home > tech tips > bruce's tips > climate control systems

Climate Control Systems

Heating, Ventilation, Air Conditioning and Climate Control

Air Conditioner/ Climate Control, a brief history

1978 -1979

Early 'cable' Slide and Vacuum and Electric Slick Control Unit. Only lasted 2 years. Air Conditioner Compressor - early Bosch type and early support brackets. Air Conditioner Condenser - early small versions Air Conditioner Condenser Cooling Fan - early small version Comment - a good early attempt

1980 - 1983

Big leap forward M573 - Manually operated Air Conditioner, not climate control, has a separate Air Conditioner push button. M563 - Full Climate Control. No Air Conditioner button Air Conditioner Compressor - large 10 Piston N/D Compressor, high quality, rebuildable. Compressor Air Conditioner Condenser - new larger type Air Conditioner Condenser Cooling Fan - new larger type Comment - 'works very well if allowed to'.

1984 - 1985

Saw the return of the separate Air Conditioner Button but this time only full Climate Control available, which is an electronic Control Unit in its own right apart from the other main S1 & S2 Control Unit, is different to the earlier versions. (Note S1 & S2 does not indicate model of car - but the control unit) 1985 First appearance of the rear Air Conditioner as an option (expensive one at that) Note - rear Air Conditioner is a separate Evaporator, TX Valve, Fan Motor, Relays, Switches and Hydraulic Plumbing. 1986 Similar to '84 & '85, but the use of more exotic materials, first appearance of lightly alloy tubing for Air Conditioner pipes and better insulation materials.

1978 - 1989

1. Same Compressor
2. Different AC hoses and pipes 
3. New different Air Conditioner Condenser. In fact a different Condenser for Manual Transmission and a different one for Automatic. The Manual one is a bit taller because the Auto version has another oil cooler above the Air Conditioner Condenser 
4. Large variable speed Twin Electric Rad/Cond Cooling Fans.

1990 - 1995

S4 & GTS Different Air Conditioner ND Compressor and different cast support brackets and new Air Conditioner hoses and fittings. 1993 - 1995 928 GTS Factory fitted system that's purposely built for R134A, which like all other types of cars that use R134A, the Air Conditioner hoses are of higher Burst Pressure. Air Conditioner Seals and O Rings are of new Material TX Valve or Valves are different Compressor Internals are optimised for R134A

top >

Some important notes:

Like most expensive cars with climate control, what comes out of the vents (air flow & temperature) is the 'only' common denominator in the mixing of hot (air from the heater) and cold dry (dehumidified) air from the Air Conditioner evaporator.

The heater core full of coolant is situated in the heater box (inside dash) below the Air Conditioner evaporator and both are almost directly behind the main centre vents.

So by definition, climate control is the mixing of hot air (if needed) and cold air (from the Air Conditioner) to obtain a set temperature inside the cabin.

Note: this only applies to cars with climate control, which is most, because there are some early 928 models which don't have climate control, which means if you want a higher temperature you slide the heater slide knob backwards and forwards.

From 1983 Climate Control is standard

If the 928 has a small round vent opening on the left of centre console that's the opening for the air (cabin temp) going to the interior temp sensor, which tells the climate control, control unit the inside temperature. The outside sensor which in early models is found in the left hand front guard, and the later ones in the left hand outer mirror body which informs the climate control unit the outside temperature.

Depending on the temperature selected the control unit will send individual voltage signals to a bank of electronically switched vacuum solenoids, which will send vacuum to the relevant flaps and vents opening and closing, and importantly the heater tap/valve.

This therefore means if there is insufficient vacuum either being supplied by the engine or there's a vacuum leak the heater tap will open when it should be closed. This will give the impression that the Air Conditioner is not working.

Note: The Heater Tap is closed fully with maximum vacuum.

So, before even thinking about doing anything to the air conditioner, the heating/ventilation system it's worth investigating to see what the problem is.

Example - using a vacuum gauge, measure the amount of vacuum at the heater tap, with the engine running the vacuum should be very close to the manifold vacuum, Approx 19" hg.

