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Testing Glow Plugs

The Baron

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Hey guys I'm sorry for the stupid question, but I've been looking through the TM's and I cant find the section on testing glow plugs. Does anyone know which one it's in?
 

Warthog

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TM 9-2320-289-20, Table 2-1, page 2-13.

or paragraph 19. ENGINE CRANKS BUT WILL NOT START; ENGINE COLD; "WAIT" LIGHT OPERATES, page 2-64


I opened the PDF and just did a find for "glow plug"

Or you can always look at the other threads listed at the bottom of the page.
 
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Skinny

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Are you looking to see if the whole system work or just checking the actual glow plug?

If just the GP, don't bother. For the cost you may as well just replace as maintenance.
 

chvss65

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test with a ohm meter, om your electric tester,,, ground negative,, touch post lead on clip end of GP,,, should be zero resistance,,, if they show resistance,, GP is bad
 

Iceman3005

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Also check voltage with a test meter, should be around 12-13 volts if more than that then one or more of them are starting to fail. If it reads 24 volts most likely all have failed, assuming your still going through resistor on firewall.
 

doghead

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RTFM. 1-3 ohms.

should be zero resistance,,, if they show resistance,, GP is bad
I sure wish people would post when they actually knew what they were talking about.
 

antennaclimber

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I bought a new Snap-On diesel compression gauge recently and finally had time to test the compression on Blazer B. This gauge requires the glow plug to be removed and a adapter to be installed to get the compression reading. Easy enough. Tests went well.

While I had the glow plugs out I tested each to see what resistance they were. All were installed new 2 years ago.

Well, 3 of them were between 1 and 4 mega ohms, most were .8 ohms and 1 was completely open. I wasn't going to check the rear passenger side one, glad I did, it was the one that was completely open. Simple testing showed that half of them failed the resistance test were possibly non functional.

I tested the 4 in question by applying power to them (12v) and none produced any heat. All of them are AC60G's and none showed any sign of swelling.

Off to the local NAPA store and picked up 8 new AC60G's. Each of them measured 1.1 ohms out of the box. Now the truck starts much faster.

Moral of the story, never assume that all of your GP's are working without resistance testing each and every one of them. It's simple and will assist you with troubleshooting a hard start issue.

Edit; The GP's are fed with 12 volts using the standard resistor bypass method.

Karl
 
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antennaclimber

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Doghead and I were curious about testing plow plugs with the test light procedure. I have never tried it and we both were curious as to weather or not this is a viable way to test them, so I offered to do a little experiment with the glow plugs that I just removed from my M1009.

I found most of the discarded GP's and tested them using a test light, voltmeter and ohm meter this morning.

This is how I tested them:
1st test.
Resistance test was made with the body of GP to negative and connector blade to positive.
Some were between .7 to 1 ohm. Others were above 2 mega ohms, one was open.
Reversing polarity on the resistance test did not affect resistance measurement. So it appears that they are not polarity sensitive.
This was done with a Fluke DMM.

2nd test.
GP blade connector was sitting on top of the positive terminal of a good fully charged automotive battery.
Wire lead of the test light was attached to the negative terminal.
The point of the test light was touched to the body of the GP.

Results.
The GP's that had a very high resistance (2-8 Meg), the test light did not light up.
The GP's that had .8 to 1.1 ohms of resistance, the test light did light up.
Switching polarity did not affect the the test results.

I then used a volt meter (in place of the test light) using the above test method.
There was no noticeable voltage drop with the low resistance GP's. Measured voltage was the same as battery voltage.
The high resistance GP's had little to no voltage reading on the voltmeter. The open GP had no voltage reading as expected.
Testing was done with the same Fluke DMM.

So it appears that using the "Test Light Method" is a viable way to test the glow plugs for a quick "Go/No Go" type of test.

I would recommend the use of an ohm meter to determine the GP resistance to see if it needs to be replaced. It would be hard to determine if a GP had 1,1.5, 3, 6 ohms or some other low resistance with a test light, an ohm meter would be a much better way to see if one is going high in resistance.
But for the high resistance or open GP's the test light method would be a quick way to test them to see if they are inoperative.

All of the above testing was done with the glow plugs removed from the engine.
There were no animals harmed and drivers were on a closed course.
Only a single cup of Dunkin coffee was used during testing.
Karl
 
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Hasdrubal

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I've only ever used the test light method for 10 years now. Take all the spade connectors off, test light hooked to positive, very fast method to check GP's. Longest part is putting spade connectors back on. Can always hear when one has failed by the engine sound at start up, look in the mirrors for the side that is making smoke. That's the bank that the failed GP is on.
 

The Baron

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So I tested my glow plugs and they all read .9 or .8 ohms. I figured that that was outside of the specs so I started replacing them. I got half of them in when I realized that they all read the exact same ohms as the other ones. Am I doing something wrong or is there a problem with the truck?
 

doghead

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What scale is your ohm meter on?

What Glow Plugs do you have?

You did unplug them from the wiring, before testing, right?
 

doghead

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Is 200, the lowest rage you have?

What do you get when you simply touch your leads together?
 

doghead

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You are touching one lead to the terminal and the other lead to the hex portion of the GP, right?
 

The Baron

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That is my lowest range. I get .1 when I touch the leads together. Originally I was grounding one of the leads and touching the terminal with the other while they were still in the truck. I just tried touching one lead to the terminal and one to the hex portion while the plug was out of the truck and am getting .9 ohms on the new ones and old ones. Do you think my voltmeter could be crap?
 
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