Recharging a Vehicle's Air Conditioning System

Have you ever been in a car during the hot summer and turned the AC on just to find that it never seems to chill the air? Sometimes, with age, a vehicles air conditioning system needs to get a boost. Here is how you can do it yourself.

Air conditioning systems in vehicles operate at high pressures. A failure to properly service the AC system can lead to injury. These instructions are intended to be used as advice only, always follow the manufacturer's instructions when possible and proceed at your own risk.

Instructions with photos submitted by Andrew Schenk

A Full AC System is a Cold AC System
Most AC system problems can be traced to one or more of three likely candidates:
  1. Low refrigerant level due to age
  2. Low refrigerant level due to leak
  3. Bad AC compressor
If your car's air used to blow cold and very gradually over the years has had diminishing effect, then your car probably falls into the first category and just needs a refrigerant refresh. However, if it seems you have to recharge the AC system annually, there is a leak in the system that should be repaired. If your car's AC compressor is bad, no amount of refrigerant will help, you are in for some costly repairs.

Since anything beyond the first category is beyond most backyard mechanic's abilities, and due to the inherent dangers of novices working with AC systems, we will stick with a simple recharging of the AC system. If a simple recharge does not fix your AC problem, then take it to a shop that is properly equipped to service AC systems.

Which Type of Refrigerant?
If your vehicle was made prior to 1994 its AC system probably uses R12 refrigerant (commonly called Freon-12). R12 was banned due to its damaging effect on the ozone layer. Later vehicles use R134a refrigerant which is CFC free and does not harm the environment as the older R12 did. If your vehicle uses R12 refrigerant you will most likely not be able to legally recharge the system. You must be EPA certified to handle R12 refrigerant and, due to its lack of manufacturing, R12 is extremely expensive. Not all is lost, however, as there are R134a retrofitting conversion kits. These kits are not usually cheap as they replace most of the AC system, but will allow you to keep your cold air.

If your vehicle takes the common R134a refrigerant, then you can recharge the system yourself. You will need to buy a couple of things from an auto parts store:
  • One or more cans of R134a refrigerant
  • A recharge kit
  • (Optional but highly recommended) a recharge kit with a pressure gauge

When recharging the AC system, you need to make sure the system has the correct pressure; too low and the AC compressor may not run, too high and the can of refrigerant you are using to recharge the system can explode in your hand! To take the guesswork out of how much refrigerant to put into your system it isn’t a bad idea to get a Smart Charger that costs about $40.

How the AC System in Your Car Works
Here is a summary of how a vehicle AC system works.
The Compressor is the heart of the AC system. It compresses and pumps refrigerant that is in its gas form. The compressor is driven by the car's drive Belt but also has a Clutch to disengage if the compressor is not needed even when the engine (and belt) is running. You can sometimes hear an audible click coming from the front of your car when it is running (especially when idling), this noise is probably the clutch activating the AC compressor. The pressurized gas refrigerant is pumped to...
The Condenser, I component very similar to the car's cooling system radiator. In fact, in most vehicles, the condenser is located directly in front of the radiator. The gas refrigerant expels its heat in the condenser and turns into liquid refrigerant by the time it reaches the bottom of the condenser. The liquid refrigerant is now pressurized as it travels towards...
The Evaporator. The evaporator is very similar to the condenser. The liquid refrigerant is warmed by the air in the car and boils (refrigerant has a very low boiling temperature). As the refrigerant turns into a gas, it sucks heat from the car's cabin, cooling it in the process. The evaporator also acts as a dehumidifier, thus on humid days when a vehicle's AC has been running, you will often find water dripping from under the passenger side of the vehicle. This is condensation and is perfectly normal.
An Accumulator is found on many AC systems. Its primary function is to store liquid refrigerant and make sure only gas refrigerant gets to the compressor. An AC compressor is designed to compress gas, not liquid, so if too much liquid refrigerant gets to the compressor, it may be damaged.

1. Find Your Low Pressure AC Valve. Every AC system has a low pressure and high pressure side. The low pressure valve will be on the hose that goes from the accumulator or evaporator to the compressor. The high pressure valve will be on the hose that goes from the compressor to the condenser or the condenser to the evaporator. The low side valve is the blue cap in the picture. Follow the instructions that come with the smart charger for hooking it up to the proper valve. In this case, the smart charger's valve won’t fit in the high pressure AC valve, so if it isn’t fitting right, you’re messing with the wrong hose! (Fig. A)
2. Install the Included AAA Batteries into the smart charger as shown. (Fig. B)
3. Start the Vehicle and Turn the AC to Max. It also isn’t a bad idea to roll down the windows so the compressor won’t shut off.
4. Once the Vehicle is Running and the smart charger is on, hook the blue hose of the smart charger to the low pressure valve. Press and hold the black button for a second or two and then release. The smart Charger will now diagnose your AC system. Consult the instructions that come with it for what to do if certain lights come on. (Fig. C)
5. If the White “Low Charge” Light Illuminates after diagnosing the system, shake a can of refrigerant and while the car is still running and the AC is still on, screw the can into the base of the smart charger. (Fig. D)
NOTE: MAKE SURE NOT TO TIP CAN UPSIDE DOWN, refrigerant will leak out. You should wear gloves and eye protection as refrigerant can cause frost bite in seconds.

6. Once the Can is Attached, squeeze black trigger for 15 seconds and then release to rediagnose the system.
7. Periodically Check the Vents to feel if the air is getting cooler.
8. If After the First Can of Refrigerant, the air isn’t any cooler, consider consulting a professional. It should take about 1 minute to empty a 12 oz can of refrigerant.

  1. Find the valve on the LOW pressure side of your AC system
  2. Turn on your car and set the AC to MAX
  3. Attach the smart charger to the LOW pressure valve
  4. Press the black button on the smart charger to diagnose the AC system
  5. If the system is low, screw a bottle of refrigerant into the smart charger
  6. Hold the black button for 15 seconds, then release to diagnose the AC system; repeat until the AC system has a proper charge

Although repairing a car's AC system is probably far out of the scope of the DIY, he or she can recharge a system low on refrigerant. It would be wise to purchase some sort of charging kit that has either a digital or analogue gauge to prevent over charging the system. If you are still having problems, or find that you need to recharge the system frequently, then take the car into a shop equipped to work on AC systems. They can add dye to the refrigerant to help find any leaks and fix any other problems.

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