Porsche has always been successful at combining form and function; a history of creating an environment that dazzles the senses yet tames the road for the few that dare. With all of the great accomplishments that Porsche has achieved through the years, it has left one aspect a bit behind. So many Porsches, yet only a few were blessed with what the masses would consider a functional AC system. At best we can hope for a reduction in humidity along with a slight cooling effect. Worse case however, is not having it work at all! Most 911’s that were fitted with factory air all share similar components. As the years went on the 911 was fitted with better condensers, lines and refrigerant. It hasn’t been until the last generation of the 911 that the AC was the real thing. All 944 owners and 928’s were equipped with an AC system that was fully functional - in some cases all too well.

To correct issues with the 911 AC system we have to first understand its inner workings. The system is created of a compressor, condenser, evaporator, expansion valve, dryer and blower. What makes the 911 so deficient in this department is the inability to force constant air over the coils of the condenser thus cooling the coils down. The system is filled with refrigerant, when you hit the switch the suction side of the compressor draws in refrigerant gas from the outlet of the evaporator.

Once the refrigerant is drawn into the suction side it is compressed and sent to the condenser where it can then transfer the heat that is absorbed from the inside of your Porsche. As air passes over the condenser in the rear of the car by drawing air through the engine lid and ultimately through the condenser, it transfers the heat. As this gas is cooled by the condenser it exits the condenser as a high pressure liquid. It then passes to the evaporator where the cooling begins. The refrigerant in liquid form enters the bottom of the evaporator as a low pressure liquid. The warm air passing through the evaporator fins causes the refrigerant to boil. When the refrigerant liquid begins to boil, it can absorb large amounts of heat. This acquired heat is then transferred off outside the vehicle. While this heat transfer occurs cooling the interior of the car, warm air passes over the fins of the evaporator coil and the moisture in the air condenses on the surface. This is the puddle of water you see when you stop or when you pull into the garage.

Now that we have established how gas changes into liquid thus absorbing heat and removing it leaving us with cool air, we need to know how to control it. The evaporator functions best at 32 degrees Fahrenheit. To maintain the desired temperature the system passes through an expansion valve that controls the flow of liquid to the evaporator. By doing so it keeps the evaporator from becoming too cold and freezing up. If you have ever noticed that you car goes on full blast blower for no reason it is because the temperature of the coil has become too cold and needs to defrost. If it is too cold the coil freezes and air can no longer pass through it to the inside of the vehicle. The temperature control **** regulates the temperature of the coil thus changing the flow to the evaporator. This system is much like a thermostat in your house.

Because the expansion valve is devised of a very small valve it is important to keep it clean and clear of deposits and moisture. This is done by the drier located in the driver’s side wheel well. This is also the location of the site glass to check the refrigerant level. The drier carries a ball in the window and as liquid passes through the drier it causes the ball to float. As it floats to the top the system indicates that it is full. Anything less, the ball will drop to the level of the refrigerant.

There are many common issues with the 911 AC systems. The most common is the style of hoses and fittings used on the system. The molecule size of R12 in general is much larger than the new R134 systems. The hoses in the earlier production of the 911 were made of a material that in time would allow the R12 to seep through the pores of the runner and past the fittings. In the days when a can of refrigerant was fifty cents in the grocery store it was not a concern. One sure way of retaining the gold-like substance known as R12 is to change the type of hoses you have. It is worth the added cost of using R12 over converting. R12 still cools better for this application.

Another known issue of failure is the front condenser in the trunk. The 911 actually carrier two condensers, one in the engine lid and the other in the trunk forward of the spare tire well. There is a blower attached to the condenser drawing air in from behind the bumper and pushing it out through the condenser to the ground. This blower is the only thing preventing the refrigerant pressure from getting too high and snapping a belt or hose. If you are continually burning AC belts up, now you know why. The motors take in a lot of road grime and do not hold up well to this. The earlier wiring also was an issue as it was un-fused. Too often the wires would get hot and burn. The easiest way to check if the condenser blower is working is with the AC on. The unit should come on immediately. You can feel air pushing to the ground just under the front nose of the bumper.

Other known issues are faulty driers that allow moisture in the system, or low refrigerant levels as a result of a leak. We have covered the lines but the AC compressor seal is often a culprit as well. This is usually indicated by a moist oily surface just under the pulley of the AC compressor. While these are the most common issues and known problems with the AC system, all functions from the interior evaporator fan to the expansion valve can fail.

Recommendations to help with longevity of your system are simple. Inspect the lines and fittings to ensure no leakage or cracking. If it is apparent that they are leaking, have them replaced. Run the AC system 5 minutes a month to allow the oil in the system to keep the seals and lines moist. This is not a myth and does work. A nice upgrade is to add yet a third underbody condenser or upgrade to the 964 style with frontal condensers and fans that constantly pull air. The upgrades do help!


Good luck and when in serious need of the AC, roll the window down and drive faster.

 

AC flow chart