2001 7.3 GPR Issues
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Junior User
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2001 7.3 GPR Issues
I have read a lot of info on here about GPR issue but I couldn't find my solution so here is the question. From what I gathered my cpu dosen't send signal to turn on glow plugs, last winter I was doing the screw driver across the posts trick to get it started. This year I decided to run a ground wire to a switch in the cab and manually activate the plugs. Well that is not working as soon as I press the switch it throws a code, not only that but it does not activate glow plugs. Also I thought that the small terminal closest to the firewall was not supposed to get current only ground, I get a 200ma reading with the key on to this post. Did I misdiagnose my problem last year and I'm dealing with a bad GPR not cpu problems btw it is the stock one 234,xxx miles. I don't want to have to pop the hood every morning this year to get it started.
Hotshot
Joined: Jul 2003
Posts: 12,177
Likes: 40
From: Newport, WA
I can't remember the proper way to test the little wires on the GPR, but if you poke around in the 99-03 7.3L forum I know this has been discussed before. With that kind of mileage I wouldn't be surprised if the GPR is TU.
Elder User
Joined: Nov 2008
Posts: 668
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From: Northfield, MI
You are mistaking voltage for current.
The GPR primary side gets supplied battery voltage constantly and is grounded by the PCM to turn it on. When the PCM grounds that circuit, you will see 0v at the terminal leading to the PCM, but there will be current flowing. Current is what does the work. You can have voltage present on a circuit without work being done, but if current is flowing then work must be taking place.
The glow plug circuits can be checked in less than two minutes with only a 12v test light. No need for ammeters, etc.
Jay
The GPR primary side gets supplied battery voltage constantly and is grounded by the PCM to turn it on. When the PCM grounds that circuit, you will see 0v at the terminal leading to the PCM, but there will be current flowing. Current is what does the work. You can have voltage present on a circuit without work being done, but if current is flowing then work must be taking place.
The glow plug circuits can be checked in less than two minutes with only a 12v test light. No need for ammeters, etc.
Jay
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Junior User
Joined: Mar 2006
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You are mistaking voltage for current.
The GPR primary side gets supplied battery voltage constantly and is grounded by the PCM to turn it on. When the PCM grounds that circuit, you will see 0v at the terminal leading to the PCM, but there will be current flowing. Current is what does the work. You can have voltage present on a circuit without work being done, but if current is flowing then work must be taking place.
The glow plug circuits can be checked in less than two minutes with only a 12v test light. No need for ammeters, etc.
Jay
The GPR primary side gets supplied battery voltage constantly and is grounded by the PCM to turn it on. When the PCM grounds that circuit, you will see 0v at the terminal leading to the PCM, but there will be current flowing. Current is what does the work. You can have voltage present on a circuit without work being done, but if current is flowing then work must be taking place.
The glow plug circuits can be checked in less than two minutes with only a 12v test light. No need for ammeters, etc.
Jay
Fleet Owner
Joined: May 2009
Posts: 21,437
Likes: 76
From: Gilbert, PA
You are mistaking voltage for current.
The GPR primary side gets supplied battery voltage constantly and is grounded by the PCM to turn it on. When the PCM grounds that circuit, you will see 0v at the terminal leading to the PCM, but there will be current flowing. Current is what does the work. You can have voltage present on a circuit without work being done, but if current is flowing then work must be taking place.
The glow plug circuits can be checked in less than two minutes with only a 12v test light. No need for ammeters, etc.
Jay
The GPR primary side gets supplied battery voltage constantly and is grounded by the PCM to turn it on. When the PCM grounds that circuit, you will see 0v at the terminal leading to the PCM, but there will be current flowing. Current is what does the work. You can have voltage present on a circuit without work being done, but if current is flowing then work must be taking place.
The glow plug circuits can be checked in less than two minutes with only a 12v test light. No need for ammeters, etc.
Jay
Elder User
Joined: Nov 2008
Posts: 668
Likes: 6
From: Northfield, MI
Previously you said you had 200k+ miles on the truck and it had the stock GPR. If you meant you had the original factory GPR then the chances that it is good are very, very slim. They only last for a set number of cycles before the contacts become worn and they stop working, and remember the glow plugs come on every time you turn the key to the "on" position.
If you don't want to replace the GPR for whatever reason at this point, at least check the circuits properly with a test light. You cannot use a DVOM to check these circuits. You need something that puts a slight load in the system or you will get false readings.
Jay
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