WE OFFER A FULL AC SERVICE
The 928 HVAC system was designed originally in 1977 and was considered cutting edge at the time. Unfortunately the ‘O’ ring sealed A/C system, R12 gas (now considered damaging to the atmosphere and discarded) and the aging bellows derived vacuum operated but fear not! We can make your cabin cool again.
There are three systems which all have to work together for the climate control to work effectively, the gas system, the electrical system and the vacuum system.
Gas system – We now use R134a gas which can cause problems with the R12 ‘O’ rings due to the different lubrication used in the system
Electrical system. The temperature is controlled using a resistance balanced circuit (modern systems use a PID type control circuit). The temperature sensors can fail and the connection to the outside sensor can suffer corrosion. The main controller rarely fails but the transistors In the flap setting motor can fail. Any of these failures results in the system tending to run full hot or full cold.
If the cabin vacuum fails due to a perished bellows in an actuator, then the heater control valve defaults to open and the heater matrix will be full on. Consequently the A/C is fighting the heater and will not be very effective at cooling the cabin.
928 HVAC IS COMPLICATED
Like most systems in the 928, the HVAC (Heating, ventilating and air conditioning) system is very complicated.
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.
The 928 A/C system initially used a Bosch/Behr compressor from 1979 to 1980 and then switched to Nippon Denso. Cars from 1980 – 1990 (S, S2, S4) use the ND 6E 171 compressor Later cars use the smaller, lighter 10PA 20C. The differences and model ranges require different flexible hoses, condensers and fittings dependent on year. Early condensers (up to the S4 have condensers that are now only available through OPC. Later condensers and filter/driers can be sourced by Ritech much cheaper
All original systems up to MY 1993 used R12 as a refrigerant, now banned. The replacement Gas is R134a and more recently R1234yf which is inert but not outwardly compatible with R12 for reasons stated below.
The compressor in an automotive A/C is lubricated by oil when the system is first charged. In the case of an R12 application the oil will have been a mineral type. R134a gas has smaller molecules than R12, and will not carry mineral oil around in suspension. For that reason a synthetic PAG oil is used These two oils do not mix but can co-exist so removing the compressor , re-sealing with PAG compatible seals and refitting with PAG oil will leave very little mineral oil in the system. Changing the filter drier is essential.
When restoring a 928 A/C system we will also remove the shreader valves from the traditional charge connectors and add the new style quick release gauge ports.
The early condensers (up to the S4) were actually very sturdy and only tend to fail due to mechanical damage. Just as well as they are only available from OPC and are eye wateringly expensive.
The later S4 condenser has a design flaw (except for the manual GT version) inasmuch as the lower mounting bracket bolt is about 1 mm too long and eventually frets the fins away and causes a failure – usually approx. 100k miles or ten years! The only option Is a replacement condenser and a shorter mounting bolt