Thanks PeterH.
I don't need to know voltage polarity, only voltage magnitude since the positive and negative half-cycles should be the same magnitude anyway. Let me run the following past you, if I may?
Choosing an arbitrary alternator output voltage, say 100VAC, could I use a transformer of 40:1 (resulting in 2.5VAC at the secondary) to feed a bridge rectifier. Assuming a voltage drop of 0.5V per diode in the bridge rectifier, this would result in ((2.51.414)-(0.52))=2.515VDC for a peak voltage to the pin, for voltage sensing. Next, as the output will be fully rectified and not feature a smoothing capacitor across the bridge rectifier, this would mean that the voltage at the pin would go to zero twice per cycle. Therefore a 5ms delay after every occurance of zero volts would be needed before a sample was taken, to ensure the peak of the waveform was being sampled and not a rising or falling edge of the sine wave.
Does that sound correct/feasible?
Additionally, I say I'm using a transformer purely for voltage isolation purposes. Instead of this, could a potential divider resistor circuit of 39k and 1k be used, connected between the phase and the return of the alternator output, to feed 2.5VAC to the bridge rectifier?