'99 Jeep Grand Cherokee Air Conditioning (a/c) Trouble

Hello,

I'm really hoping that someone out there can help me find a solution to
this problem that I'm having with the A/C in my Jeep. I recently
purchased this Jeep, so I don't know much about its service history.

The problem is that the A/C compressor only kicks on when the control
is set to 'defrost' mode. In any other mode, the A/C will not come on.

I took the Jeep down to a local quick lube shop to have the system
evacuated and recharged, hoping that low refrigerant might have been
part of the problem. This, however, has done nothing to solve the
problem. The guy told me that he pulled out just over 16oz. of
refrigerant and the total capacity is 24oz. so, it was a little low. It
might just be placebo, but the air does feel cooler. It is diffifult
to feel that cold air on these hot days, however, when it is coming out
on top of the dash.

Any help or suggestions will be greatly appreciated.