Like most systems in the 928, the HVAC (Heating, ventilating and air conditioning) system is more complicated than most. The Air conditioning equipment is very similar to any other vehicle system except for the use of ‘O’ rings - most new systems use compression joints, but in the earlier days of A/C in Europe, ‘O’rings were considered better at absorbing vibration and temperature extremes found in cars.

Air Conditioning General Principles

The principles of Air conditioning have not changed since the ‘fridge was designed - an inert gas is compressed and passes through a condenser - basically a radiator - which cools the hot gas and turns it from gas to liquid at a raised pressure. Any moisture is removed in a dryer (water in the system will freeze and cause all sorts of problems) and the liquid passed through pipework to an expansion valve and directly into an Evaporator. The expansion valve is essentially a restrictor that causes the gas to partially vapourise , and then, as it passes into the evaporator it expands as the pressure drops, and turns from a liquid to a very cold gas.

The 928 A/C system initially used a Bosch/Behr compressor from 1979 to 1980 and then switched to Nippon Denso. Yup, Japanese! The most typical 928 Nippon compressor type is the 10PA 20C with various pipe connections dependent on year. All original systems up to MY 1993 used R12 as a refrigerant, or Freon which is now banned throughout Europe as it is a principle Ozone destroying Gas. It is still used in USA and Switzerland, but is becoming less and less common. The replacement Gas is R134a which is inert but not outwardly compatible with R12 for reasons stated below.

The compressor in an automotive A/C is lubricated by oil originally injected into the gas inlet port when the system was first charged. In the case of an R12 application the oil will have been mineral. R134a gas has smaller molecules than R12, and will not carry mineral oil around in suspension, and for that reason a synthetic oil is used  (Usually called a PAG oil). These two oils do not mix so traditionally the only way to change gas was to completely clear the system of mineral oil and start again. A very expensive process. For this reason ‘drop in’ gasses were produced (R413a, confusingly similar to R134a) being the most common. These gasses are compatible with mineral oil because they are a blend of gasses which will carry mineral oil around with them. To confuse matters even more, there is now also a ‘drop in’ oil which is a blend that will mix with the mineral oil, will be carried around the system, and will also swell the ‘O’rings to improve the chance of sealing.

Now we come to the problem and grey areas.

 1.  When a system leaks gas, oils leaks with it (because the oil is being carried around, remember) and quite   often a leak area can be identified by the presence of oil around a joint. Unless this oil is replaced when the system is gassed it is lost. A 928 should have 350cc of oil in the A/C system, 250 of which should be in the compressor. Further more, there is no way to tell how much oil there is in an automotive system.

 2. The ‘Drop in’ gasses are 80% R134a and a blend of other gasses - if the gas leaks, which constituent has leaked? basically one cannot tell how much oil, and of what type, and how much gas, and of what type one has in the system unless it has never been touched, or has been completely converted to R134a!    

 3.  R134a has smaller molecules, and to stop it leaking out of the hoses, a new ‘barrier’ hose was developed, basically a dual walled hose. If you took a brand new R12 hose, and used it with R134a it would undoubtedly leak. However, a used hose does not (necessarily) leak. Why? Conventional wisdom suggests that the R12 and the oil forms a skin on the inside of a used hose that stops the R134a from escaping

4. There is a school of thought that it doesn’t matter that the mineral and synthetic oils don’t mix. The Mineral oil drops down to the bottom of the system (ie. the compressor) and stays there. Fine, that’s just where it is needed. The PAG oil then circulates around the system and lubricates the other bits. Simple!

So, where do we go from here? Put simply, we can remove any existing R12, identify and fix the leaks, modify the system to R134a (which includes changing the charging connectors) replace as much oil as possible with a dual compatible oil and recharge the system with R134a (We only charge toe 80% using the new gas).

See next page for details.