Car A/C Suction Throttling Valve System
Some older air conditioning systems use a suction throttling valve or STV as the main evaporator control. This type of system is common on older General Motors and Ford vehicles. The valves themselves differ in configuration but, functionally, they’re all pretty similar. Basically, all of them maintain proper evaporator temperatures by modulating evaporator pressure. The compressor in an STV system runs continuously instead of being cycled on and off. This type of system is no longer in favor with the manufacturers because it requires too much energy (fuel) for operation.
The suction throttling valve, also known as a pilot operated absolute (POA) valve on some systems, is mounted on the outlet side of the evaporator. Because refrigerant flow from the evaporator is being regulated by the valve to maintain evaporator temperatures, this device allows the compressor to run all the time.