Thoery: If the mechanic has told you that the compressor is bad due to lack of "magnetism",then why would it work on the defrost mode like normal?. To me , it's either broke or it's not. The clutch has a series of wound up coil thats insulated with a thin coat of clear insulation to keep the copper coil from shorting. when energized , it creates a magnetic field thus engaging the clutch surface to the metal face of the opposing side. It would also have a clamping diode in the circuit to prevent an exessive surge on the electrical system when the clutch is energized. If that clutch is in fact faulty, it shouldn't work at all even if you energize the circuit directly to the power source, however it does work on defrost?, that indicates a fault on the electrical circuit prior to the compressor. I've never seen or heard a symptom like this so im intrigued with the issue on this compressor. If the low or high pressure switch is faulty, it will cause the compressor clutch not to engage. If you jump the terminals across, it should switch the clutch on.