A tach is a counter. Obviously the signal to it has to be something that can be counted. In this case it is a DC pulse with no particular duty cycle. Unless you understand what I just said you are going to have trouble with this.

IF you place a voltmeter on a wire containing a DC pulse of 12v the voltage that you will read will be determined by the duty cycle as the volt meter just averages. Thus a 12v DC pulse with a 50% duty cycle will read a constant 6v.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician