I have a 2000 SLT, 4.7, 4x4, automatic.

The A/C works well. However, it runs when it is in the 3rd position going clockwise. Or, the 4th if you include the off position. I think it may also run in other positions like the defrost position. I can hear the compressor turn on and off, the engine idle changes etc....

I've pulled the fuse to the A/C and the engine runs smoother.

The heater also blows barely warm when idling (at full engine temp) and blows warm when running. I think this is a separate issue based on the reading I've done. But, thought I would throw it in the mix in case I'm wrong.

Any help on how to fix this would be appreciated.

 

yes, the AC is supposed to run in any defrost mode, to cool the window and reduce the condensation.

 

Your heater core is likely restricted. It's a common problem. You can try to flush it out, disconnect both heater hoses and flush it out, backwards. It may help.

After replacing mine, due to a leak, it will cook you out of the cab.

 

I have had the same problems with my 2000 3.9L truck when it comes to low heat output during winter. AC runs great in the summer and I always heard the compressor kicking on with the defroster setting. I found if you set the dial to defrost/floor, the compressor runs very little or does not kick on like on straight defrost. I tried back flushing the heater core and that really did not help much, so this fall I did the flush method. I poured a bottle of prestone flush and run the truck for 4 days and then took it to Sears for a coolant exchange where the coolant is sucked out using vacuum pressure while the engine is running. This method did help increase my heat temperature on cold mornings. It usually took 4 to 5 miles of driving before my guage would finally move above cold. Now it starts to move after 2-1/2 miles. Always had the problem of long warm up time, but once the gauge moves up, the single cab will toast you.