GP & GPR System
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Hotshot
Joined: Jan 2008
Posts: 13,144
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From: Rochdale MA
GP & GPR System
The glow plug system on our truck really is'nt as complicated as it seems. I have tried to break it down as much as possible. I hope that I covered everything, because it took me a long time to do this. With a ton of research!!
This system consists of a glow plug relay (GPR) for 49 states or glow plug control module (GPCM) on 1999 and newer California emissions vehicles, eight glow plugs, a "wait to start" light, related wiring and the powertrain control module (PCM) to operate the system.
The "wait to start" light is independent of the actual glow plug on time. The PCM will turn the "wait to start" light on just for three to five seconds to allow sufficient cylinder temperatures to be reached for ease in starting. However, even though the light is off, the PCM will keep the glow plugs energized for a much longer time to reduce smoke and to improve cold driveability. The amount of on time can be as long as 180 seconds, and is determined by the PCM based on inputs from the Air temp. sensor, the engine oil temperature sensor and available battery voltage.
Symptom: Hard start caused by the glow plug. To diagnose (the following applies to non CA models only.)
Make sure the battery is fully charged.
At the GPR, located on the right valve cover, verify B+ at the battery feed wire. This is a large diameter wire, usually black/yellow or black/orange.
If OK, use a DMM to check for vehicle power to the GPR on the smaller red wire with key on, engine off (KOEO).
If OK, have someone cycle the key off and on, while you check the other small wire (generally pink/orange) for less than 0.5 volts. This is the control wire the PCM grounds for 30 to 180 seconds to energize the GPR relay. If the circuit is being grounded, an audible click should be heard confirming activation of the relay.
If no sound is heard, touch a standard 12-volt test light to the output terminal of the relay containing the two large brown wires (some later vehicles and California models will likely have one brown and one yellow) and check for voltage. If there is no voltage present, replace the GPC relay.
If the GPR checks OK, turn off the key and allow the glow plugs to cool.
There are different ways to rest the glowplugs. This is just one of them.
Note: This test is done using an amprobe capable of reading DC Amps
Place the ampprobe around the two output wires of the GPC.
While turning the key on, watch for the initial draw.
Depending on the manufacturer, glow plugs will draw 17 to 26 amps each with an average draw of 20 amps (20 x 8 = 160).
Move the clamp to each glow plug supply wire at the valve cover connectors, cycle the key on and check the amperage each. Any glow plug that is substantially ower than the others may need replacing.
Allow time to cool between testing each cylinder. On the 1999 and newer SuperDuty, the supply wires will be in terminals 1-2-10 and 11 of the two 11-cavity connectors. Note: The wires on the driver's side of the engine may be yellow.
California vehicles operate identically to the 49-state systems. Instead of a 4-pole relay controlled by the PCM, the glow plug system contains an actual module. This module is in the same location but has more wires. The output terminal of the 49-state relay has one large wire for each bank that branches off to individual cylinders. The GPCM has one output wire for each cylinder. As with its 49-state cousin, it still contains the B+ wire, the vehicle power wire and the pink/orange to energize it. It also contains a communication line to the PCM. This module "monitors" individual glow plug draw and reports any errors to the PCM. It can flag a code P0670 if there is a control line fault, a P0683 if the diagnostic circuit is faulty and any combination of P0671 through P0678 for a glow plug fault in cylinders 1-8.
Since this module monitors actual current to each cylinder, the malfunction indicator light will illuminate and a code will be generated for any glow plug that draws less than 20 amps, even though no starting issues are present
The "wait to start" light is independent of the actual glow plug on time. The PCM will turn the "wait to start" light on just for three to five seconds to allow sufficient cylinder temperatures to be reached for ease in starting. However, even though the light is off, the PCM will keep the glow plugs energized for a much longer time to reduce smoke and to improve cold driveability. The amount of on time can be as long as 180 seconds, and is determined by the PCM based on inputs from the Air temp. sensor, the engine oil temperature sensor and available battery voltage.
Symptom: Hard start caused by the glow plug. To diagnose (the following applies to non CA models only.)
Make sure the battery is fully charged.
At the GPR, located on the right valve cover, verify B+ at the battery feed wire. This is a large diameter wire, usually black/yellow or black/orange.
If OK, use a DMM to check for vehicle power to the GPR on the smaller red wire with key on, engine off (KOEO).
