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