A frequency counter can work fine on AC or pulsed DC (which is what the ignition creates). The counter just filters out the DC component. Once the DC is filtered, all that's left is AC.
The alternator does not put out sinewaves unless it is disconnected from reg/rect. While connected, it basically looks like square waves, but should still work fine.
The only problem is, if some phases start to shunt, the freq. counter may not acknowledge those pulses. It should work at idle with the lights on, since there probably won't be any shunting, but to work at higher RPMs you'll have to disconnect the alternator from the reg/rec and run the bike off the battery or an external power supply (or charger).
If the alternator will be connected to the reg/rec while testing, I would use one yellow wire and ground as the test connections. It should be a nice square wave, like Fig. 5 on this link:
www.geocities.com/loudgpz/GPZAltntrWaves...rnatorWaveforms.html
Post edited by: loudhvx, at: 2007/07/13 08:29