Car did this in the spring, fixed itself. Doesn't make a sound when you turn the key. I'm thinking the switch is bad that keeps car from starting when shifter is not in park or nuetral. What would that switch be called? Can't look it up with out a name.
Thanks.

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1984 MB 300SD Turbo

is your shifter sloppy? If so you might need to replace the bushings for it. Otherwise its called a neutral safety switch

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'85 300D - federal spec, built in late 84. 85 300D Complete AC System Rebuild

Agree if sloppy. I replaced my bushings in about five minutes and it has performed perfectly since. Thought it might be the neutral safety switch but was just the shifter bushings. Before doing that I never knew when I turned on the key if anything would happen or not; but this resolved the problem.

Under the battery tray is a wire junction block. You will have to find a thread with a picture of it and where you can use a jumper wire to crank the Engine and start.
I am at the Library and have no pic of the junction block to post.

If you can start from the Wire Junction Block it is as previously stated likely to be the Neutral Safety Switch and or the Shifter Bushings.

Try starting in Neutral.

Added: See post #3
http://www.peachparts.com/shopforum/diesel-discussion/340967-300sd-starter-wont-crank-while-hot.html

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84 300D, 82 Volvo 244Gl Diesel

Ditto. Unlikely the NSS is bad, and more likely the plastic shift linkage bushings fell out (youtube videos & posts incl. mine). They usually make a rattle as you drive.

In "N" position, you can wiggle the stick back & forth to hopefully get the tranny into neutral. The "P" position is up against a stop so may not get it into park. You should always set the parking brake, especially now that it appears you can't access the park position.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans