C40TLRX tach output signal

[99yam40]

Is there someone out there that could help me find out what kind of a signal output there is and where it comes from, for a tach for this C40TLRX yamaha motor?
MV, Voltage, etc, and could it be read by a Digital VOM on a hz setting to reprsent the RPM?
How about a scope or any other meter besides a tach?

[boats.net]

Between the green and black wire at gauge you should get an AC voltage reading and voltage will increase as rpm's increase. The tach signal comes from alternator.

[99yam40]

Thanks, for the reply.but Can you give me a little more insite to the way the tach works
does this mean that the tach uses the increase of voltage to give the RPM or is it the amount of pulses from the alternator that represents the RPM?
I have read there is a need to set the tach to the proper number of poles, so this makes me think that the HZ or number of pulses gives the RPM.