Understanding the (6.9) GP controller

[rockbender]

I was fortunate enough to receive a file folder full of receipts and miscellaneous information when I got my van, and amongst that information is a copied page which looks to be an excerpt from a service manual for a 6.9. The page that I have describes how the glow plug controller works and it is a bit different than I've read here in my some of my searches so I thought I would share. Feel free to chime in if you know this to be true, false, or otherwise. It's a bit long, so I apologize in advance:

"The 'quick start; afterglow' system is used to enable the engine to start more quickly when the engine is cold. It consists of the flour* (???) glow plugs, the control module, two relays, a glow plug resistor assembly, coolant temperature switch, clutch and neutral switches and connecting wire. Relay power and feedback circuits are protected by fuse links in the wiring harness. The control module is protected by a separate 10A fuse in the fuse panel."
"When the ignition is turned to the on position, a Wait-to Start signal appears near the cold-start knob on the panel. When the signal appears, relay No. 1 also closes and full system voltage is applied to the glow plugs. If engine coolant temperature is below 86 deg F, relay No. 2 also closes at this time. After three seconds, the control module turns off the WTS light indicating that the engine is ready for starting. If the ignition switch is left in the ON position about three seconds more without cranking, the control opens relay No. 1 and current to the plugs stops to prevent overheating. However, if coolant temperature is below 86 deg F when relay No. 1 opens, relay No. 2 remains closed to apply reduced voltage to the plugs through the glow plug resistor until the ignition switch is turned off. "
"When the engine is cranked, the control module cycles relay No. 1 intermittently. Thus, glow plug voltage will alternate between 12 and zero volts with relay No. 2 open. After the engine starts, alternator output signals the control module to stop the No. 1 relay cycling and the afterglow function takes over."
"If the engine coolant temperature is below 86 deg. F, the No. 2 relay remains closed. This applies reduced (4.2 - 5.3) voltage to the glow plugs through the glow plug resistor. When the vehicle is under way (clutch and neutral switches closed), or coolant temperature is over 86 deg F, the control module opens relay No. 2, cutting off all current to the glow plugs."

*flour - this was copied verbatim from the sheet. I'm not sure if this was suppose to be 'four' making it for some other application? Everything else including the adjacent schematic points towards the 6.9 system.

In a nutshell, what this tells me is that the stock controller would hold the plugs at 12V for no longer than 6 seconds, and then reduce the voltage. I never realized that the system changed the voltage output. I had heard information about appropriately sized wire to force a voltage drop to the plugs, but don't recall reading that it worked in this manner. All of a sudden it makes since as to why we have plugs that are rated for 6 volts and how we would rarely burn them out off of a good functioning controller but roast them all the time with our pushbutton systems. It also makes me think that it would be better for the plugs on a manual system to hold for 6 seconds, and then alternate a bit - on 3, off three, on 3 off three.

Ideally, I'd like to have a second pushbutton (or a three way momentary toggle would be perfect!) so that I could apply 12v for 6 seconds or so to get things fired up, and then hold them in their warmed up state with the 6V for another 6-10 seconds without worry about burning them up.

I'm hoping that Mel might be able to chime in or update his thread on the new 6.9 controller that he put it. Now that I understand the actual logic of the controller I'm thinking it is going to be smarter than me and my big fat thumb!

 

[rockbender]

Additionally, here is an interesting note I found in the wiring diagram:
"Each glow plug lead contains a piece of brown. In case of short circuit, the wire will burn out without damaging the insulation."

 

[Kevin 007]

"understanding" the 6.9 controller is very similar to trying to understand women
This is a very interesting posting! I never realized voltage was reduced to maintain the glow. I am currently trying to master a manual glow system to work for me and its frusterating.

Just as you mentioned, this explains why manual switchs cook plugs all the time even though we only cycle them for the same amount of time or less, compared to the controller.

 

[The Warden]

VERY interesting...and, I noticed a few things that don't seem right:

"When the ignition is turned to the on position, a Wait-to Start signal appears near the cold-start knob on the panel."

First thing that's worth noting is that, on '84-'86 trucks (what most people accept as the "standard" 6.9l system), the light isn't marked "Wait to Start". It's actually marked "Glow Plug". With that said, '83 trucks ARE marked "Wait to Start".

