From the owner's manual on my 300SD, page 37, "at ambient temp. below 32F and with the engine cold, completely depress accelerator while starting. Actuate starter until the engine fires regularly and engine speed rises. Then ease off the accelerator slowly. Cranking times of up to one minute will not do any harm to the starter. At ambient temps. of less than -4F, depress the accelerator three times prior to cranking."
Just as a side note. I can't believe it's ok to crank the starter for up to 1 minute. Every gas car I have owned has always said not to crank for more than 15 seconds at a time for fear of over heating the starter. These have to be some seriously strong starters to handle cranking a diesel for 1 minute. Over-engineering, you got to love it.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member