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964/993 HVAC fault overview

Common 964/993 HVAC faults
 

No A/C cooled air
This is the most common HVAC problem in the 964/993, and almost always caused by too low refrigerant level. The refrigerant pressure control switch will stop the A/C compressor to avoid damages if the pressure is too low. It is also possible that the compressor clutch 12V lead has been disconnected.
Get an A/C specialist to check and refill the A/C system and ensure they use an electronic sniffer for leaks or put in dye to spot them. Be especially aware of leaks in the evaporator, this is a weak spot in the 993. A sniffer can be poked through the pollen filter housings to reach the evaporator, as well as through the resirculation opening under the dashboard. Here is a DIY for changing the evap. It is possible to measure the pressure switch signals, see the HVAC Details page for details. 

Always heated air, or no heated air from dashboard air nozzles
The mixing flap servos, one or both, have probably given up the ghost, stuck in either open or closed position. These two servos control the heated air flow on each side of the dashboard.
Check flap servo movements, see this page. It is also possible to use a Porsche compatible OBD tool to read out any fault codes.

Always heated air or no heated air on one side of the dashboard
See previous fault.

No cold (outside) air
This is either due to a faulty fresh air intake flap servo, or the mixing flaps may be stuck in open position allowing heated air to overshadow any fresh air.
Check flap servo movements, see our HVAC test page. It is also possible to use a Porsche compatible OBD tool to read out any fault codes.

Air temperature decrease over time
The cabin air temperature sensor gets dirty and clogged with dust over time. The CCU fan is also known to fail after many years of service, this is necessary to maintain an airflow over the sensor.
The CCU enclosure gets quite warm during use, and if there is no air flow over the cabin temperature sensor the CCU will be fooled to think it is very hot in the cabin. It will therefore gradually close the mixer flaps as the CCU warms up. The user often turns up the temperature setting to compensate, ending up at a very high or max setting after an hour or two. This results in an almost binary temperature control, very cold or very hot.
Check CCU fan operation and/or clean the temperature sensor in the CCU front panel. See the CCU details page for details. It is also possible to use a Porsche compatible OBD tool to read out any fault codes.

Sudden change of air temperature

Sudden changes of air temperature without moving the temperature knob can be caused by a worn temp knob potentiometer in the CCU, or a disconnected or faulty inside temp sensor. The sensor can easily be damaged when you dismantle the CCU front. See our CCU details page for more info.

Front fans turn off intermittently or will not work on maximum
This could also be experienced as too low air volume or weak fan operation.
This is most often due to the rear fan is not operating as it should, and the CCU is shutting down or slowing the front fans as a inbuilt safety measure. See our HVAC test page for a test procedure for the rear fan. It is also possible to use a Porsche compatible OBD tool to read out any fault codes.

Erratic front fan behaviour can also be caused by a worn CCU fan control potentiometer. Tryto slightly wiggle the knob. See the CCU details page for details.

Front fans turn off when pressing Defrost button
The Defrost button is a shortcut for max fan speed and max temperature. The rear fan is therefore supposed to start running at high speed. If it does not, you will expereience problems exactly as in the previous fault.

Mixer servo(s) does not move proportional to temperature setting
The mixer servos shall move simultaneously and according to the setting of the temperature knob. If one or both servos are moving only between extreme positions, even with small changes on the temp knob, this might be due to a defective mixer chamber sensor.

No recirculation of air
This is only a problem in the 993. The vacuum operated recirculation flap is not moving, most probably due to a leak in the vacuum system. A common problem is a disconnected hose, maybe done by a careless car stereo installer. Check the vacuum hose connection to the recirculation flap actuator as well as the electric valve connections. See our HVAC details page for more on this.

Wheezing or whooshing air sound in dashboard
Also experienced as too low air volume or weak fan operation.
This is most probably due to a faulty fresh air intake servo or an inert recirculation flap. The front fans are trying to draw air through closed flaps or other orifices. Then, a sudden opening of the fresh air, recirculation or mixer flaps would cause a rush of air, thus a whooshing sound. This could especially be experienced when turning the temperature knob up and down, since high fan speed is causing a low air pressure in the mixing chambers.
Set fan speed to maximum and temperature knob to minimum. Set recirculation to off. Both mixer flaps should close and the fresh air intake should open. Also, try maximum temperature: Both mixer flaps should open and the fresh air intake should close. Press the recirculation button: The fresh air intake should close and the recirculation flap open. If it does not, the same wheezing occurs. You can also use a Porsche compatible OBD tool to read out any fault codes.

Pulsating wheezing air sound
This is most probably caused by failing mixer flap or fresh air intake servos. It makes it difficult for the CCU to control the heated and fresh air flows to control the temperature, depending on what position the flaps are stuck in. This can cause an oscillating action on the working flap servos. See No cold (outside) air above.

A/C condenser fan intermittently run on high speed
The low speed fan resistor/ballast is damaged.
The condenser cooling fan is supposed to start running at low speed as soon as the A/C button in pressed on the CCU. It should only run on high speed if there is need for excessive cooling. If low speed does not work, the pressure in the A/C refrigerant system would increase due to the lack of cooling effect in the condenser. The three-level pressure switch will eventually turn on fan high speed. This cools things down fast, and fan stops again. The cycle then repeats. This is not a good situation, you do not want such high pressures in your system, especially in a 993. The 993 evaporator is prone to failure and leaks, a faulty low speed fan resistor may be one of the reasons. See our A/C/Oil Cooler page for details.

Condenser fan pulsating on high speed when turning on heat
Fan is pulsating on high speed when the temperature dial is taken off minimum. Maybe the Climate Control Unit is not original to the car? If you install a Porsche 993 CCU in a 964, the car wiring must be changed to accomodate the different CCU harness wiring. At least six wires need to be changed. The 993 unit is easliy discernable by the double A/C switch. 964 CCU's have only one big A/C button.

CCU fan whine, rattle or unpleasant sound in dashboard
Some might think they have a very noisy clock in the dashboard, but this is often a failing CCU fan. The fan needle bearings wear out, and the fan may be very dirty. You could try to clean the fan rotor, but be prepared to change it, a repair may not last very long. See the details page for more on this. We stock a spare fan solution.

Battery draining, CCU fan does not stop
A faulty transistor in the CCU causes the unit to not shut down properly at ignition off. It shall stay on for up to 20 minutes at ignition off. See our CCU details page for details.

Front fans always on, even when ignition is off
A faulty "Final Stage", the high current driver for regulating the front fan speed. This unit is mounted just behind the fuse box in the trunk, under the black plastic cover. Pulling fuse 1 will stop the fans.

Rear fan always on, even when ignition is off
The rear fan is supposed to run at ignition off for up to 20 minutes to cool the engine, depending on the temperature in the fan air tunnel. If the fan does not stop, the rear fan temp sensor may be disconnected or faulty, or the fan relay may be damaged. (the relay sticks due to wear) The connectors for the air inlet vacuum actuator and the temperature sensor can be mistakenly reversed. (993 only)
See our HVAC details page for more info.

Rear fan pulsating on high speed
The low speed ballast resistor is faulty, or the rear fan relay may have failed. Pop open the relay to inspect it for damage. See our HVAC details page for more info.

No front or rear fan operation, no servo movement
Check fuses: Fuse 1, (Terminal 30) Fuse 17,(Terminal 15) Fuse 28. (Terminal X) See our HVAC details page for more info.