Or maybe it should be why don't glow plugs. Thanks for reading this I know there's a lot of info and I've read nearly all of it but I don't quite get this part.

87 300SDL

My Glow Plug light decided not to come on anymore... of course, exactly when we have our first night below 50F. The car won't start... well not the way it should anyway.

I took a voltmeter to every GP and found 11.4+ volts going to each one. I have a six pin GPC. It stays on for about 20 seconds and then clicks off. However I still can't start it... just crank. I'm sure it would eventually start, but the results would be dismal until it warmed up.

I don't have any problems putting in new GPs or even a GPC if I need to, but why do the GPs have voltage, no indicator on the dash, and no start?

I feel like I'm missing something.

Thanks,
Bill

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'87 300SDL, intensely modified Greasecar setup, with nickle plated heat exchanger, 3 valve setup, 312,000 miles
'95 Suburban diesel, Greasecar basics, 16 plate heat exchanger, mercedes fuel lines, Racor filter
'85 Renault Alliance converted full electric vehicle

voltage will go to the plug as long as the fuse has not failed on the GPrelay... to test the plugs, you have to measure the resistance to ground. from each plug in the 6 pin connector to ground. the ones that measure less or more than 1 ohm are bad. more than two bad and the light on dash quits.
http://www.dieselgiant.com/glowplugrepair.htm

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I just had the same problem with my '86 SDL. More than one plug had died, but it was warm and starting was ..... well, getting harder. Then I thnk the last one died and it just would not start. Bought/installed 6 plugs and all is well. Well, I did also replace my 6 year old battery. BTW, total time to replace all 6 plugs: 75 minutes. Only thng I removed was one end of a piece of throttle linkage.

I just pulled #1 glow plug... It Ohms out at zero so... I'm just going to order a new set and test them out along the way. Bummer this couldn't happen a little earlier on when it was still warm out.

Thanks for the small lesson in electrical theory, not that I really understand it, but it accomplishes the mission.

Bill

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'87 300SDL, intensely modified Greasecar setup, with nickle plated heat exchanger, 3 valve setup, 312,000 miles
'95 Suburban diesel, Greasecar basics, 16 plate heat exchanger, mercedes fuel lines, Racor filter
'85 Renault Alliance converted full electric vehicle

There are several current sensors in the glow plug relay. I'm not sure about your engine, but on our old '83 with a 617, losing #1 or two of #2-#5 will cause the lamp to fail to illuminate. The remaining plugs do glow.

Yep, I think we have this issue on our 300D...it misses on 1 cylinder when started cold...so one of the #2-#5 plugs must be out....as the glow light still comes on. I'll test them today....

On my car the light stopped coming on and it was cranky (pun!) when starting....so I just replaced all 5.

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I have about the same thing happening to mine. The light on the dash does not come on, but the glow plugs still work and the car still starts right up. I'm guessing from previous posts, that it may have a certain plug out that causes the light to not work. I replaced the g.p.'s not long ago, but I understand that its not uncommon for them (or any electrical component for that matter) to randomly quit working no matter the age.

I went through 3 starters (the third one being the "charm") before I got my car to crank over correctly, and it turned out that the first two, although new (rebuilt), were just not working. And if anyone has changed the starter in a 617, you know that its not the easiest job...its not hard, but its tedious. I finally figured out if I turn the wheel all the way to the right (or left?) I had an easier time getting the starter out of the engine bay through the bottom.

Anyway, yea...sometimes things don't work although they're new...

Zero or infinite resistance? Zero would indicate that you have continuity; infinte would be an open circuit.

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