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Old 03-08-2011, 05:30 PM   #6
Penta
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City: Sidney, BC
Vessel Name: Penta
Vessel Model: 36' 1961 Sather ex-Gill Netter
Join Date: Dec 2007
Posts: 118
RE: Alternator AC Voltage

Oh boy did this post ever go sideways !!

Reefdrifter - you are correct with the assumption that the "TACH" terminal of your alternator is AC but is actually more a pulsed voltage than an alternating one. It is years since I last worked on such a system and the old brain has very little recall back that far as to what a normal voltage would be however you should be able, with a decent AC voltmeter see some increase in voltage when you increase the engine speed.
What is actually happening is the alternator is producing these voltage "pulses" in relation to the speed it is rotating but not actually changing the voltage at the same time. Most AC voltmeters will read a slightly higher voltage when the frequency goes up and that is what you should see when you increase the engine RPM's
Hope this helps you out and I am currently looking in some of my old manuals to see if I can locate the relative "TACH" voltage for your series of alternator.

Good luck
John "Penta"
Sidney, BC
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