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Old 09-20-2003, 09:14 PM
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Join Date: Sep 2003
Location: Bastrop Tx -Central Texas
Posts: 6
Question Glo plugs shorting out

Hello and thank you in advance for any help here.

The glo plug light was not coming on in my 77 300D. My dad and I checked out the glo plugs to find that one was shorted out (not working -- Not sure which is the correct terminoligy) we replaced the plug (actually replaced all 5) and the light still would not come on. By accident we found that the fuse for the glo plug - not the big fuse but the one in the fuse box labeled caurberator heater had lots of corrosion. After we cleaned the fuse, everthing worked for about a week.

This morning - same thing - glo light not coming on. After checking glo plugs we found two plugs shorted out again.

Thing is, I had a mechanic working on it - he said when he was working on it, the element that attaches to the plugs was staying hot when it should not. He was stumped. He did not even try to fix it after I just spent $2000.00 for some work he did for me. Guess what? not going back there again.

My question is-- If the timer in the relay was to get stuck in the on position some how, would that be why the glo plugs are shorting out. Or is there something else that can cause this.

Sorry for such a long message but I wanted to give you as much info as possible.

Thanks!! Cendy
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Old 09-20-2003, 09:30 PM
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Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150
Shorts would indicate that there is too much carbon in the pre-chambers and that the glow plugs are grounding there and burning out.

Definitely, if the relay is staying on too long the plugs will burn, these are the series loop type.

Find a better mechanic, these aren't that hard to figure out. However, I'd make sure he was 50 ish or older, a young guy may not have ever seen this type of starting system!

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Old 09-20-2003, 09:44 PM
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Join Date: Sep 2003
Location: Bastrop Tx -Central Texas
Posts: 6
Peter, Thanks for your reply

"Shorts would indicate that there is too much carbon in the pre-chambers and that the glow plugs are grounding there and burning out. "

Is this an easy fix - something we can do or is is something a specialist would need to do and would it be very costly?

Thanks again, Cendy

Sorry about this question. After posting I found some info in the search on this. Should have looked there first.
Thanks anyway.

Last edited by Lady Benz; 09-20-2003 at 11:48 PM.
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Old 09-20-2003, 10:40 PM
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Join Date: Dec 2000
Location: Republique de Banana
Posts: 3,484
Peter is describing two different possible problems.

One possibility is carbon buildup in the prechambers. There is an MB special tool (glow plug reamer) for removing the carbon buildup. I don't remember exactly how much the tool costs, but seems like it was a hundred bucks or so.

The second possibility is that the glow plug relay is staying on after starting for some reason. It sort of sounds like this is what the guy gave up on. Like Peter said, this is probably not difficult to fix. Anybody with basic electronics skills and an understanding of how this circuit works should be able to troubleshoot it. He should be able to find a wiring diagram from Alldata. Your best bet is to seek out a mechanic who is familiar with the pre-1980 glow plug system. If I was going to just throw parts at it, I would probably start with the glow plug relay itself. Should be inexpensive at your local MB wrecking yard. Just be sure you get one from about 1977 to 1979. Later models would be different.

It's also possible that some previous owner has butchered the wiring in some way. If that is the case, it would be more understandable why the mechanic is having trouble figuring it out. I think this is unlikely, but a possibility worth mentioning.

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