Q: Does non-climate control car AC cut-in & cut-off automatically?

A/C systems that used fixed displacement compressors will cycle on and off (the compressor that is) depending on cooling demand. If you have a variable displacement compressor in your vehicle, though, the compressor may hardly ever cycle simply because the compressor is capable of load following. In fixed displacement systems, without automatic climate control, the rate of cycling is usually determined by pressures within the system. An example of this type of system is what’s known as a CCOT (cycling clutch orifice tube) system. The compressor clutch will cycle on and off depending on cooling demand, usually at around 21-23 psi on the suction side, but of course there is variance from car model to car model. On newer car, the vehicle’s PCM is involved in the control strategy, too, as the PCM can command the compressor off during periods of high engine demand, such as rapid acceleration. Insofar as climate control heating and HVAC systems, those are electronically controlled and, in the main, merely "automate" all the mechanical controls that formerly were used to control the position of blend doors and the heater control valve. Control strategies are electronics-based. If you need a diagnostic of a vehicle HVAC problem, performed by a certified Mechanic, dispatched by YourMechanic right to your location, please request HVAC inspection/testing and the responding certified mechanic will address your issue(s). If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.