This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.
The Porsche 911 has never been renowned for its air conditioning systems. At best, most of the systems on the 911 can be described as marginally cool. Part of the problem stems from the fact that the air conditioning systems on the early cars were a dealer-installed option, and never fully adopted by the factory. As it happened, the A/C systems were basically patched into the car, and weren't really well integrated into the 911.
On any car, the A/C system is a complicated beast. This project is not intended to be a repair manual for your A/C 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 condensers, which are located in the front and rear of the 911. These condensers cool the refrigerant and turn 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 cold 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 A/C 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.
The late model factory A/C systems from 1978 used a dual condenser system to try to remove more heat from the system. It is important to check that the blower motor in the front of the front trunk is working in order to achieve the maximum cooling from the system.
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 help 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.
A belt that runs off of the main crankshaft operates the A/C compressor. Make sure that you don't tighten this belt too tightly, or you may place undue pressure on the bearings inside the compressor. If you think that you might be having problems with your compressor, then check 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. Sometimes the A/C system will not turn on the compressor if the system is not charged with refrigerant.
Finally, if the rest of the A/C system appears to be functioning correctly, and you are still not getting cold air output, then you might have to replace the compressor. The compressor contains a piston with seals that may deteriorate if the system is not run for a while. Also make sure that when you are running the A/C system you keep the engine lid closed. You can open it to perform pressure tests on the valves, but at all other times keep the lid closed.
On the receiver-dryer, there is what is commonly called a sight-glass. The sight glass is an excellent indicator of the condition of the A/C system. Find the sight glass and clean it. After running the system at maximum for about 5 minutes, increase the rpm to 2000, make sure the engine lid is closed, and check the sight glass. If the glass is completely clear, the system is either fully charged or completely empty. Have an assistant turn the system on and off while you watch the glass. If the system emits cool air, or if bubbles appear when the system is off, then it is fully charged.
A few bubbles in the sight glass mean that the system needs a charge, or the compressor is not functioning properly. Make sure that the clutch is on, and check the sight again. If the bubbles remain, then the system needs to be charged.
If the sight glass is foamy, or if oil streaks appear, then the system is very low on refrigerant and needs to be recharged. If the sight glass is cloudy, then this is an indication that the desiccant in the receiver-dryer is breaking down, and the unit needs to be replaced.
For environmental reasons, it is illegal in some states to perform your own A/C refrigerant recharging with the older R12 refrigerant, but you can check the overall charge (amount of refrigerant) of the entire system. The system's pressure can be checked using a pressure valve that is located on one of the hoses that is attached to the compressor. Before checking any of the valves in the system, make sure that you put on a pair of thick gloves and wear eye protection. If Freon is released from the system, it will rapidly expand, and could cause frostbite if it gets on your hands. On a similar note, the system is pressurized, and the Freon could discharge into your eyes if you are not careful.
There are two test valves on the compressor. These Schrader valves are very similar to the valves that are used to inflate your tire, and the pressure of the system should be checked with an air-conditioning pressure gauge. Make sure that you check the pressure of the system while the engine is on, and the compressor and A/C system is running. Make sure that the clutch is engaged when taking the readings, and the system is set to maximum cooling.
The compressor will have two markings on its housing where the two hoses connect to it. One is marked 'S' for suction, and the other 'D' for discharge. The valve on the suction side should read about 10-30 psi when the system is running. The discharge side should be about 140 psi if the compressor is functioning correctly. Be careful again that you wear hand and eye protection, as the refrigerant can easily give you frostbite.
Take another set of readings when the system is off. If the system gives readings that are equal, and the gauge indicates a temperature/pressure value equal to the outside air, then the system may need a recharging.
If your A/C system needs a major overhaul, this can be a difficult and time consuming process. In order to work on it, the entire system must be evacuated of any Freon. The process of recharging the system from empty is not an easy task either. In order to prevent water vapor and other impurities from entering the system, it needs to have a vacuum drawn on it for about a half an hour. Needless to say, the repair and replacement of most A/C components are beyond the average weekend mechanic.
The original Freon that was used in the older style air conditioned cars is no longer being manufactured. Around the year 1995, auto manufacturers started phasing out the use of the Freon based A/C systems, and started implementing the newer R134 systems. The cost of the replacement freon is skyrocketing as the current supplies disappear. This Freon, which was once sold to the public in do-it-yourself kits, can now only be purchased by dealers who are trained in recharging these systems. If your A/C system needs a major overhaul, it's a wise idea to upgrade to the newer R134 kits available for the 911. Particularly if your compressor is broken, it doesn't make much sense to replace it with a rebuilt original one. Although the R134 system is not as efficient and doesn't cool as well as the original system, the refrigerant can be purchased inexpensively at your local auto parts stores. The typical non-factory installed 911 A/C system uses about 28 oz of R12 freon. The factory A/C systems with both a front and rear condenser uses about 39-41 oz of freon.
The workhorse of the system is the compressor. The white arrow points to the high port side of the compressor. This line comes out of the compressor and goes to the condenser, which is located on the inside of the engine grille. The low part of the system is located directly beneath. The valve for testing the pressure in the system is covered with a small black cap that needs to be removed.
The receiver/dryer is commonly located in the inside rear of the front left fender. There should be a small cap covering the sight glass, which contains a small white ball. This ball helps to tell you whether or not your system is full. When adding refrigerant to the system, simply check the level of the ball. When the ball rises to the top of the glass, you should have more than enough fluid in your system.
The A/C blower motor located in the front trunk is an important part of the system that occasionally fails. The blower motor helps the front condenser cool the refrigerant in the system by blowing air over it. Check the proper operation of the motor when the system is running, and replace it if necessary.