What is the correct procedure for charging an A/C system with the engine running?

Charging an A/C system with the engine running should be done through the low side service port. This procedure is essential for several reasons. First, charging on the low side allows for the refrigerant to enter the evaporator directly, which is critical for proper absorption of heat and efficient cooling. When the engine is running, the compressor is actively circulating the refrigerant, which helps to ensure that the refrigerant is being properly mixed and utilized within the system as it is charged.

Additionally, charging through the low side avoids the risks associated with charging through the high side, which operates under much higher pressures and can pose safety hazards. Charging on the high side may also lead to overcharging the system, which can cause damage or inefficiencies.

Charging the system when it is off is not advisable, as it would not allow for immediate observation of system performance or identification of potential issues. Properly charging with the engine running ensures that technicians can monitor the necessary parameters, such as pressure and temperature differentials, to verify that the system is functioning correctly.