Engine Stall When AC is turned on.

First, check to see if BOTH the low speed and high speed engine cooling fans are working.

The PCM maintains two idle trim settings. AC on and AC off. This could be a simple matter of the PCM not having enough time to "learn" new AC on idle trim values.

Of course this assumes that the IAC is working and that the AC is not drawing an excessive load.

Note, if the AC is short cycling, then the AC will not run long enough to allow the PCM to learn. Is your AC short cycling? Causes for short cycling include low on Freon.

If low on Freon, it may be possible to get around this by jumping the low pressure switch thus forcing the AC to run. Or better yet, fix the AC.

Note, when the battery is disconnected, the PCM will "forget" all idle trim settings and thus need to be relearned.

For more information on how to trouble shoot idle issues:

http://www.allfordmustangs.com/forums/2260649-post.html