Glow Plug Current
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Junior User
Joined: Oct 2010
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When the truck has sat overnight, it doesnt start like it does after it has run and is warm. Kinda acts like a flooded gas engine. It just is not a crisp start.Thought I would check to see if maybe had a glow plug dead. Checked on the main line of the gpcm with a Fluke 337 amp clamp on DC amps.
Post Fiend
Joined: Oct 2009
Posts: 12,123
Likes: 35
From: N. Fort Worth, tx
ahhh ok. not really how to test a glow plug but I guess in theory it would work.
check the glowplugs using the ohms seletction on the meter.
go from the buss bar 4 pin connector (glowplug side) to the battery negitive readings should all be 3 ohms or less. any that are over that should be removed from the engine and then retested. also test the buss bar if you have any that fail.
I suspect you may have a stsiction issue with the injectors, a voltage issue with the batteries or a ficm concern vs. a glowplug concern.
check the glowplugs using the ohms seletction on the meter.
go from the buss bar 4 pin connector (glowplug side) to the battery negitive readings should all be 3 ohms or less. any that are over that should be removed from the engine and then retested. also test the buss bar if you have any that fail.
I suspect you may have a stsiction issue with the injectors, a voltage issue with the batteries or a ficm concern vs. a glowplug concern.
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Junior User
Joined: Oct 2010
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ahhh ok. not really how to test a glow plug but I guess in theory it would work.
check the glowplugs using the ohms seletction on the meter.
go from the buss bar 4 pin connector (glowplug side) to the battery negitive readings should all be 3 ohms or less. any that are over that should be removed from the engine and then retested. also test the buss bar if you have any that fail.
I suspect you may have a stsiction issue with the injectors, a voltage issue with the batteries or a ficm concern vs. a glowplug concern.
check the glowplugs using the ohms seletction on the meter.
go from the buss bar 4 pin connector (glowplug side) to the battery negitive readings should all be 3 ohms or less. any that are over that should be removed from the engine and then retested. also test the buss bar if you have any that fail.
I suspect you may have a stsiction issue with the injectors, a voltage issue with the batteries or a ficm concern vs. a glowplug concern.
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Hotshot
Joined: Jul 2005
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From: Middle Tn.
Just a little back up to physics. Lets say the glow plugs are made of tungsten. It has a given resistance when cold. As the tungsten heats up the molecules speed up and the resistance drops thus the current, amps, will start dropping. Refer to posts #1 thru #3. 
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Junior User
Joined: Oct 2010
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The way I see it, they are a resistance heater and if supplied with a constatant voltage then the amperage would remain realtively close. I was going by the coffee table book page 68. Not trying to start an argument. Just asking questions.
Hotshot
Joined: Jul 2005
Posts: 13,834
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From: Middle Tn.
The current will only drop a minut amount and then stay steady once the temp. stabilzes. You may or may not be able to read the drop depending on the grade of meter that you are using.
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Junior User
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Fluke 337 Digital Amp Clamp Meter capable of 1000 A DC. Very High Quality. The part I dont get is the glow plug has 2 resistance coils in series, one for heating and one formaintaining heat at its peak. I think tonight i will dig deeper, Its like one bank starts and shuts down quickly.
Lead Driver
Joined: Sep 2007
Posts: 8,180
Likes: 669
From: Scio, OR
I'm still a little confused about what you're measuring. Are you measuring one glow plug wire at a time? Or an entire bank at a time? Or all 8 at once.
If you're clamping over the wiring bundle going to one bank, for example, you're seeing the amperage of 4 glow plugs. So 85 to 45 amps is an average of 21.25 amps initially, down to 11.25. This doesn't sound like a problem to me. When the glow plug is cold it's going to draw more current than when it's hot (see post #9).
As cheezit said, though. The correct way to test the glow plug is to unplug them and measure their resistance individually when cold.
If you're clamping over the wiring bundle going to one bank, for example, you're seeing the amperage of 4 glow plugs. So 85 to 45 amps is an average of 21.25 amps initially, down to 11.25. This doesn't sound like a problem to me. When the glow plug is cold it's going to draw more current than when it's hot (see post #9).
As cheezit said, though. The correct way to test the glow plug is to unplug them and measure their resistance individually when cold.
Hotshot
Joined: Jul 2005
Posts: 13,834
Likes: 25
From: Middle Tn.
Fluke 337 Digital Amp Clamp Meter capable of 1000 A DC. Very High Quality. The part I dont get is the glow plug has 2 resistance coils in series, one for heating and one formaintaining heat at its peak. I think tonight i will dig deeper, Its like one bank starts and shuts down quickly.
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Junior User
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I am reading the whole bundle for all gp's. The bible (original version) on page 68 states that when gp's are on for approximately 40 seconds the amperage should be stable and you should see 10-12 amps per gp. So I clamped on to the whole bundle to see if it was steady and what I found was it was initially 85 amps which would represent all working and it started dropping down rater quickly to 45 amps. I would think they all should be on and steady until controlled off. I will check each bank individually and possibly each gp tonight. I deal with immersion heaters on a daily basis and have never seen one vary amperage, switch turns them on and off, amperage doesnt change. Dont know what the GPCM does to these.