Note: remember if the vacuum is tested on engines at higher altitudes that reading will drop. At Sea Level = 19" hg, at every 1,000 feet the Vacuum will drop an inch.

Measure the vacuum at the Heater Tap in series and in parallel to check if the vacuum diaphragm of the Heater Tap is leaking. The Heater Tap is worth removing and applying a separate vacuum pump to close the tap and checking to see if its internal coolant stop seal is sealing, because it could look closed, but if old will allow hot coolant to get past slowly.

Next if the vacuum to the Heater tap is low, anything below 15" hg is too low and the Heater Tap will creep on, check Vacuum from the engine to the Vacuum solenoid bank of valves, sometimes the vacuum one-way valve restricts vacuum a bit too much.

Next check that the Vacuum has restored to the bank of solenoid valves, check and isolate any vacuum leaks that go off to the individual vacuum operated Flap diaphragm units, remember of course that if any of these leak vacuum, then the vacuum to the heater tap will be low depending on how many are leaking.

I have found over many years, which one's will and do leak. Be it the Recirculating Flap or the Centre Comb or other Vacuum Diaphragms. There is an inexpensive way of rectifying the problem without having to remove the entire dash and heater box and replacing which is not viable due to the cost involved.

top >

Common items for improvement for cars that were designed for R12 Gas or Switched over to R134A:

The first item to know is that the R134A is not as efficient as R12 and will expand more and the condenser will get hotter - which means it will need as much cooling as possible. 928S4 onwards is no problem because of their twin large electric fans, but earlier cars the earlier cars ('79-'86) need;

a) Properly working Viscous Fan Clutch (main radiator cooling fan), easy to check when hot because if worn out during idle, particularly Automatic Transmission models at idle in gear the fan is running very slowly and with a worn Viscous Clutch it will drag almost no air at all through the radiator and Air Conditioner Condenser.

b) The R12 system was designed with a temperature switch on top of the receiver drier, which would in turn, turn on the Electric Condenser Fan - usually too late and would build up too much pressure and blow off hose or safety valve within the drier. The solution is to fit a separate relay to delete the old switch and using the wiring for the switch and extra wiring the relay will allow the fan to run when the compressor is on. (Similar to most modern cars).

top >

Main, age related problems

The common item is an electrical disconnection of power supply to the Air Conditioner clutch when they get between 12-15 years old on average (all models) even though the Air Conditioner button illuminates the Air Conditioner clutch doesn't engage;

a) Most common one is a worn out Evaporator Switch (which is easy to diagnose and replace)

b) Same symptom as above but the Evaporator Switch is functioning and of course the Air Conditioner is fully charged, the Air Conditioner main Slide Control Unit has deep within it a circuit that will become intermittent in operation because of the amperage it has to cope with over many years, replacement of entire unit is not needed usually. By removing the amperage from this circuit by wiring up another separate relay and knowing which wires to interrupt you can make this electrical circuit survive for many more years, simply by removing the amperage, that over many years will damage the circuit. This option is inexpensive and effective.

c) Like all Air Conditioner systems on all cars and all makes the hydraulic side of things don't get better as they get older. All Porsche cars up to '93 were on R12 designed systems, that means the hoses (multi layer), "O" Ring Seals, TX Valves and Seals within Compressors and even condenser design is different to R134A design from '94 onwards. Remember the Air Conditioner hoses can be remade with R134A hoses if needed TX Valves (2 different types) can be replaced with R134A types. N/D Compressors can be overhauled with R134A compatible Seals.

You pay for what you get, if you don't mind having ongoing leaks and paying for Air Conditioner gas over and over again but if the car is over 12 years old then maybe the replacement of all the common perishable items like the above may be a good idea.

d) Systems that have over the years been converted from R12 to R134A and by now most have the proper removal of R12 Lubricant, so the R134A Lubricant doesn't mix and turn to sludge (most that I have seen have been good - chemically cleaned). But, I have seen a couple of cases where the 'clean outs' have been poor and from there the problems flow out and it can take a year or two, but Semi blocked Condensers (internal) and Compressor problems.

Buchanan Automotive
Unit 2/2 Paton Pl, Balgowlah NSW, 2093
(02) 9948 2651