techguy512 has produced a tach amp replacement that uses a small microcontroller (computer) to do what the old analog part did. It may be that he can adapt that to convert the signals. The issue is that the tachometer expects a steady stream of pulses, and not bursts - so his algorithm would have to be somewhat complex. It would have to behave like a Phase Locked Loop.

It will be much, much, much easier (not to mention cheaper) to retrofit a diesel tachometer. It looks to be a straight conversion with no extra holes. Some rewiring is required, of course.

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1984 300TD