If OK, have someone cycle the key off and on, while you check the other small wire (generally pink/orange) for less than 0.5 volts. This is the control wire the PCM grounds for 30 to 180 seconds to energize the GPR relay. If the circuit is being grounded, an audible click should be heard confirming activation of the relay.
If no sound is heard, touch a standard 12-volt test light to the output terminal of the relay containing the two large brown wires (some later vehicles and California models will likely have one brown and one yellow) and check for voltage. If there is no voltage present, replace the GPC relay.
If the GPR checks OK, turn off the key and allow the glow plugs to cool.
There are different ways to rest the glowplugs. This is just one of them.
Note: This test is done using an amprobe capable of reading DC Amps
Place the ampprobe around the two output wires of the GPC.
While turning the key on, watch for the initial draw.
Depending on the manufacturer, glow plugs will draw 17 to 26 amps each with an average draw of 20 amps (20 x 8 = 160).
Move the clamp to each glow plug supply wire at the valve cover connectors, cycle the key on and check the amperage each. Any glow plug that is substantially ower than the others may need replacing.
Allow time to cool between testing each cylinder. On the 1999 and newer SuperDuty, the supply wires will be in terminals 1-2-10 and 11 of the two 11-cavity connectors. Note: The wires on the driver's side of the engine may be yellow.
California vehicles operate identically to the 49-state systems. Instead of a 4-pole relay controlled by the PCM, the glow plug system contains an actual module. This module is in the same location but has more wires. The output terminal of the 49-state relay has one large wire for each bank that branches off to individual cylinders. The GPCM has one output wire for each cylinder. As with its 49-state cousin, it still contains the B+ wire, the vehicle power wire and the pink/orange to energize it. It also contains a communication line to the PCM. This module "monitors" individual glow plug draw and reports any errors to the PCM. It can flag a code P0670 if there is a control line fault, a P0683 if the diagnostic circuit is faulty and any combination of P0671 through P0678 for a glow plug fault in cylinders 1-8.
Since this module monitors actual current to each cylinder, the malfunction indicator light will illuminate and a code will be generated for any glow plug that draws less than 20 amps, even though no starting issues are present
This next write up is the compliments of PonyboyChris:
How to check Glow Plug System
To check the Glow Plug Relay (GPR)
· Be sure the engine is cold, so that the PCM will tell the GPR to turn on. If the engine is hot, you won’t have as much time to check.
· Locate the GPR – Its behind the fuel filter on top of the engine, a little bit toward the passenger side of the valley. There may be two relays there. If so, the rear one is the GPR. It will have two fairly large wires (yellow and brown) connected to one of the large posts.
· With your multitmeter set to DC volts, and 15 V range (if not autoranging), clip the positive (red) lead to the output terminal (with yellow and brown wires connected), and the negative (black) lead to a good ground point (like the battery ground terminal or someplace metal directly on the engine block.)
· Turn the key to ON (do not start)
· If your GPR is good, it should click, and you’ll see 11 volts or so on your meter, then, depending on temperature, it will click off up to 2 minutes later. You should do this a couple of times to make sure it consistently makes the connection.
· If you don’t get voltage with this test, confirm by retesting as follows.
· Remove the two small wires from the smaller two of the four GPR terminals.
· With jumper wires, apply voltage from the battery across the two small terminals. If your voltmeter now reads voltage on the output terminal, your GPR is OK, and your problem is in the PCM circuit that tells the GPR to activate.
To check Glow Plugs.
· Remove the electrical connector on the inboard side of valve cover at the gasket. Press down on the top of the connector latch and pry gently with a screwdriver.
· There will be 9 pins on the valve cover gasket where you removed the connector. The two pins furthest forward and the two pins furthest back are for your glow plugs.
· With your multimeter set to resistance (ohms) and low range (single digits) if not autoranging, clip the negative (black) lead to a good ground point.
· Probe each of the 4 outer pins individually with the positive (red) lead, noting the resistance. Good glow plugs will have a resistance between 0.6 and 2 ohms. If you get infinite resistance on any glow plug, that one is either bad or the connector under the valve cover has come loose.
To check the Glow Plug Relay (GPR)
· Be sure the engine is cold, so that the PCM will tell the GPR to turn on. If the engine is hot, you won’t have as much time to check.
