The tach amp gets it's signal from a sensor mounted near the front of the crankshaft. All you need to do on ANY engine you choose to install in your 300d is to install the sensor on the new engine and also install a single trigger for it on the crankshaft pulley or harmonic balancer. I did this on the 4.3L V6 I installed in my '82 300D Turbo 123. It has worked real well for 4 or 5 years now. I did have to resolder all the connections on the tach amplifier board, which is mounted inside the cap that screws onto the connector on the drivers side inner fender to get the tach amp working reliably, though. (A common problem with all the lead-free soldered connections in our Benz's.)

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C