Early 617s with tachs use a sensor at the crank pulley and a tach amp on the left wheel mound. Pressing firmly the connectors in the tach amp cap onto the receptacle usually gets the tach working again. Do a search on "cigarette butt" (seriously). The rpm signal is only for the tach. Later 617s (IIRC, CA 84 and all 85) take an engine rpm signal from the flywheel, route it through the EGR controller then to the tach, ACC, cruise, etc.

This is from memory so someone with better info please provide corrections.

Sixto
95 S420
87 300SDL