· Locate the GPR – Its behind the fuel filter on top of the engine, a little bit toward the passenger side of the valley. There may be two relays there. If so, the rear one is the GPR. It will have two fairly large wires (yellow and brown) connected to one of the large posts.
· With your multitmeter set to DC volts, and 15 V range (if not autoranging), clip the positive (red) lead to the output terminal (with yellow and brown wires connected), and the negative (black) lead to a good ground point (like the battery ground terminal or someplace metal directly on the engine block.)
· Turn the key to ON (do not start)
· If your GPR is good, it should click, and you’ll see 11 volts or so on your meter, then, depending on temperature, it will click off up to 2 minutes later. You should do this a couple of times to make sure it consistently makes the connection.
· If you don’t get voltage with this test, confirm by retesting as follows.
· Remove the two small wires from the smaller two of the four GPR terminals.
· With jumper wires, apply voltage from the battery across the two small terminals. If your voltmeter now reads voltage on the output terminal, your GPR is OK, and your problem is in the PCM circuit that tells the GPR to activate.
To check Glow Plugs.
· Remove the electrical connector on the inboard side of valve cover at the gasket. Press down on the top of the connector latch and pry gently with a screwdriver.
· There will be 9 pins on the valve cover gasket where you removed the connector. The two pins furthest forward and the two pins furthest back are for your glow plugs.
· With your multimeter set to resistance (ohms) and low range (single digits) if not autoranging, clip the negative (black) lead to a good ground point.
· Probe each of the 4 outer pins individually with the positive (red) lead, noting the resistance. Good glow plugs will have a resistance between 0.6 and 2 ohms. If you get infinite resistance on any glow plug, that one is either bad or the connector under the valve cover has come loose.
With the amount of amperage that the GP system draws you can see why good batteries and battery maintanence are vital to the starting of your truck
In my other thread: https://www.ford-trucks.com/forums/787866-battery-q-and-a.html I didn't include any definations of what the battery abbreviations meant. So here they are:
<TABLE cellSpacing=0 cellPadding=0 width=576 border=0><COLGROUP><COL width=576><TBODY><TR><TD vAlign=bottom width=576>Cold Cranking Amps
Cold Cranking Amps is a rating used in the battery industry to define a battery's ability to start an engine in cold temperatures. The rating is the number of amps a new, fully charged battery can deliver at 0° Farenheit for 30 seconds, while maintaining a voltage of at least 7.2 volts, for a 12 volt battery. The higher the CCA rating, the greater the starting power of the battery.
</TD></TR></TBODY></TABLE>
<TABLE cellSpacing=0 cellPadding=0 width=576 border=0><COLGROUP><COL width=576><TBODY><TR><TD vAlign=bottom width=576>Reserve Capacity (RC)
Reserve Capacity, (RC) is a battery industry rating, defining a battery's ability to power a vehicle with an inoperative alternator or fan belt. The rating is the number of minutes a battery at 80 degrees F can be discharged at 25 amps and maintain a voltage of 10.5 volts for a 12 volt battery. The higher the reserve rating, the longer your vehicle can operate should your alternator or fan belt fail.
</TD></TR></TBODY></TABLE>
Hot Cranking Amps (HCA)
<TABLE cellSpacing=0 cellPadding=0 width=576 border=0><COLGROUP><COL width=576><TBODY><TR><TD vAlign=top width=576>HCA is hot cranking amperes. It is the same thing as the CA or the CCA, except that the temperature at which the test is conducted is 80°F
Here are a few pics compliments of camodown of a dissambled glow plug relay. Note the very pitted and arched out contact.
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</TD></TR><TR><TD vAlign=top width=576>
</TD></TR></TBODY></TABLE>
<TABLE cellSpacing=0 cellPadding=0 width=576 border=0><COLGROUP><COL width=576><TBODY><TR><TD vAlign=top width=576>
</TD></TR><TR><TD vAlign=top width=576></TD></TR><TR><TD vAlign=top width=576></TD></TR></TBODY></TABLE>
<TABLE cellSpacing=0 cellPadding=0 width=576 border=0><COLGROUP><COL width=576><TBODY><TR><TD vAlign=top width=576>
</TD></TR><TR><TD vAlign=top width=576></TD></TR><TR><TD vAlign=top width=576></TD></TR></TBODY></TABLE>
Post Fiend
Joined: Jun 2004
Posts: 9,293
Likes: 10
From: Parker, CO
Very nice writeup.
On the glow plug systems, the baro sensor also has input and will help determine how long the glow plug light stays on, as well as how long the glow plugs stay energized. For example, my house is at 6000 ft elevation, and my wait to start light typically stays illuminated for 4 seconds, and a little longer when temps are below freezing. If I drive up to 10,000 ft, and if temperatures are the same, the glow plug light will stay illuminated sometimes for twice as long. So the baro sensor does have quite a bit of influence.
Here is a quote from the Ford manual:
I think that's the only sensor you missed. 
Everything else looks good. 
On the glow plug systems, the baro sensor also has input and will help determine how long the glow plug light stays on, as well as how long the glow plugs stay energized. For example, my house is at 6000 ft elevation, and my wait to start light typically stays illuminated for 4 seconds, and a little longer when temps are below freezing. If I drive up to 10,000 ft, and if temperatures are the same, the glow plug light will stay illuminated sometimes for twice as long. So the baro sensor does have quite a bit of influence.
Here is a quote from the Ford manual:
Barometric Pressure Sensor
The Barometric Pressure (BARO) sensor is a variable capacitor sensor that processes a signal indicating atmospheric pressure. This allows the PCM to compensate for altitude. The PCM uses this information to calculate injection timing and glow plug control. With the 1999-1/2 model, the BARO sensor is no longer a stand alone component. It is now contained in the PCM.
The Barometric Pressure (BARO) sensor is a variable capacitor sensor that processes a signal indicating atmospheric pressure. This allows the PCM to compensate for altitude. The PCM uses this information to calculate injection timing and glow plug control. With the 1999-1/2 model, the BARO sensor is no longer a stand alone component. It is now contained in the PCM.
Everything else looks good.
Hotshot
Joined: Feb 2007
Posts: 10,499
Likes: 10
From: Sterling Heights,Mi
Great Post Jim and Poneyboy, I am also glad you suggested using the meter. When mine had gone bad, everything checked out, until I put the DVM on it, and found the voltage drop, the contact on relay was shot. It made contact, but not a great contact..higher amp, lower volt, blah blah blah...Great Write up.
Thread Starter
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Hotshot
Joined: Jan 2008
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From: Rochdale MA
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Butt-Head
Joined: Apr 2004
Posts: 9,454
Likes: 2,938
From: Reed Point, MT
It's a very common problem. You wouldn't believe how many GPRs I've taken apart for customers to show them why they weren't working. I've had a couple with contacts similar to the one in your pics.....and a couple a lot worse. Usually, the ones that were REALLY burned up were changed at the end of the summer when the truck wouldn't start in the morning once it got cooler at night. They'd ignore the puffs of white smoke all summer (afterall, the truck still started) and that GPR would burn, and burn, and burn.
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Cargo Master
Joined: Mar 2008
Posts: 2,975
Likes: 19
From: Archdale,N.C.
If a GPR(only about a year old) was working(I can tell by the lights getting brighter after about 90 secs. give or take) could it be weak and not working enough?I know this might be a retarted question so yall can rag me if ya need to...lol
Post Fiend
Joined: Feb 2007
Posts: 14,541
Likes: 2
From: Dallas-Ft. Worth
It could, Phil. As the contact becomes more and more pitted, it becomes more & more of a resistor, dropping the voltage that actually makes it to the GPs. But a year old GPR *should* be okay. If it's not, check out the monster Stancor guzzle put in:
Welcome to guzzle's Stancor GPR replacement Mod Web Page
Welcome to guzzle's Stancor GPR replacement Mod Web Page
Thread Starter
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Hotshot
Joined: Jan 2008
Posts: 13,144
Likes: 11
From: Rochdale MA
Thank Youvery much Bob. I guess that you could say that I "got printed"!!
Elder User
Joined: Feb 2005
Posts: 896
Likes: 1
From: Chattahoochee Hills, Ga.
Yep, you are in print ! I started printing off some of the great stuff that folks like you put on our site. After I knocked my laptop off the truck and got it good and greasy I figured that'd be a better approach. Even with a nice, big, dry, heated shop that laptop just kept ending up balanced on the radiator or battery. Notebook is better !!!
