I think your gas tach must have been bad. If you try to plug a gas tach into a diesel 124 the needle will be buried above the red line at idle. BTDT

This is because the gas and diesel engines get their signal from completely different means.

Gas uses the ignition coil trigger from the distributor, so 3 pulses per revolution (assuming a 6 cyl).

Diesel uses the ring gear via a Hall sensor with 144 pulses per rev.

The ring gear signal goes to the EDS module which does rely on the OVP working. The car will run without the OVP, but you'll have poor idle control, and also the ABS won't work, as the ABS computer also uses this protected voltage source.

Where did you mention this was a gas conversion? How did a gas tach get in your car, and how did it work before the manual trans was there with a diesel motor?

Thanks for following up. Great you got it going. The diesel tach operates on a hall sensor. The gas probably works off the ignition coil. I bet the gas wants to see a higher voltage than the diesel tach.

anybody know of a source for a wiring diagram for my 1992 200
td? The AC quit working and we cannot find the wiring piece by piece.