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Vacuum is an important part of any auto A/C service. The system must be vacuumed before charging in order to remove air and moisture from the system. Without a proper vacuum, you will have excess pressures and erratic gauge readings. In addition, for the long term, it’s important to remove the moisture. Moisture and refrigerant (freon) will mix and cause the formation of harmful acids. Those acids will corrode A/C components like the condenser and evaporator, causing them to leak.
In order to vacuum the system, you fist must start with the manifold pressure gauges connected to the system (See the information on How to connect manifold pressure gauges to an A/C system). Starting with the system empty and all control valves on the manifold gauges closed, connect the gauges.
Then connect the yellow (center hose) of the pressure gauges to the vacuum pump. Start the vacuum pump with all valves closed. Once the vacuum pump is operating, open BOTH the high and low side control valves on the pressure gauges. This will assure that vacuum is being pulled throughout the entire A/C system.
Typically, you should reach a level of 29.9" of vacuum (at sea level). More importantly, you should be checking the level of vacuum using a micron vacuum gauge. That’s because the compound low pressure gauge is not really accurate for reading vacuum. As an example, 28.9" of vacuum would only be about 25,000 microns. In order to boil moisture, you require levels of vacuum below 1,500 micron. Deep vacuum is considered to be below 750 micron.
To properly vacuum the A/C system, you should be able to attain and hold deep vacuum (under 750 microns) for at least 1/2 hour. Generally, the longer you can maintain vacuum, the better.
Once the vacuum is finished, be sure to close all control valves on the pressure gauges before turning the vacuum pump off. That will assure the vacuum in the system is maintained.
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