Ahh, but Pat is right about the 85. I can't verify the point but 85 617 motors began using the engine speed sensor for the tach and EGR control and for some reason A/C clutch control even though the belt wasn't the problem and the compressor was still a GM R4 with no speed sensor.
The earlier 617 TDs used a sensor on the front balancer. The 85 used a sensor on the flywheel. I have bypassed these control relays when necessary. Does the tach work?
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician