Ford 7.3 relay module

Doug Owner/Technician California Posted   Latest  
Resolved
Driveability
Electrical
1992 Ford F-350 Custom 7.3L (M) 4-spd (AOD)
Crank / No Start

Greetings Technicians, I have a question about this '92 7.3 where its glow plug module is not grounding properly activating the relay. On these earlier modules where the PCM doesn't control the ground, the module will ground it until the glow plugs heat up to start the motor and then releases the ground? It currently has too much resistance on the ground causing it not to activate the relay. 

Thank you, Doug.

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Kyle Mechanic
Illinois
Kyle
 

Check the resistance of all 8 glow plugs. It only takes one bad one for the relay to work improperly. The module uses the impedance of the glow plugs to determine glow plug on-time.

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Glenn Owner/Technician
Texas
Glenn
 

Hi Doug, Wow a classic. 92' should be a pre turbo model unless it had a turbo added on. These have a glow plug module mounted at the top rear center of the intake, it has a cover over it. It also has a coolant temp electric switch on the front passenger head for the GP module. That switch typically goes bad with age. That switch is what activates the module when the key is turned on. That…

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Doug Owner/Technician
California
Doug
   

Ahh, I was wondering what that switch was SOL (Smiling Out Loud). That makes sense with the CTS activating the ground control…. I don't mean to doubt your understanding of how it works but here is what the service information says. “The glow plug controller determines the glow plug temperature by electronically measuring the resistance of the glow plugs,… A ‘Wait To Start’ lamp on the…

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Doug Owner/Technician
California
Doug
 

Well that didn't seem to work. I purchased a new module and going to hook it up tomorrow and see if that fixes it. If not we'll go from there. And I know I know I tried my best not to be a parts changer but nothing else worked. 😒

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Albin Diagnostician
Washington
Albin
 

That glow plug system is a stand alone system that uses the resistance of the glow plugs to calculate the on time of the glow plugs. Take a look at the wiring diagram. diag​.​net/file/fy6s924aj… ​ If you are experiencing the problem of the glow plugs not staying on long enough, you will notice the GP relay will rapidly click on and off. This is caused by one or more of the glow…

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Doug Owner/Technician
California
Doug
   

Yeah I figured that out. Looks like to me that this diagram shows that if the wait to start light bulb in the dash is burnt out, like Glenn said, it's not going to properly activate the module relay. So it looks like I have to get in the dash to check the bulb & the new module wouldn't work either.

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Glenn Owner/Technician
Texas
Glenn
 

Hi Doug, You may not see one of these again for a long time, but you will remember they can be a challenge. As Albin indicated, go with Motorcraft if at all possible. Geek Auto and Rock Auto are usually good sources, even Ebay surprises me from time to time with OE new parts. When it comes to the bulb(s), use quality HD ones like Sylvania. For the temp switch, I was recalling the earlier 6.9…

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Kevin Owner/Technician
Arizona
Kevin
 

Doug, That temp switch does not control the glow plugs. They are controlled only by their collective resistance. That switch is for timing advance and fast idle. Replacing 1 or 2 is usually a waste of time, do them all. Use only Ford/Motorcraft! You've been warned.

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Matthew Owner/Technician
Oregon
Matthew
 

Some of those old coked up glow plugs are pretty hard to get out. Those like to swell and coke pretty badly if I remember. I always unscrewed them until the thread were free and then put a vacuum line or fuel line over the plug and put it in a drill and spun it while tapping the glow Plug back towards the Threads and the cooking grinds off that way and you can pull them right out.

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Albin Diagnostician
Washington
Albin
 

You can also start the engine up and unscrew it and let the compression do all the hard work.

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Rex Technical Support Specialist
Ohio
Rex
   

Make sure the grond connection on the block that the controller connects to is clean and there are good grounds from the block to the batts. That conroller is fussy about it's ground. It should still cycle on even with several bad plugs. For testing glow plugs I have alway used an incandescent bulb test light. Hook the test light lead to the pos batt post. Touch the tip of the glow plug to the…

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Doug Owner/Technician
California
Doug Resolution
 

So during my continued testing I could not get it to operate the relay so I installed a new module with relay and it works beautifully now! Thank you all for your very helpful posts. Maybe next time I'll get a newer diesel to play with LOL.

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Glenn Owner/Technician
Texas
Glenn
 

Hi Doug, Newer 7.3 in 94.5 and up were a Power Stroke and they are a different animal to learn about. I worked for a few years on early 6.9 and 7.3 none-turbo diesels, and it was a definite learning curve with the newer ones. Glad you got this one resolved.

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