Air Cond

Technical Information and Restoration Parts for the 928

Overview

My experience with air conditioning is relatively recent. Most cars I worked on in the past did not have air conditioning. The 78 I bought in 1979 has a fully functional system never needing any attention in all the years I have owned the car. However it is extremely common to find 928s with non functional air conditioning systems. In fact a lot can be learned about the overall condition of the car from the state of the air conditioning components. An owner neglecting maintenance on the air conditioning is probably an owner who has neglected a great deal of other items.

To start lets review the components of an air conditioning system and then move on to the specifics of the 928. The 928 has a fairly simple and straight forward system consisting of the condenser (that radiator in front of the engine's radiator), a dryer (the round can with the site glass attached to the condenser), the compressor (located on the lower right side of the motor so it can be hammered on by every road hazard possible, the evaporator (inside the car and hard to see) and the expansion valve (the silver block on the back of the evaporator just behind the engine well.

The air conditioning system is driven by the compressor. An electric clutch pulley on the front of the compressor spins constantly. When cooling is required an electro magnet on the clutch connects to the compressor causing it to compress the Freon in the system. The compressed Freon is pumped into the dryer then the condenser before reaching the evaporator in the dash. A fan blows over the evaporator sending cool air into the cabin. The expanded gas is then circulated into the compressor where the cycle starts again. An electric fan and the air blowing into the engine compartment removes the heat from the compressed Freon. The dryer contains a desiccant which removes any potential moisture.

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Auxiliary Fan: There are a few other components to the system that vary with different model years. The auxiliary fan in front of the radiator is switched on from two sources. If the radiator temperature rises above the limit switch temperature at the bottom front of the radiator the switch will close operating the fan. On most models the auxiliary fan will come on as soon as the A/C is engaged. All the early models start the auxiliary fan should start when the A/C is turned on.

Sensors: On the drier there are two sensors. One is a high end temperature switch that will open if the temperature on the dryer becomes too hot. The temperature switch can be removed without bleeding out the Freon. The other sensor is a low pressure switch. The compressor clutch will not switch on if there not any pressure in the system. Less than a single can of Freon is enough to trip the switch. The switch cannot be removed with draining the system of Freon.

Inside and Outside Temperature Sensor: Starting in 1980 Porsche introduced climate control where the HVAC system senses the outside and cabin temperature and operates the A/C based on the differential between the two and the set desired temperature on the dash controller. The inside sensor is embedded in the center console with a small fan that blows the cabin air over the sensor. The exterior sensor is located under the front of the left fender inside the alternator cooling tube. Air pumped through the alternator for cooling passes air over the sensor inside the tube. If the exterior sensor is bad or disconnected the A/C will only operate in the extreme cold setting.

Secondary Auxiliary Air Valve: Since the A/C compressor puts a load on the engine the idle speed has to be adjusted so the engine will not stall on idle. The LH injection system adjusts idle speed electronically. The older cars pre 1985 use an electric solenoid to allow additional air into the intake system. The valve is located on the engine cross bar.