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?