a/c comp. works only when defrost is swithd on, if defroster is off a/c stops

Hello, Since the defroster turns on the compressor, I would start by checking the normal a/c button (switch). If your vehicle turns on the A/C automatically, I.e you have no way to turn the air conditioner on except for turning the knob to cold, the issue could be your ambient air temperature sensor. The ambient air temperature sensor determines how hot or cold it is outside, and that's how it knows how hard to run the A/C system. Defrost forces the system on because it is dehumidifying the air in the vehicle. There is also an evaporator temperature sensor, which measures the temperature of the A/C evaporator coil - the part that cools the air going to the vents. If the evaporator temperature sensor has failed, it may think the evaporator is too cold, and shut off the A/C. It could also send no reading, so the A/C would have no signal to run.