Alternator AC Voltage
"Even though it is an AC tap it has to be measured using the DC scale of your multimeter."
*Actually it is NOT an AC signal at the Tach terminal and I think that is what is confusing you. It is DC.
Yes, the alternator generates AC initially but that AC goes through a set of diodes or a rectifier which turns that original AC into a pulsing DC. That is the case for both the main power terminal and the tach terminal.
Once it goes through that rectifier, it is no longer AC. In the case of the Tach terminal the rectifier diodes, usually a smaller set separate from the main power set, are set up so only half of the pulses get through so the signal is a lower voltage AND highly pulsing. It is that highly pusing signal that the tach reads and converts to the rpm. The rate of pulses changes as the alternator/engine speed changes.
*So yes you read the Tach terminal with the DC scale.
The alternator guys may be correct. In your case your Dmm worked, in some cases it may not have. Even though DMMs have mostly supplanted analogue guages there are some functions that the older meters can actually do better at DEPENDING on the circuit and how sensitive it is. Also DMMs have gotten much, much better at reading things like this than they were many years ago so maybe their advice is just out of date a bit and also they KNOW the analogue meter will work without guessing about the particulars. Doesn't matter here.
-- Edited by C lectric on Thursday 24th of March 2011 10:57:33 AM