Don't know about the 84 SD but on my 85 300D, the "tach amp socket" is actually a diagnostic port/ socket. The difference from a real tach amp socket is that the diagnostic socket does not have an output pin that goes to the tach in the cluster. The 85's tach uses a sensor on the flywheel (whereas the diagnostic port's sensor is on the harmonic balancer). The 85 300D's tach circuitry is in the EGR computer (as opposed to be in a round black module known as the "tach amp" in 83 300D's). In the 85 300D, the OVP controls the power to the EGR computer. It is an unnecessarily complicated design IMO..

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked