The Volvo MD (series) diesels all have a electrical pickup on the timing
gear housing. Not sure what type it is, only that it has two wires and they
go to the OEM tach.

Since these engines are intended to run without any dependance on a
electrical system (can be hand cranked) the tach circuit seem to be on it's
own as well.

Since the engine and OEM instrument are new I would suspect that the tach is
correctly matched to the engine.

Not much use talking to a Volvo dealer about this, since they don't deal
with these engines since production stopped 25 years ago.. Anytime I mention
this model to a dealer they 'recoil' and start bad mouthing it as 'old
technology' and try to sell me one of their "Turbo Charged Tractor" engines.

Me? I love them! This being my second of this model. I have accumulated
enough spare parts to keep it running for the next 25-30 years.

Steve
s/v Good Intentions