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.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech