I've been through the same thing with the 3 prong tach and can't figure out a way to make it work. You'll need a 2 prong tach.

It seems that the 3 prong tach uses a pick-up from the transmission. I grabbed a sensor from the PNP but our trannys don't have the necessary mounting location.

I've leads connected from the EGR computer and all ready for the 2-prong one... now I just have to find it.

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'83 240D