Evacuation, Charging, Performance Testing and Commissioning
Evacuation, charging, performance testing and commissioning
This domain tests the final technical stages: removing air and moisture, charging to specification, proving performance and handing over a verified repair.
Core rules
- Evacuation is not just pulling a vacuum; it removes air and moisture. Use a two-stage vacuum pump and a micron gauge connected at the system, not the manifold gauge set.
- A common target is 500 microns (about 0.67 mbar). Isolate the pump and hold the vacuum. A rise above about 1,000 microns in ten minutes suggests a leak or moisture; investigate rather than charge.
- Charge by weight whenever possible. Use the vehicle label or manufacturer data. Liquid charging into the high side with the compressor off is common for initial charge; avoid liquid slugging the compressor.
- Performance test at a known ambient and humidity: vent temperature, high and low pressures, blower speed, airflow, blend door operation and clutch or control valve behaviour. Record the result.
Worked example
A system specifies 550 g R134a. Recover, replace the drier, evacuate to 500 microns, isolate and hold for ten minutes; the gauge rises to 800 microns, which passes. Charge 550 g by weight, run the system and check vent temperature and pressures against the ambient condition. The reasoning is that a vacuum hold proves both leak-tightness and moisture removal before the charge is committed.
Traps
- Using manifold gauges to judge deep vacuum.
- Charging until it feels cold.
- Overcharging to chase vent temperature.
- Skipping the vacuum hold test.
- Testing performance with a blocked cabin filter or incorrect airflow.