from one of the relay terminals. This is what limits the voltage to the 6v glow plugs.

It is important to understand how these systems work differently. The early 6.9 system uses a temperature switch in the cylinder head coolant jacket (in the controller itself, separate from the cold advance temperature switch). This switch was prone to failure in the “on” position, which would burn out all of the glow plugs. Often the tips would swell when this happened. They wouldn’t fit out of the glow plug hole. The tips would break off and fall into the combustion chamber. This means the engine needs removed and the heads pulled. Or many owners just ran it and hoped the piston would pulverize the glow plug tip and spit it out the exhaust valve 😖.

The 87-94 solid state system actually uses the glow plug resistance to determine temperature (each one is essentially a thermocouple). As they heat up, the resistance increases, and the controller shuts them off. If you have one, or more than one dead glow plug, the controller “thinks” they are hot and you get a very short glow cycle.

You have probably read this, but avoid aftermarket glow plugs. Especially Autolites... they are notorious for swollen tips even without controller failure. Motorcraft (Beru) is OEM, the spade 12v plugs are ZD1’s and the bullet connector 6v plugs are ZD9’s. I personally won’t run an early 84-86 controller. It’s not worth the risk. The later style controller can be twitchy but is an acceptable system if you want automatic operation. Both controllers cycle on and off after the engine starts to clean up smoke, and this shortens glow plug life. A manual momentary switch can be a good solution, but is only as good as the person pushing the button! If someone other that you needs to drive the truck keep that in mind. If you choose to convert from 12v system to 6v system this will help:

I should have kept quiet and waited. Very good info. F834!

Especially on the brands. I remember having swollen tips hoping they didn’t break off!

[Ford F834]

I should have kept quiet and waited. Very good info. F834!

Especially on the brands. I remember having swollen tips hoping they didn’t break off!

Dane, I try not to insert brand bias other than to say “this worked well or didn’t work for me”... but there are so many horror stories about Autolites that I feel remiss not to warn people. I have also read enough accounts of disappointment with other off-brands to make me want to avoid them. The one exception that comes to mind here is Jan/Romel77 on this forum. He bought Motorcraft and says his did not last. He replaced them with Bosch which have treated him better. He uses a manual momentary switch and 6v plugs. I intend to run Motorcraft in mine when I get it back together since many, many owners swear by them and claim years of service out of them. My glow plugs quit working and I discovered the previous owner changed the connector terminals and was running 6v plugs with the 12v controller 🥴. They were also Autolites. I didn’t want to deal with (likely) swollen Autolites that had been subjected to 12v, so I unplugged the controller and resorted to starting it with ether. Then other things happened, but I digress.... buyer beware!

[Jagg82]

To check the glow plugs, use a multi meter and test the resistance between the wire terminal and the threaded collar/hex collar. The resistance needs to be less than 1 ohm. If it’s more, the glow plug is burnt out. They are pretty much like little light bulbs with no glass.

While testing the plugs is a good idea, you need to double check what controller and system you have. I say this because short glow cycle is more common on the later glow plug system. The 84-86 system was so bad that most are converted to manual push button or have been rewired to the 87-94 control system. The two are very different. Here is how you can tell them apart: The early system is 12v with flat spade connectors and the later system is 6v with round bullet connectors. The early system has a large round thread in piece threaded into the cylinder head back by the firewall. It has a multi pin connector to the wire harness on top. The late style controller is a solid state block with the relay mounted right on top of it instead of on the fender. This is mounted to the aluminum intake on 87+ trucks, but is often fender mounted on conversions since the earlier intakes lack the threaded holes to mount it. You will see a metal “Z” strap coming from one of the relay terminals. This is what limits the voltage to the 6v glow plugs.

It is important to understand how these systems work differently. The early 6.9 system uses a temperature switch in the cylinder head coolant jacket (in the controller itself, separate from the cold advance temperature switch). This switch was prone to failure in the “on” position, which would burn out all of the glow plugs. Often the tips would swell when this happened. They wouldn’t fit out of the glow plug hole. The tips would break off and fall into the combustion chamber. This means the engine needs removed and the heads pulled. Or many owners just ran it and hoped the piston would pulverize the glow plug tip and spit it out the exhaust valve 😖.

The 87-94 solid state system actually uses the glow plug resistance to determine temperature (each one is essentially a thermocouple). As they heat up, the resistance increases, and the controller shuts them off. If you have one, or more than one dead glow plug, the controller “thinks” they are hot and you get a very short glow cycle.

You have probably read this, but avoid aftermarket glow plugs. Especially Autolites... they are notorious for swollen tips even without controller failure. Motorcraft (Beru) is OEM, the spade 12v plugs are ZD1’s and the bullet connector 6v plugs are ZD9’s. I personally won’t run an early 84-86 controller. It’s not worth the risk. The later style controller can be twitchy but is an acceptable system if you want automatic operation. Both controllers cycle on and off after the engine starts to clean up smoke, and this shortens glow plug life. A manual momentary switch can be a good solution, but is only as good as the person pushing the button! If someone other that you needs to drive the truck keep that in mind. If you choose to convert from 12v system to 6v system this will help:

Wow. Thanks. The glow plugs have the spade connector so that clears that question. I did the ohm test by just unplugging the glow plug and one tester to spade and other to a ground. Most were over one but 2 were .8. Great thread and thank you so much. Awesome info!!!

[Ford F834]

Wow. Thanks. The glow plugs have the spade connector so that clears that question. I did the ohm test by just unplugging the glow plug and one tester to spade and other to a ground. Most were over one but 2 were .8. Great thread and thank you so much. Awesome info!!!

You are welcome! So your GP’s are fried, and if you have the original controller I would say it is suspect. If it looks old, it’s probably a good idea to replace it, convert it, or wire a manual switch. The aluminum body of the 83-86 controller acts as an anode in the coolant jacket and they just don’t last.

Interestingly, I just now noticed that there are new replacement 6.9 controllers that look identical to the originals but are solid state instead of electro-mechanical. I know nothing about them, but I would consider this over the original design:

https://www.accuratediesel.com/shop/60.html