I am supposed to be able to check my rpm's on my '79 300 SD with my tach diagnosis socket. If I unscrew the top transmitter, the manual says to connect a digital tester to the socket. The book says to use "e.g. Bosch, MOT 001.03"- which I bet has the proper hook up . Is it possible to check without this special service tool? If so, does anyone know which pins to hook up to or how to do it?

TAch at diag connector
 

Patsy , sometimes it can be read with a good DVOM, ie. a Fluke 88. This is hit or miss. The bosch tool is a faster capture rate and always works.And has a dedicated connector, so this may be fuzzy. Use pins 1 and 3,I think. or follow the grey lead from the front of the motor up to the diag connector. it goes to the three pins at the outbord side. one is the sheild, other two are the pulse signal. Set meter to RPM, or hz and do the math. You cant hurt anything poking around with a meter in that plug.
Should be 750 rpm, and its governed inside the pump. Drop off throttle rods and you get 750. If the paint seal on the
idle screw on inboard side of pump is un broken, I bet the idle is 750.