Second, this is the first I've ever heard of a "cold-start knob" on one of our trucks. Since the article didn't go any further, I can only guess what this means based on past experience...but, on early Mercedes diesels (all naturally-aspirated engines, and turbo engines in '78-'79 300SD's), there was a knob that you could turn to increase the idle speed to keep a cold engine running smoothly. I'm wondering if this article's describing something similar? ...if so, I also wonder if the "flour" isn't as much of a typo as one may think? Is there any chance that this article could be meant for either the Ranger or the ****** diesel, which were both 4-cylinder engines?

For what it's worth, I just pulled out my 1984 "Emission Diagnostics" Ford shop manual that covers the design of the '84-'86 6.9l glow plug system. Here's what it has to say about the glow plug system (page 32-12 in this manual):

The 6.9l diesel engine utilizes an automatic electric glow plug system to aid in starting the engine. The function of this system is to pre-heat the combustion chamber to aid ignition of the fuel.

The system consists of eight glow plugs (one for each cylinder), control switch, power relay, wait lamp and a wiring harness which incorporates eight fusible wires (one for each glow plug) located between the wiring harness and the glow plug terminal. (Fig. 17). The system is activated by the ignition switch being turned to the On position.

Figure 17 shows the glow plug controller, with a "controller cycling switch", an after glow timer, and a circuit breaker as integral parts within the controller. The only thing feeding into the controller is switched power, to two points. There's a connection from the controller cycling switch to the left-hand glow plug bank, I guess to tell the controller that the plugs are in fact coming on...and there's the trigger wire to the relay. This diagram isn't showing anything else...certainly not the additional relays that your article mentions.

So, without seeing it, my best guess is that your article is either for the '83 system, or it's for a different engine (likely either the Ranger or ****** diesels)...

 

[plywood]

If you switch to the 7.3 beru/motorcraft plugs, they limit voltage as they get hotter.

I hold mine on about 15 seconds around freezing and it starts up and purrs. Been doing this for the last 4-5 years.

Usually 6-8 seconds does it on a cold start in the summer, that's straight 12v.

 

[plywood]

So, without seeing it, my best guess is that your article is either for the '83 system, or it's for a different engine (likely either the Ranger or ****** diesels)...

I agree, not much sounds right. I did drive an old DT466 that had a cold start knob like a pull out throttle on an old truck or tractor. Don't know but I kind of think it was a timing advance?

My recollection of the 6.9 controller is it is mostly a bi-metal spring that heats up and bends and shuts off and that's supposed to be about the same timing as the GPs. I believe it's in the water jacket so it's hot when the engine is hot.

 

[The Warden]

My recollection of the 6.9 controller is it is mostly a bi-metal spring that heats up and bends and shuts off and that's supposed to be about the same timing as the GPs. I believe it's in the water jacket so it's hot when the engine is hot.

That's what I've always understood, although I haven't dissected a controller. FWIW the same section I quoted from before describes the controller this way:

Control Switch: The control switch is located on the rear of the LH cylinder head. It senses the temperature of the engine coolant and determines the length of time that the glow plugs will be on. The length of time varies between 4-10 seconds during initial cycle.

Rather vague, but certainly the description of the bimetallic spring fits with my experience and with the official description...when I first got my truck, the spring was stuck in the disconnected position, then eventually unstuck itself and actually worked properly for about 2 years before the spring stuck in the ON position. I was able to get the purple wire unplugged from the relay before the glow plugs burned themselves out, fortunately.

I firmly contend that the 7.3l system is the better system (compared with a manual control as well as with the 6.9l system)...I just need to take the time to put in the proper harness and get mine working properly *sigh*

 

[rockbender]

The header of the sheet/page says 'Full size Trucks, Vans, Bronco' and there is a schematic that is clearly labeled 6.9 system with 8 glowplugs in the system.

From what I am interpreting, it seems that the pre-glow and after-glow switches may be in the controller itself?

It would be really interesting if someone with a functioning 6.9 controller could hook a volt meter up to one of the glow plugs (with the plug still hooked up) and monitor the voltage to see how it is behaving.

 

[The Warden]

The header of the sheet/page says 'Full size Trucks, Vans, Bronco' and there is a schematic that is clearly labeled 6.9 system with 8 glowplugs in the system.

From what I am interpreting, it seems that the pre-glow and after-glow switches may be in the controller itself?

It would be really interesting if someone with a functioning 6.9 controller could hook a volt meter up to one of the glow plugs (with the plug still hooked up) and monitor the voltage to see how it is behaving.

According to the diagram I'm looking at, the controller has three portions: a circuit breaker, an afterglow timer, and a controller cycling switch. I found a copy of the diagram I have here:

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If I'm looking at it correctly, I don't see any way that this system can reduce the voltage to the glow plugs, unless the mechanism to reduce the voltage is in the glow plugs themselves. The plugs receive their power strictly from the relay, which in this system is simply an on/off switch triggered by the controller. According to the diagram, there is a wire going from the controller cycling switch to the left-hand glow plug bank, but I don't recall any wires at the controller even remotely being big enough to carry the kind of current that the glow plugs take, so the only other use I can think of for that wire is as a signal wire to the controller. Maybe the wire's meant to tell the controller how hot the plugs have gotten and that's supposed to tell the controller when to cycle the plugs off?

Is there any way you can scan your sheet in? My best guess is that it's for the '83 system, which IIRC is significantly different from the '84-'86 system, and I haven't seen much documentation on it...

 

[rhkcommander]

Additionally,
"Each glow plug lead contains a piece of brown. In case of short circuit, the wire will burn out without damaging the insulation."

Piece of brown, they mean brown fusible links. Its true, ive torn a harness up

In a nutshell, what this tells me is that the stock controller would hold the plugs at 12V for no longer than 6 seconds, and then reduce the voltage. I never realized that the system changed the voltage output. I had heard information about appropriately sized wire to force a voltage drop to the plugs, but don't recall reading that it worked in this manner. All of a sudden it makes since as to why we have plugs that are rated for 6 volts and how we would rarely burn them out off of a good functioning controller but roast them all the time with our pushbutton systems. It also makes me think that it would be better for the plugs on a manual system to hold for 6 seconds, and then alternate a bit - on 3, off three, on 3 off three.

Ideally, I'd like to have a second pushbutton (or a three way momentary toggle would be perfect!) so that I could apply 12v for 6 seconds or so to get things fired up, and then hold them in their warmed up state with the 6V for another 6-10 seconds without worry about burning them up.

We have 6v glowplugs because 12v'ers would take wayyyy too long. Afterglowing is to smooth out cold idling and emissions. With a pushbutton most people have problems because they either: cant count seconds accurately and dont use a watch/stereo/timer/cellphone, havent tested to see how long it takes the glowplugs to get hot, or double glow back to back

We dont really need afterglowing with pushbuttons, and AFAIK theres only 1 relay. Also it doesnt adjust voltage.

 

[johneich]

It would be really interesting if someone with a functioning 6.9 controller could hook a volt meter up to one of the glow plugs (with the plug still hooked up) and monitor the voltage to see how it is behaving.

I have a functioning 6.9 controller and would be glad to hook up a volt meter to it this weekend or next.

FWIW I think I have a late 83

I have no cold start knob as far as I know

 

[Jake_IN]

this is the first I've ever heard of a "cold-start knob" on one of our trucks. Since the article didn't go any further, I can only guess what this means based on past experience...but, on early Mercedes diesels (all naturally-aspirated engines, and turbo engines in '78-'79 300SD's), there was a knob that you could turn to increase the idle speed to keep a cold engine running smoothly. I'm wondering if this article's describing something similar? ...if so, I also wonder if the "flour" isn't as much of a typo as one may think? Is there any chance that this article could be meant for either the Ranger or the ****** diesel, which were both 4-cylinder engines?

My ranger has a knob down at the bottom of the dash that i can pull out to advance the timing and it gives it a high idle. Just throwing that out there.

 

[rockbender]

johneich - that would be great if you could report what the controller is doing!

Warden - thanks for posting the diagram. That is what the one looks like in my wiring diagram, but I need to review the one on the page with the description that I posted and see if it is the same or not.

I'll try to get my sheet scanned in later today or tomorrow.

 

[vegas39]

The 6.9 controller sends a true 12 volts to the plugs.

VW also had that cold start knob on the old rabbits and jettas, it advanced the timing and made a hell of a difference on starting.

 

[rockbender]

Sorry about the delay guys - here is the sheet that I copied the original post from. You can see clearly that it shows a 6.9 GP system diagram with both a power relay and afterglow relay.