I have the same problem with my 300D right now, it will not start. I have narrowed it down to the glow plug system. I have tested the resistance across the plugs themselves and they test ok, so I am guessing the plugs are good. The glow plug light on the dash does not come on but did last week. I have noticed that when I turn the key, the light does not come on, but after about 30-45 seconds I hear a click comming from the glow plug relay(the black box on the driver side fender well). I have also tested to see if the plugs are getting voltage which they are. I just ordered the OEM glow plug relay from the MB dealer for $130, and the fuse as well for $.95(figured I'd get it just in case). I will be installing it tommorow morning and will post on how it goes.

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1980 300D European
"The Red Fox"
200,000 miles

2003 Chevrolet Silverado

609 Certification