The
most common cause for this is the rear blower not running. The Climate Control
Unit in the dash monitors the air flow and if there is insufficient air pressure
from the rear blower the cabin fans will be switched off. The rear blower
has a dedicated 30 amp fuse (uprated from the original 25 amp part with no
other changes in 1990) in the rear electrical panel together with a relay.
The relay used is identical to the one (or two) fitted in the main electrical
panel in the luggage compartment for the oil cooler and air conditioning condenser
fans. The relays can be swapped for diagnosis purposes.
Any
commercial air conditioning servicing agent can re-gas the 964 system. Although
originally filled with R12 refrigerant the system must be converted to the
more environmentally-friendly R134a refrigerant which requires the filling
valves to be replaced and the system evacuated and refilled with the new refrigerant
and the correct R134a-compatible oil. The correct charge is 660g of R134a
with 19ml of PAG46 oil.
There
is a small sampling fan mounted on the back of the Climate Control Unit which
continues to run for approx. 10 minutes after the ignition is switched off.
This fan allows the CCU to make decisions regarding engine cooling by sampling
ambient temperature. One problem seen very often is the driver transistor
in the CCU fails and leaves the fan running constantly - running down the
battery in a short time.
The
main cabin fans permanently run at the slowest speed - even when 0 is selected.
Switching to speed 1 causes no increase in fan speed. This is as designed.
The
fuse for the rear blower fan (along with those for the oil cooler and air
conditioning condenser fans) was uprated by Porsche in 1990 to a 30 amp part
with no other changes being made to wiring or relays.
A 25 amp fuse SHOULD NOT BE FITTED.
Technical Bulletin 8904 (here)
refers.
The
rattle you hear is caused by the air diverter flaps in the rear wheel wells
being flipped by the reverse air pressure in the cabin created by the shutting
of the door.
It only occurs if the mixer flaps are left in a certain position leaving an
open air path from the cabin back to the heat exchangers. This explains why
you may not ALWAYS hear the noise.
The problem is caused by the deterioration of the heater tubes hidden behind the footwell trim on each side. The tubes are made from an aluminiumised fibre and deteriorate over time to create leaks. The hot air from the heat exchangers then escapes into the footwells and under the dashboard.
Replacement tubes are available from your local OPC - part numbers 964.572.527.01 (L) and 964.572.528.01(R) for around £12 each. Although simply fitted with clamps at each end they are awkward to fit due to the limited space available.
Frequently
Asked Questions - Heating & Air Conditioning |
Why do my cabin
fans stop after a few seconds when heat is required?
|
|
|
|
|
How can I get the
air conditioning re-gassed?
|
|
|
|
|
There is a noise
from behind the Climate Control Unit. What is it?
|
|
|
|
|
There doesn't sound
any difference between fan speed 0 and 1. Why?
|
|
|
|
|
The 25 amp fuse
for the rear blower keeps blowing. Why?
|
|
|
|
|
Why do I hear a rattle at the back of the car when I shut the door?
|
|
|
|
|
Why do I get hot air delivered to the footwells regardless of the controls?
|
|
|
|
|
|
|
|
|
|
| |
|
|
|
|
|
