Bought a 124 E300D 1994 recently. Ran fine except for faulty tach. Removed the OVP and re soldered the joints as per this you tube clip: https://www.youtube.com/watch?v=zPSSXSVMiJk

Started the car, Tacho working fine and it started easier from cold. Took it up the road. All seemed fine for a couple of miles then it started running rough when under load and throwing what I take to be diesel smoke out the back. Drove it slowly back home. Removed a working OVP off another car. Same problem. Tach still working but rough running under load. Worried if my repair on the first OVP was rubbish, and it's failed wrecking the electronics it's supposed to protect. Wife will be going ape in a moment as she is supposed to be driving it to work tomorrow