If the A/C's are anything like other cars, there is a freon pressure switch in the system such that when the feon levels are too low, the compressor will not engage as a means of preventing the destruction of the compressor under conditions when freon is too low. May have a charge that is below minimal standards at this point and once you recharge (look for a leak too), the compressor is likely to become energized once again by virtue of the safety switch permitting a competed circuit. Good luck!

JBeechcraft