When I check one of my glow plugs it shows "OL"or open on my ohm meter. I pulled it out to check and put power to it. It fired up hot in about 10 seconds. But when I check it with the meter out of the block it still reads like it's bad. I just bought the meter today so I think I'm probably doing something wrong in my testing. I have had this happen several times where I thought for sure the plug was bad after several tests and trouble starting only to find it's good after I take it out.

Here are my testing procedures.
Glow Plug in the car
place one lead on the block and the other on the threaded end of the plug

Glow plug out of the car
one lead on the brass ring and one on the threads that go into the block. or one on the threads at the end and one on the brass ring. (Which is correct?)

I also checked my wires and they all seem good and I'm getting voltage across the fuse. How much should I see on that fuse?

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"Gerwalt" 1984 - 300TD

I've never had luck testing them in the car with the ohm meter. I pull them and give them 12v.

Another way to test them is to get an Amp Meter. Each glow plug at peak power pulls about 20amps. If one is bad it won't get anywhere near that,

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If it is heating up and stays hot in the center of the heating Element it is a bad Glow Plug.

My though is get a new one so you do not have to pull it out again later.

Another though is have you reamed or otherwise cleand the Carbon out of the Glow Plug Hole?

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84 300D, 82 Volvo 244Gl Diesel

Do you have the harness unplugged at the relay when you test them in the engine? It should be if the wires are attached at each plug.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

are you leaving out the rouge "t" in your wording now?

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

And if you don't have an amp meter handy Just do a spark test:
1) clip a wire to the battery or hot source (like the one on the glow plug relay) with one end stripped
2) pull plug that goes to the glow plugs on relay
3) Touch each hole with your hot wire and see if you get a spark (IE, you are energizing each glow plug one at a time and seeing if they suck juice) . If it sparks your glow plug is good.

You can tell a good spark from a bad one. And amp meter tells you the same thing. But all the experienced mechanics around here just do the spark test.

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What Would Rudolph Do?
1975 300D, 1975 240D, 1985 300SD, 1997 300D, 2005 E320 , 2006 Toyota Prius

Just a quick question (probably dumb too, but.....) Can a car battery charger/starter be used to bench test glow plugs? I would think so, but is there a preferred method?

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2012 Mercedes ML350 Bluetec 91K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 231K (mine)
1998 Volvo S70 T5 Turbo 196K (kids)
1994 Ford F150 4WD 249K (firewood hauler)
1983 Mercedes 300D 376K (diesel commuter)

I've used a car battery many times, I usually use one on the bench and not in the car though.

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- Details on how to test glow plugs.

Mercedes Diesel Glow Plug Repair

Spell checker does have its limitations.

does anybody have MS word with the grammar corrector? I remember reading on that and remember humorous "corrections" it would suggest...

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

heh, I like red

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

I did not unhook the harness while testing them in the car. I thought you could test the continuity if you put the probe on the threads if the wire was attached?

The plug lit up from the tip down. It took about 10 sec to get red hot at the tip and about 20 to get red hot about halfway down.


I do not have a reamer. I did stick a drill bit in cyl #1 one time but haven't popped any other plugs out until now.

Why would the plug read like it's bad while out of the car and still heat up?

I guess I can put a new one in there just for good measure. I'm not exactly sure how old it is but I'm guessing about 2yrs & 20k miles.

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"Gerwalt" 1984 - 300TD

Your conclusion is accurate.

All of the subsequent testing was a waste of time caused by a meter that doesn't function properly or an operator that doesn't understand the meter.

Nothing else is possible.

I am an awful speller and not very good at typing either.

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84 300D, 82 Volvo 244Gl Diesel