Climate Control Systems
Heating, Ventilation, Air Conditioning and Climate Control
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
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
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
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
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
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.
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.
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).
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
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
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.
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.
Unit 2/2 Paton Pl, Balgowlah NSW, 2093
(02) 9948 2651