Air Conditioner Vaccum and Recharge

Easy to follow step be step guide on how to vacuum down and recharge an automotive air conditioner, this guide is meant for repairs where the system has been opened for repairs ie: new condenser, compressor etc., but it also can be used as a maintenance procedure as well, though appearances may vary, this process is the same on most vehicles.

Each system has a specific amount of refrigerant needed to operate correctly and is located in the owners manual or on the a/c system itself, this guide will successfully recharge the system even if this information is not available.

Air conditioner systems deal with refrigerant under high pressure so use caution, read completely before beginning.

Difficulty Level - 4 of 10

Tools and Supplies Needed

  • Air conditioner recharge gauge set
  • 134a refrigerant
  • Vacuum Pump
  • Protective eyewear and gloves
Before beginning, park the vehicle on a level surface with the engine "OFF", apply the emergency brake with the transmission in park, wear protective eyewear and gloves.

Refrigerant Recovery

Step 1 - A charged system contains refrigerant under high pressure (70 to 100 psi static - system not running), this liquid/gas can be recovered. (Note: Allowing the refrigerant 134a to leak into the atmosphere is illegal, though is less harmful than its predecessor r12, which was slowly bled out of the system by loosening a hose fitting at the gauge set.) This recycling machine gathers refrigerant along with a small amount of compressor oil which is then separated into individual tanks.

Refrigerant Recovery Machine
Step 2 - Connect an air conditioner gauge set to the recovering machine and open the low side valve to start the recovery.

Low Side Pressure Connection
Vacuum Down and Recharge

Step 1 - An air conditioner gauge set or recharge kit is needed to connect to the system on both high and low side pressure ports, a gauge set will also be used to vacuum down and recharge the system, inspect all hoses and connections. These units must be "air tight" and not have any leaks at hose fittings or valves. The red colored gauge and connector valve represents the high pressure side of the system, while the blue color represents the low pressure side. The center hose (yellow) is connected to a recovery machine or a new refrigerant supply bottle, both gauge valves should be closed.

Air Conditioner Refrigerant Pressure Gauge Set
Step 2 - Beginning with system pressure at zero, by the way of recovery, repair or leak, start by locating both high and low side service ports, these locations will vary and can be obscure. (Note: a high pressure port is always larger than the low side port.)

High/Low Side Pressure Service Ports
Step 3 - Once located, remove both high and low side dust caps.

Remove Dust Cap
Step 4 - While pulling the retainer ring backwards, press the valve over the service port, repeat this for both service ports. (Note: High and low side fittings will only work on their respective ports.)

Attaching Gauge Valves
Step 5 - Once secured, tighten the valve which activates the internal plunger that opens the service port valve to the refrigerant lines.

Tighten Valve
Step 6 - With both gauge valves closed, there should be little to no pressure in the system.

No Pressure
Step 7 - Connect the center hose (yellow) to the vacuum pump or recovery machine and turn "ON", this step is used to remove any moisture and static air from inside the system.

Connect Hose
Step 8 - Slowly open the low side gauge valve, the pump will now start pulling vacuum throughout the system.

Opening Low Side Gauge Valve
Step 9 - Once the vacuum has been on for 25 to 30 minutes, close the low side valve, the system should hold at 29 in., turn the vacuum pump or recovery machine "OFF" (Note: If the system loses vacuum there is a leak and must be rechecked. This could be a faulty O ring installation or a loose hose on the valve set). If the system hold vacuum proceed to the next step.

Holding @ 29 inches of Vacuum
Step 9 - Disconnect the yellow hose from the vacuum pump and attach it to a new 134a refrigerant source. (Note: Many recovery machines have a charge feature that can be used.)

134a Refrigerant
Step 10 - Open the valve on the Keg.

Open 134a Refrigerant Keg
Step 11 - While the gauge valves are closed and once the refrigerant source has been opened, the gauges will respond with equal reading on both high and low side, this is static pressure which will vary depending on outside temperature.

Static Pressure
Step 12 - Turn the keg over which expels liquid refrigerant instead of gas, this will help expedite system charge time, most systems hold about 2 pounds, a digital bathroom scale under the keg can aid in the recharge process.

Turn Over 134a
Step 13 - Next, start the engine and turn the air conditioner "ON" to the highest settings including fan speed.

Air Conditioner On
Step 14 - Slowly open the low side (blue) valve (never open the valve completely.) Refrigerant will start to flow to the low pressure port. (CAUTION: Never open the high side (red) valve, this will fill refrigerant container with high pressure and may cause a rupture.)

Refrigerant Flow
Step 15 - The compressor operation will begin which will be followed by the low side pressure dropping, while the high side pressure starts to rise.

Low Side Drop - High Side Rise
Step 16 - Continue adding 134a until the gauges start to look like this. If both gauges are too high the system is over charged or the cooling fan is not working. If high side pressure runs way up quickly, (300+) and the low side goes into a vacuum, the system has blockage; such as a plugged expansion or orifice tube. If the compressor engages, and neither gauge pressures move (stay the same) the compressor has failed. - Replace air conditioner compressor

Typical Gauge Readings
Step 17 - As these pressures rise monitor the temperature of outgoing line of the evaporator and air vents in the passenger compartment, both should be cold to the touch. (Note: Monitor the refrigerant scale weight to help determine a complete charge.)

Monitor Outgoing Temperature
Step 18 - Once the system is performing properly close the low side gauge valve (Note: Do not overcharge).

Close Low Side Gauge Valve
Step 19 - Turn the ignition switch off, the A/C system will shut down along with the engine.

Turn Key Off
Step 20 - With the engine off, turn the connector counterclockwise to release the plunger effectively closing the service port, repeat this procedure for both valves.

Loosen Valve
Step 21 - Firmly grasp the valve retainer and pull up, this will release the connector from the service port, repeat this procedure for both valves.

Release Valve
Step 22 - Once both valves have been removed, reinstall the dust cap for each service port. Once the job is complete, close the refrigerant supply valve and store the gauge set properly for the next repair. Enjoy the cold air!

Reinstall Dust Caps
Helpful Information

Automotive air conditioner systems have a mixture of refrigerant and oil to keep the air conditioning compressor lubricated during operation. Measuring the amount of (pag) oil is difficult in the system because it's spread throughout the system, in other words if there is a major leak and oil is dripping out, the system will need oil, unlike a very small leak which releases little to no oil. Oil can also be drawn out from the system during the evacuation process, if this occurs add new oil in its place, never re-install used oil.

There are two separate occasions in which an air conditioner system needs to be recharged, a normal condition of residual seepage, or when a repair has been performed such as replacing a leaky compressor, condenser or evaporator. Once the system has been opened it must be held under vacuum for a period of time (usually 30 minutes) to remove moisture.

The AC system will not work correctly if the engine is overheating or running hot.

If further assistance is needed, our certified car repair technicians are ready to answer your car repair questions for free.

Common Problems

  • Failed compressor
  • Refrigerant leak
  • Plugged orifice tube/expansion valve
  • Failed vent control system
  • Shorted control switch or relay
  • Plugged condenser or evaporator (cooling fins)
Best Practices

  • Use a garden hose or compressed air to clean the condenser at the front of the vehicle to ensure maximum system performance.
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