gas the compressor clutch won't engage, to prevent damage to the compressor itself. I believe the same thing can happen if the system has too much gas in it, or too much pressure. I'd suggest trying out Hako's method, and see how you go. Might also be worth checking out the switch in the dash, perhaps this could have a faulty contact? The quickest way of finding out the issue would be to go to the auto elec though, as they can hook up the gauges and check out the pressure right away, and then go from there.