This article is one in a series that have been released in conjunction with Wayne's new book, 101 Performance Projects for Your Porsche Boxster. The book contains 312 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 950+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Boxster owner's collection. The book is currently available and in stock now. See The Official Book Website for more details.
On any car, the AC system is a complicated beast. This project is not intended to be a repair manual for your AC system, but to serve more as a guide on how the system works, and the maintenance involved with its upkeep.
Almost all air conditioning systems work on the theories of thermodynamics, whereby heat flows from a warmer surface to a colder one. Heat from inside the car is transferred to the cold metal fins of the evaporator. The refrigerant in the system picks up the heat from the evaporator and takes it to the compressor. The gas is then pressurized which concentrates the heat by raising the temperature of the refrigerant gas. The gas is then sent to the condenser. The condenser cools the refrigerant and turns it back into a liquid from a gas. The liquid is then sent to the receiver-dryer, where any water vapor that may have formed in the system is removed. The receiver-dryer also acts as a storage container for unused fluid. From the receiver-dryer, the liquid flows into the expansion valve, which meters it into the evaporator located inside the car. Here the liquid absorbs heat, and becomes a low-pressure gas. This evaporation, or boiling of the refrigerant, absorbs heat just like a boiling pot of water absorbs heat from the stove. As heat is absorbed, the evaporator is cooled. A fan blows air through the evaporator and into the cockpit of the car, providing the cooling effect.
The compressor pumps the refrigerant through the entire system. An electromagnetic clutch on the compressor turns the AC system on and off. In addition to cooling the car, the system also removes water vapor from the ambient air via the cooling process. It is not uncommon to find a small puddle of water underneath your car from the condensation of the air conditioning system. A thermostat control on the evaporator keeps the condensation in the evaporator from freezing and damaging the unit.
So what can be done to maintain and protect the system from deterioration? First and foremost, the air conditioning system should be operated at least once a week, if the outside temperature is above 50 degrees Fahrenheit. This will circulate the refrigerant in the system, and helps to keep all the seals in the system from drying out. Most failures are caused by refrigerant leaking out of the system and can be prevented by making sure that the system is run frequently.
The belt that runs off of the main crankshaft operates the AC compressor. If you think that you might be having problems with your compressor, check the condition of the belt first. Turn on the system, and check to make sure that the electromagnetic clutch is engaging. If not, then you may need to replace it. Check the power connection to make sure it is live before replacing. The system also has a pressure switch located right next to the high pressure port in the front cowl area which will shut it off if the pressure inside the system is too high or too low. Check the pressures in the system, and/or the operation of this switch if you're having A/C problems.
On the 1997-00 Boxsters, there is also a set of diagnostic codes that the AC control unit can output, to aid you in diagnosing problems. Check the 101Projects.com website for more details on this.
Refilling AC Systems
The biggest problem with AC systems is a loss of refrigerant. Luckily, the replacement and top-off of refrigerant is a relatively easy task. All Boxsters use R134 refrigerant which can be can be purchased inexpensively at your local auto parts stores. The Boxster air conditioning system capacity is 850 grams (30 oz) of R134. In addition, the compressor needs a synthetic lubricant for proper operation. If you're filling a completely empty system, add 195 ml (6.6 oz) of ND 8 refrigerant oil.
The kit I used to refill the car in this project is manufactured by Interdynamics (see Figure 1). Start the car outside of your garage, turn on the AC system and fan to full blast, and let the car run with the system on for about three minutes. Following the instructions included with the kit, connect a new can of refrigerant to the hose/gauge assembly. Connect the gauge assembly to the low-side port on your AC system (see Figure 2). Be sure to wear eye protection and heavy leather gloves when handling the coolant and gauge assembly: if coolant leaks out at any time, it can freeze a small patch of skin on your hands quite easily and give you frostbite.
With the car running and the AC system turned on full blast, take a reading on the pressure gauge. If your system is properly charged, it should read between 25 and 45 psi. If the pressure is low, then turn the valve on the can to release more refrigerant into the system. Be sure that you shake the can for about 30 seconds and turn it upside down when you connect it to the gauge assembly. Also be aware that the pressure gauge reading will automatically elevate as you are adding more coolant - periodically close the valve on the can to check if the pressure is rising in the system. If the pressure doesn't increase after adding one complete can, then you most likely have a major leak in your system, and you should seek the help of an AC system professional mechanic.
With the system properly filled and measured with your gauge, you should head to the passenger compartment and check the temperature of the air exiting the vents. On a system that is operating really well, the temperature will be in the mid-thirties Fahrenheit. For systems that are older and weaker, then the temperature readings will mostly like be higher. Also keep in mind that if your system is cooling air in the 30 degree Fahrenheit range, the compressor on the car will tend to turn itself on and off, and the temperature will rise up and down slightly. This is not a defect of the system: the compressor turns itself off as the temperature in the evaporator nears the freezing temperature of water. This prevents the evaporator from becoming frozen and clogged with icy buildup.
Shown here is a great starter AC kit from Interdynamics. This kit contains three cans of R134a refrigerant and oil, and is specifically designed to replenish older cars that may have a few small leaks in the o-rings of the air conditioning system. Included with the kit are a can adapter valve, an in-line pressure gauge, and several adapters that are not required for use with the Boxster. The kit is available for about $35 at most general automotive stores, and contains everything that you need to recharge your R134a air conditioning system.
This photo shows the location and orientation of the AC ports on the Boxster. The AC ports are normally covered with plastic covers that simply screw off (inset). The low-side (the side that you attach the gauge and refrigerant to) has the smaller port adapter, and is attached to the larger pipe (yellow arrow). The high-side (used primarily for checking the compressor during diagnostic testing) has the larger adapter (green arrow), and has a smaller diameter pipe.
With the engine running and the system engaged, connect the gauge to the low-pressure port on the AC system. The high-side has a larger adapter, so that you can't accidentally attach the gauge to the wrong port. With the gauge attached, you can now turn the valve to add more refrigerant to the system. In the photo inset, you can see that the pressure for this AC system is exactly where it should be: in the middle of the white range. Remember to use heavy-duty leather gloves and eye protection when working around AC components: it's possible that a fitting or a valve may break or leak refrigerant on your hands.
Your hand is a pretty poor indicator of relative temperature. In order to get an accurate reading, I recommend that you use a digital thermometer, like the one shown in this photo. Final temperature performance of your AC system will vary based upon a number of factors: age, quantity of refrigerant in the system, the condition of the compressor and associated components. This car is a 1999 Boxster with 80,000 miles on the odometer. The vent reading is 46° F (8° C) with the AC at full blast. Outside temperature when this reading was taken was probably around 70 degrees Fahrenheit. You should expect at least about a 20 degree drop from the outside ambient air.