Are you saying the starter won't even turn? Or the starter turns the engine but the engine does not start?

If the starter does not even turn the engine over, and the battery is fully charged, I'd start by cleaning and tightening the battery terminal connections, the ground connection and checking voltage from the positive terminal strip to frame ground, static and while attempting to turn the engine over. If you go to start it, and you get no voltage drop when the key is turned to the crank position, I would suspect the nuetral safety switch. Did you try it in nuetral?

If the engine turns but does not fire, search the diesel forum for glow plug troubleshooting - there are pages and pages of posts on the subject. You'll find info on how to test the gp light circuit, the gp fuse, individual gps, wiring, etc. It is all there.

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Thank You!
Fred
2009 ML350
2004 SL600
2004 SL500
1996 SL600
2002 SLK32
2005 CLK320 cabrio
2003 ML350
1997 C280 Sport