Resistance on glow plugs aren't consistent. Possible bad relay. Which to replace?

baby g

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Hi everyone,

You can see below the results of the resistance check I did on my glow plugs on my 93 7.3 IDI NA. My relay clicks about 10 times after the wait to start light turns off. To test the glow plugs, I removed each connector and placed the positive lead of my ohm meter on the top of the glow plug and had the negative lead placed on the negative terminal of my battery (to ensure I had a solid ground).
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I also tested my relay, and found that with the key on, I wasn't getting voltage to the glow plug harness, but was getting voltage to the relay as well as to the controller.
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So, is this a glow plug issue, a relay issue, or both?

Thanks for the help!
 

DaveBen

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Zero ohms is a no resistance. This means that glow plug is shorted from + volt to - volt (ground). This will cause the glow plug controller to not work. You need to recheck the glow plugs and replace the glow plugs that read 0, 5, and 6. By the way; Welcome to Oil Burners!
 

baby g

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Thanks for the reply! So I suppose it makes the most sense to replace all of the glow plugs as a set?
 

IDIBRONCO

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Ultimately, that would be up to you, but I don't think that I would. I would just replace the bad/out of spec ones. Also, the controller is supposed to click after the light shuts off so that's nothing to worry about.
 

Nero

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You also want to measure the ground as close as possible to the glow plug, which I'd touch the side of it to get a more accurate reading. Dirty threads can cause resistance.
 

captain720

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I would replace only the bad ones if I have done them all since buying. If you have never done them than I would say it’s a good idea but that’s my thoughts.
 

baby g

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Ok, I'll test again tomorrow and see if grounding closer to the plug changes anything.

I recently bought the truck and don't know when the last time the glow plugs were changed. That's my reasoning for changing them all at once.
 

Nero

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I'm more lazy, I use the go no go method. Just use a test light on the glow plugs. If the light lights up? They're good.
 

IDIBRONCO

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True but that won't tell you if they are out of spec. It only tells you if they're burned out. I only use the test light method for a quick check.
 

baby g

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Alright, I replaced all 8 glow plugs with Motorcraft ZD29 glow plugs. Cylinder 4 is a real pain in the butt, but I got it out with a extended 3/8th socket on a 1/4" u joint and a 1/4" extension.

Thanks for the help y'all! No more clicks, she starts right up.
 
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