Porsche 964/993 Climate Control Unit repair

CCU removal

Before you begin, tape an old towel or similar to the dash just beneath the CCU. This will protect the surface and avoid damage. Remove the CCU from the dash with normal Blaupunkt radio removal tools. You will be able to get your hand in behind the unit and gently push it out. Set the wiper switch to it's up most position so it is not in the way as you extract the CCU.
Disconnect the two connectors on the back. They are mounted in opposite directions.

Front panel removal

The front panel is removed by pressing the three brown tabs on both sides of the CCU. Gently, pull the front out of the frame, taking care not to damage the connector inside. Unplug the connector and remove the front face.
You can either leave the rubber air tube in place, or pull it out. It's very easy to refit.

Front panel disassembly

Pull out both rotary knobs and the two slider knobs. Remove the two nuts on the rotary knobs, see picture. Depress the two plastic clips at the centre of the PCB and remove the board. Remember to pull out the temperature sensor assembly and clean it. Be careful not to break the leads. Check the two light bulbs while you're in there. See picture.

Front panel controls disassembly

The potentiometers for temperature and fan speed control can wear out, you could try to clean them with proper electronic contact cleaner. Do not use any 5-56 or orher oily stuff, this will only make things worse. The switches can also be disassembled and cleaned. See pictures. Spare part switches are available, at least for the resirc/defrost switch (944 653 921 00).

PCB removal

Remove the four screws on the back connectors, see picture. They can be either allen or normal phillips type. The two sandwiched PCB are mounted in slots on the side walls of the CCU frame. Slide out the PCB through the front. Be sure to fit the PCB in their side wall slots when refitting. Failing to do so can have catastrophic results due to electrical shorts from the CCU metal parts.

Cabin air temperature sensor

When dust or grime gathers around the sensor that measures the air temperature inside the car, the temperature will gradually fall during longer drives. To compensate, the temperature setting could be increased, but could eventually end up at the max setting giving either cold or very hot results. Air is sucked through a duct by the CCU fan, a thermistor inside the duct measures the air temperature. To ensure this works accurately, disassemble the front, remove the PCB and clean the thermistor and surrounding assembly. Be very careful, the thermistor leads can be brittle!
The same temperature decrease symptom can also be caused by a inert CCU fan due to electric failure or wear. This will stop the air flow through the sensor, and the normal heating inside the dash/CCU unit will be detected as air temperature increase causing the CCU to gradually stop the hot air flow to compensate for this. A defective CCU fan will be flagged as Truble Code 45 (Inside sensor blower motor) when reading the OBD interface.

No CCU fan shutdown

The CCU fan is supposed to stop within 10-20 minutes after ignition off. An old fan can be rather noisy and if so, replace it. Any mechanical repair on it will not last. I've tried, but here's a Rennlist thread with how-to.
The current drain on the battery at ignition off should be around 50mA. If the CCU and fan does not shut off the current drain is about 500mA. Pull Fuse #1 to check if the CCU is the item in question. The transistor controlling the power control relay can fail, it's a common BC 337 transistor. An easy fix without having to split the sandwiched PCB boards.
While in there, check the two CCU front panel lights 12V 0,9W.(Porsche part# 964 641 920 00) Also clean the CCU inside air temperature sensor.
The process is explained in this thread on Rennlist.

Servo driver failure

The servo mechanisms are driven by high power opamps in the CCU and the electric servos are equipped with a potentiometer for position feedback. The servo motors are powered directly from the CCU opamp outputs to enable them to turn either way. If a servo gets stuck due to mechanical or electric failure the driver IC inside the CCU can be overheated and/or damaged.
The CCU opamps are TCA 2465 Single-in-line integrated circuits, and somewhat difficult to obtain these days. To point out any defective opamp ICs you may have to do a little fault finding.
If you are "lucky" you see a very faint brownish colouring on the PCB around the solder leg row under one of the TCA's due to overheating. This may be visible from the solder(outward) side. Also, the solderings can get somewhat dull or grey due to the heat, and not have the shiny solder "blankness". My web shop Bergvill F/X stock the TCA 2465 Opamp.
It is possible to test the otputs of the driver opamps while the CCU is in the car. The voltage should swing from minus to plus 12V (polarity should be reversed) if you turn the temperature knob from min to max and back. This would be the movement required by the flap, opening and stopping the airflow. Connect a voltmeter between ground and one of the outputs. The voltage should be either +12V or GND. (or rather 11V or 0.7V due to the internal transistors) Beware that if there's no or a faulty servo connected to the tested output in normal CCU operation, the CCU will shut down the servo as soon as the potentiometer feedback is not reported correctly according to any applied motor movement signals.
You will therefore only see a short glimpse of the driving signal if the servo is damaged. 993 CCU units have an inbuilt test sequence program that can exercise all servos and actuators, and this is done without the feedback signal. This is therefore very convenient to detect any faults. You'll have to use an OBD tool to activate the test program.
The CCU has two connectors at the back, a 25 pin K-connector and the larger 35 pin larger G-connector. See picture above. Pin numbers are printed on the connectors. See the picture for the internal connection diagram. Note that K6 and K14 share the same opamp output as well as K15 and K3.