I have an '85 full size Jimmy. I spent today finally replacing the alternator and power steering pump. It steers like a dream and it stays above 9 volts with the ac and radio on at idle or cruising down the road. However with the ac on the alternator is continually cycling on and off in 5-10 second intervals. With the ac off it holds steady at about 14 volts cruising or idling. As soon as I turn the ac on to max the voltage stays in constant fluctuation between 11ish and 14 volts. As soon as the alternator kicks off it starts dropping REALLY quickly then the alternator cycles back on and gets it back up to 14 volts the repeat. Any ideas as to what the problem is? I assume all the ac system is original or close to it.

 

What counts is what the voltage is at the battery. Get a meter and watch voltage at the battery terminals while AC on.