2001 2.5L Ranger XL started have idle issues when the air conditioning is on. It doesn't display any of these issues otherwise during any other driving/idle conditions. Truck has a newer battery and plugs, new fuel filter, and cleaned MAF. In max a/c on the lowest setting, no issue, but when you move the flow to positions 2-4 and the truck is stopped it begins to fluctuate in RPM. Usually drops to about 300 and revs up to about 1200. It'll do this repeatedly until I begin to drive again. Its especially noticeable when the truck isn't warmed up to operating temp yet. Unfortunately for me, I live in Florida and so the A/C not working isn't really an option. Any ideas?

 

Welcome to the forum

MAX AC and "regular AC" only have two difference in 2001, earlier just had one difference.

On all vehicles when MAX AC is selected the Fresh air vent is closed, and that is all that changes.
So inside air, which is cooler, is recirculated for Maximum cooling, MAX AC.

On later models a heater shut off valve was added in the engine bay, on most newer vehicles coolant is circulated thru the heater core all the time, this make heater cores last longer since coolant has anti-corrosive chemicals.
But for Maximum cooling the hot coolant flowing thru heater core is stopped.
And thats all that happens/changes from AC to MAX AC in your 2001 Ranger

All AC compressors have 2 speeds, OFF or ON, lol, so nothing changes there.
Blower Fan does have different speeds, so different AMP draws.
So your alternator may have a bad field(it has 3 fields) so can only produce 2/3rds of it power at idle, so it is drawing down idle and system voltage trying to compensate for it.

After driving vehicle for 10 minutes or so, leave it running and open the hood.
Test battery voltage, should be 13.5-13.8volts
Now turn on Blower fan to HIGH, voltage should drop but then come back up to where it was, thats the voltage regulator responding to the extra AMPS needed.
If voltage just drops and doesn't come back up then you have a failed field

You can not do this test with restarted engine, won't get results you need, engine needs to have been running for at least 10minutes to recharge battery after starter up

An alternator can't produce it's full rated amps until engine is above 1,500 RPMs
At idle it can produce vehicles rated amps if ALL 3 fields are working, and no extra Radios or Lights were added.

Come example of a failed field is head lights dimming at idle, you can run a vehicle for years this way, because engine will spend most of its running time above 1,500rpm so battery won't be drained.