Glow plug control module
Cross-Country
Joined: Jun 2007
Posts: 96
Likes: 0
From: Just outside Chicago, IL
GPCM seems to work all right
Okay, disconnected the black connector off my GPCM and tried to start the truck -- turns over with no start. I then disconnected the green connector as well -- turns over with no start. I also noticed the lack of voltage drop associated with glow plugs firing (voltmeter also on board)
I then re connected them both and saw the familiar voltage drop and the truck started pretty well. It's 10* F here today. I also got a P0683 code once both connectors were in place again and read/cleared them with my scan tool. This is the one I have. I bought it for about $40 at Pep Boys last summer.
ACTRON OBD-II POCKET SCANNER - JCWhitney
Do you think I wasn't getting fuel before and the repeated startings put fuel in for me so it started pretty easy once all was hooked up again???
I then re connected them both and saw the familiar voltage drop and the truck started pretty well. It's 10* F here today. I also got a P0683 code once both connectors were in place again and read/cleared them with my scan tool. This is the one I have. I bought it for about $40 at Pep Boys last summer.
ACTRON OBD-II POCKET SCANNER - JCWhitney
Do you think I wasn't getting fuel before and the repeated startings put fuel in for me so it started pretty easy once all was hooked up again???
Hotshot
Joined: Jul 2008
Posts: 12,782
Likes: 2,142
From: N.GA Mountains
I was about to swap GP's and figured I'd measure resistance first. ALL GOOD. Waiting for Ed to get me my GPCM...
Its been 2 days already! 
FWIW, here's this:
QB11 CHECK FOR DTC P0683
DTC P0683 is set when the PCM detects an open or short to ground or power diagnostic line.
Possible causes:
open diagnostic line
diagnostic line shorted to ground or power
faulty PCM circuit
faulty GPCM circuit
Perform KOER On-Demand Self Test and retrieve/clear continuous DTCs.
Is DTC P0683 present? Clear DTCs and retest, make sure there is no DTC P0670 present. If DTC P0670 is present, perform DTC P0670 pinpoint test sequence and fix problem before performing DTC P0683 pinpoint tests.
Yes GO to QB12 .
No END test if no further glow plug DTC detected.
QB12 CHECK FOR OPEN IN DIAGNOSTIC LINE
Key off.
Disconnect wiring at PCM and GPCM.
Connect breakout box; leave PCM harness disconnected.
Check resistance between PCM test pin 8 and GPCM test pin 9 at GREEN connector.
Is the resistance greater than 5 ohms? (Typical resistance value is less than 1 ohm.)
Yes REPAIR open in circuit 1277 (WH/LG). RESTORE system. CLEAR DTCs and RETEST.
No GO to QB13 .
QB13 CHECK FOR SHORT TO POWER IN DIAGNOSTIC LINE
Key on, engine off.
Measure voltage between PCM test pin 8 and ground.
Is the voltage greater than 5 V? (Typical voltage should be 0 V.)
Yes REPAIR short to power in circuit 1277 (WH/LG). RESTORE system. CLEAR DTCs and RETEST.
No GO to QB14 .
QB14 CHECK FOR SHORT TO GROUND IN DIAGNOSTIC LINE
Key off.
Connect PCM and GPCM.
Key on, engine off.
Measure voltage between PCM test pin 8 and ground.
Is the voltage a constant 0 V? (Diagnostic signal that is not shorted to ground is a pulse train; 0 V - 12 V.)
Yes REPAIR short to ground in circuit 1277 (WH/LG). RESTORE system. CLEAR DTCs and RETEST.
No GO to QB15 .
QB15 CHECK GPCM CIRCUIT
Install breakout box.
Make sure PCM and GPCM are connected.
Set up scan tool as follows:
choose digital measurement system
choose frequency meter
choose level and change to 6-V scale
connect only the scan tool signal to PCM breakout box pin 8 (GPCM Diagnostic input)
Disconnect EOT sensor connector, (this will maximize the glow plug ON time).
Key on, engine off.
Measure frequency at PCM breakout box pin 8, (take the measurement when frequency stabilizes, approximately five seconds).
Is the frequency in the range of 5 to 10 Hz? (Typical frequency for diagnostic signal with all plugs functioning is maximum 7 Hz. Typical frequency for the signal with one or more glow plugs failed is maximum 15 Hz.)
Yes INSTALL a new PCM. RESTORE system. CLEAR DTCs and RETEST.
No INSTALL a new GPCM. RESTORE system. CLEAR DTCs and RETEST.
Cross-Country
Joined: Jun 2007
Posts: 96
Likes: 0
From: Just outside Chicago, IL
Okay, okay, you win. Your scan tool is better than mine by FAR! lol 
I am pretty sure I got that code because I had the key on just before and both GPCM plugs out. It caught it on the next keying and all is good. I drove quite a bit after this with three key starts since and all is good. I just can't figure out what the deal is when the truck sits for a few days and has trouble starting. It turns over great, batteries are working fine.
I also wonder if there is a relationship between cold weather and something to do with the battery cables? What is between the positive cables and the starter. I assume these start on positive so there must be some sort of relay system triggered off the key, right? Could foul weather affect that too? Where is "that" located?
I am pretty sure I got that code because I had the key on just before and both GPCM plugs out. It caught it on the next keying and all is good. I drove quite a bit after this with three key starts since and all is good. I just can't figure out what the deal is when the truck sits for a few days and has trouble starting. It turns over great, batteries are working fine.
I also wonder if there is a relationship between cold weather and something to do with the battery cables? What is between the positive cables and the starter. I assume these start on positive so there must be some sort of relay system triggered off the key, right? Could foul weather affect that too? Where is "that" located?
New User
Joined: Jan 2011
Posts: 3
Likes: 0
New guy here. Having the same hard start problem here. Traced it to the GPCM. Couldn't find the part at Parts Guy Ed. Can someone give me the part number for the GPCM. (search the Parts Guy Ed site under Glow plug control module. GPCM to no avail) Thanxs david
Postmaster
Joined: Dec 2008
Posts: 3,340
Likes: 2
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