Manifold Gauge Readings

Troubleshooting the A/C Compressor

An A/C manifold gauge set is used to get a look at what's going on inside the A/C system. The low side or suction pressure, the high side or discharge pressure along with a few vent outlet temperatures will tell a technician a lot about an air conditioners performance. It's important to listen to the customer, understand the type of air conditioner, and have a good understanding of how an automotive air conditioner works.

An automotive air conditioner has a high and low side divided by a metering device and the A/C compressor. With the clutch engaged, a normally functioning compressor pulls vapor in from the suction side and discharges the vapor at a higher temperature and pressure through the discharge side. These pressures and temperatures are highly predictable and affected by the ambient or outside temperature. The pressures in the system increase as the ambient temperature increases and decrease as the ambient temperature decreases. Always compare readings with manufacturers specifications and charts when diagnosing A/C performance.

When the low side is high and the high side is low, the A/C compressor (reed valves) are most likely the problem whether it is a CCOT (cycling clutch orifice tube) or a TXV (expansion valve) system. Note that when a system is static and not in operation the system pressures are equalized and are very close to each other. It's normal for some vehicles to make a slight hissing sound while equalizing just after a vehicle is turned off.

As the A/C compressor wears and gets weak the high and low side readings of the system also start getting closer to each other. As the compressor gets weaker the customer may notice it runs cooler at higher speeds. This is because as the compressors piston seal wears it looses its compression. When the engine is revved the compressor is able to compensate for the loss in compression and increase system pressure.