All Volvos worldwide came with Bosch alternators until the early 90s when some got Denso units.
I wonder if the tach just measures ripple on the alternator output.
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16 years ago
'84 240 tach
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16 years ago
That is exactly what it measures. Thus, since the the alternator revs are proportional to engine revs, the tach reports the proper value. I installed the tach back in 1985, and can't recall the details. There was sheet of paper that explained the hookup. I don't remember if the "W" terminal was mentioned. The brand of alternator does not really matter. A good Volvo dealer should be able to tell you the connection requirements.
For what it is worth, there are after market tachs that derive a signal from the either the camshaft pulley or the fuel injection pump pulley. I believe they are used on some VW engines?