AC running but AC light is off + inconsistent
 

2000 Suburban AC will run and the AC light will be off. AC will not cycle...runs 100% full out.
Happens every time I start the car with the FAN SWITCH in OFF position.
If car is started with FAN SWITCH ON (no AC) the AC will not run on it's own.
AC light will work when AC button is pushed without a problem. Not sure if it will cycle like it
Seems that during winter the Heat will suddenly turn cold (as though the AC is running).

Problem is 90% repeatable...as shown above but I can't find any threads anywhere that are like this problem.

Is this one of the dash switches (AC or FAN) or something bigger ? HELP !!