Manifold Gauge Readings
An A/C manifold gauge set measures refrigerant pressures inside an air conditioning system. An automotive air conditioner has a high and low side, divided by a metering device and the A/C compressor. A compressor pulls low-pressure vapor from the suction side with the clutch engaged. It then compresses and discharges it at a higher temperature and pressure through the discharge side.
These pressures and temperatures are highly predictable and affected by ambient or outside temperatures. They increase as the outside temperature increases and decrease as the outside temperature decreases. When the low side is high, and the high side is low, the compressor is not functioning properly, whether it is a CCOT (cycling clutch orifice tube) or a TXV (thermostatic expansion valve) system.
When a system is static and not in operation, the pressures are equalized and very close to each other. A slight hissing sound occurs while the pressures equalize immediately after shutting down. As the compressor wears and weakens, the system's high and low side readings start getting closer to static.