Just forget about resistance testing pin type glow plugs. It's a waste of time. They can show acceptable resistance readings and still not work properly.

I expect that the reason for this is that they corrode internally which prevents heat trasfer to the shell.

When your pin type glow plug car gets hard to start or won't start at all, verify that there is voltage at each plug and then replace ALL glow plugs.

I have almost twenty years experience with pin type glow plug MB diesels and I can promise you that this is the case.

Merry Christmas