hi All, can you measure tach pulses, from the W & R posts on the alternator, with a volt meter? a friend told me I can measure it on the AC side, but I'm looking for a number range to measure my 8V92 from idle 600rpms to 1800rpms, on the volt meter, if possible, lvmci...
If you can read the signal on the R terminal with a volt meter depends on the meter, and what you want to know. The alternator pulses on the R terminal are half-wave rectified DC, and since there are three diodes they pulse at three times per rotation of the alternator. If you have a 24 volt system (nominal 24 volts, it's really 28 volts or whatever your regulator is actually set to regulate to) they will pulse from zero to plus 24 volts. The reason it's called 12 volts is you are getting only half the pulses, so the average voltage is half of the normal output. You can read them with a volt meter but what you read will depend on the meter, if I put my meter on RMS AC I would expect to read somewhere around 9 or 10 volts. A typical digital voltmeter would not have a frequency range, but if it did it would probably read the frequency of the pulses, that's effectively what the tachometer is doing, of course. If you know, or can calculate, the drive ratio of the alternator compared to the crankshaft you know the frequency anyway.
The main output of the alternator is through six diodes, and is full-wave rectified DC pulsating at six times the alternator rotation frequency, but because it is connected to the battery it is effectively a smoothed out plain old DC voltage.
What is it exactly you want to read?
Brian
thanks Brian, wanted an explanation, I could have just hooked up another tach at the alternator, but wanted to understand it, thanks, tom, lvmci...
Glad it helped.