The compressor activation button is built into the knob that selects the fan speed.The above button enables operation of the air cooling circuit, but actual compressor activation is also dependent upon activation of:- the four-stage pressure switch located on the coolant circuit that locks air conditioner activation if coolant pressure is too low or high;- the engine control unit that excludes compressor operation in accordance with engine management mode; e.g. in the case of high engine temperatures or high engine load (sudden acceleration);Enablement is also required from the vehicle interior fan adjustment knob (at least in position 1) and the demister thermostat. See E6020 AIR CONDITIONINGThe compressor supply circuit is controlled by a special relay located in the engine bay junction unit. The relay supply line is protected by a special fuse.