Need some help... This morning I went to start my 81 300td its about 40 F and the glow plug light did not come on. I proceeded to wait tell the relay clicked (about 30 seconds) then I glowed them again... light comes on. Tried to start and it was rough cranked for about 30 20 seconds... didn’t start. Waited 5 to 10 minutes and repeated... it took I think 3 times... But every time after starting the car... and reglow the plugs... the light would not come on until the after the relay clicked and a glowed them for the second time...

Do I have a faulty glow plug I’ve replaced 2 recently...The others all tested fine

Point me in the right direction

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81 300td turbo 212,000

Yes, you most likely have a faulty glow plug. How did you determine the others test fine?

http://www.dieselgiant.com/glowplugrepair.htm

The above link is a good starting point to learn how to properly test glow plugs. I don't do exactly as stated. I find it hard to reliably test resistance through the plug at the relay. Instead I put one probe on the back of the GP and the other as far up towards the threads as possible. For me, this gives me the best readings without actually pulling the GPs. With that said, pulling them is actually the best test because you can apply 12V and see how well they glow. Sometimes they have a lot of carbon on the tip which hinders the performance of the GP.

The way you are describing it you could also have a faulty GP relay or bad/loose wiring.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD

I would check them again. It only takes 5 minutes or less. You may have had another one fail. What glow plugs are you using?

I remember replacing 1 and then 6 months later 2 more went out. Everything is fine now.

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AJ

1985 300D (SOLD)

I used the method described on diesel giant... when one didn’t ohm correctly I pulled it and connected it to the battery didn’t glow... I pulled another one just for fun and it glowed red...

let me describe it too you maybe better...

turn key to glow... no light... 30 seconds... relay clicks…. turn key off position

turn key to glow ... light comes on ... 30 seconds... relay click turn key to start ... cranks for awhile... turn key off

turn key to glow ... no light... repeat 2 more times like above....

why light turn on for the second glowing and not the 1st...

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81 300td turbo 212,000

Looks like the same issue with my '86 SDL.

Turn key to glow...no light, 30 sec glow light comes on for 3 seconds, then off, key to off position.

Turn to glow second time, glow light comes on and stays till relay clicks, turn key to start, car starts...

This issue started after I replaced 6 glow plugs. Didn't do this before I replaced the glow plugs because car was telling me some glow plugs are about to go bad.

Any idea??

Thanks

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1987 300SDL 314K mi, counting; #22 Head
1999 E300Turbo 405K original miles
1997 E320 180K mi (Son's)
2002 ML320 (Son's)
1998 S320 140K mi (Son's) sold
1985 300SD 311K mi
1984 300D 324K mi (killed by a hit-n-run turd)
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I am not sure but here is my guess-
The resistance on one or more GPs are getting to the point where they are starting to get to high. The GP light will come on some times because they are so close and sometimes not.

If it were me I would start by replacing the other 3 glow plugs and see if the problem goes away.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD

I had similar issues recently as described above.
I changed out all 5 glow plugs at once for the effort of dealing with changing out at least two that I knew were bad beforehand and now everything is fine. Turns out 4 were bad, and the 5th on its way out.

Mine were Bosch and now are Beru from here.

I also got a new battery and the car is now garaged, but driven at least weekly.

It's just like new.

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85' 300D No inspection, No registration fees, Cheap insurance
"If my calculations are correct, when this baby hits 88 miles per hour, you're going to see some serious %$&^."