Porsche 964/993 HVAC fault overview

Common 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's 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's a DIY for changing the evap. It's possible to measure the pressure switch signals, see the HVAC Details page for details. This Rennlist thread explains the air conditioning refrigeration system.

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 mixer flaps operation by turning the CCU temperature knob to maximum and minimum. Both mixer valves should simultaneous open and close accordingly. See details below on where the servos are placed. You can also 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 fresh air intake flap and mixer flaps operation by turning the CCU temperature knob to maximum and minimum. Both mixer valves should simultaneous open and close accordingly. The fresh air flap shall open on minimum temperature setting, and close on maximum. See details below on where the servos are placed. You can also 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, it is used to keep an airflow over the sensor.
The CCU enclosure gets quite warm during use, and if there's no air flow over the cabin temperature sensor the CCU will be fooled to think it's 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. You can also use a Porsche compatible OBD tool to read out any fault codes on the fan or temp sensor.

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 my 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. 
Set temperature knob to minimum and operate the fan speed knob. The front fans should run normally. Set to fan speed 1 and slowly increase the temperature setting. The rear fan should kick in at low speed as soon as heated air is needed. (depending on ambient and cabin air temperature). If there's a fault with the rear fan, the front fans will either go down to low level or shut down completely. Check the rear fan fuse, relay, motor and low speed resistor. You can also 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. Try fiddling with the control, and maybe press or pull the knob a little. 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. See 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 the 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 series 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 don't 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.

Test and repair: Turn on A/C, the condenser fan should start running on low speed immediately. You can also use a knife and pop the cap off relay R14 and press the two relays inside manually with your finger. The fan should then run on low or high speed. The low speed series fan resistor is known to fail due to corrosion and overheating. It is mounted on the condenser assembly. Change the resistor, and be prepared to change the oil cooler fan resistor as well.
Beware, a faulty resistor and resulting high pressure problems can cause leaks in the A/C system. See this 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. Have you changed your Climate Control Module lately? 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.

CCU fan whine 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.

Battery draining, CCU fan does not stop

A faulty transistor in the CCU causes the unit to not shut down properly 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. It's 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 the 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 the 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 the details page for more on this.

Quick guide to test most HVAC components

Use these test steps to determine faults without using a OBD tool. Details on where to find components is found elswere on this page or the HVAC details page.

  • Mixer servos: Turn temperature knob to max and min, the mixer servos shall open and close
  • Fresh air servo: Turn temperature knob to min and max, the fresh air intake should open and close
  • Footwell servo: Move lower slide knob to the right and left, the footwell servo should open and close
  • Defrost servo: Move upper slide knob to the right and left, the defrost servo should change air flow between middle air nozzles and windscreen
  • Main interior fans: Set to minimum temperature, turn fan speed to min and max. Fans should run accordingly
  • Rear (AUX) fan low speed/resistor: Set fan speed to 1 and maximum temperature. The rear fan shall run on low speed.
  • Rear (AUX) fan high speed: Set fan speed to 2 or more and maximum temperature. The rear fan shall run at high speed.
  • Condenser fan: Remove relay R14 and short terminals 30 and 87 to start the fan at high speed
  • Condenser fan low speed/resistor: Remove relay and short terminals 30C and 87C for low fan speed
  • Oil cooler fan: Remove relay R04 and short terminals 30 and 87 to start the fan at high speed
  • Oil cooler fan low speed resistor: Remove relay R04 and short terminals 30C and 87C for low fan speed
  • CCU fan: Turn ignition on, CCU fan shall run
  • Resirculation flap: Press Resirc button on CCU, flap shall open (993: vacuum operated, 964: spring loaded)
  • A/C compressor clutch/refrigerant level: Minimum temp setting, press A/C button, (993: either small or big snowflake) compressor shall engage and run in intervals