'84 240 tach

[James Sweet]

Great! Thank you for that information
I looked at my alternator and found that it is at BOSCH (don't know if you
use them in the U.S.). It has an unused output that is signed "W". Could
that be for the tach-signal maybe...?

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.

 

[jch]

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.

_____
Jacob/James,

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?