Well I got a nice ammeter
and took some measurements.
With the 16" spal fan on high, it consumes about 11 Amps. The total system load on the alternator with the fan running I measured at 25 Amps. Without the fan it's 14 Amps.
The alternator is able to maintain 14v output under a 14 Amp load at idle. However, 25 Amps is too much for it to handle at idle and the output drops to around 12.5v. If I nudge the throttle so the rpm goes up to about 1100 then the alternator is able to maintain 14v output.
I think maybe it's simple, the vehicle was not originally designed to handle that extra 11 Amp load at idle that the fan adds.
I am going to see if I can install an overdrive pulley on the alternator. I'm not sure what the pulley size is and the shaft size, dimensions, etc, I'll try to figure that out.
note: with a relay fan switch this voltage drop isn't a big deal, everything on the car still works. It's just that the spal controller doesn't like to see that voltage drop and will sometimes throw an error code, and even that only happens occasionally. There might also be other issues, I still need to make sure all my wiring to the controller is solid.