Tore Bergvill

Porsche A/C repair in general

Here's some links to fault finding on the A/C system:

Removal and disassembly

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.

PCB removal

Remove the four screws on the back connectors, see picture. They can be either umbraco 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.

Air temperature decreases over time

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.

CCU fan does not stop, battery draining

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".

Littlediode still stock the TCA 2465 Opamp, I've bought several there.

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)

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.

Airflow control servos

There are five electrically operated servos in the Porsche 993:

2 servos--one for each side--for the temperature mixing flaps that feed the mixing chambers. These servos are in the foot wells for a 993, in the trunk (with the rest of the servos) for a 964.

1 servo for foot well flap control.

1 servo for the fresh air flap

1 servo controls the amount of air going to the center or defrost vents.

The above three servos are in the back end of the luggage compartment, under the windshield beneath a black plastic tray.

In addition there is a vacuum-controlled resirc flap in the dash behind the CCU unit.

Here's a link to a site explaining the servos on a 964 and their operation.

Fan operation

There are a total of 4 fans (5 for a Turbo):

-Rear (or aux.) blower, left side of the engine compartment. This supplies forced heat when running, also helps cool the engine under certain circumstances.

-Taking the interior fan control to over 2 will cause the rear blower to switch to fast-speed, if heat is being requested (two speed fan).

-Left and right interior fans, in the luggage compartment. Speed is controlled using the knob on the climate control unit (CCU).

-A small fan mounted at the back of the CCU for measuring temperature. This fan is known to become noisy and fail over time. See column above.

Interior fan cut out

If the rear auxillary blower fan is seized, or its fuse blown, the CCU will stop the interior fans when temperature control is taken off "blue dot"--say, greater than 21C. The defroster will also cut out shortly after starting.

CCU connector G19 and G1 are the blower motor supply voltage feedbacks.

A new blower motor can be bought separately here and here. This Rennlist thread explain how to do it.

If you have a cup bypass pipe installed and therefore no heater fan, two resistors must be installed to mimic the blower fan feedback signals. Here's a how-to.

Vacuum faults and resirc flap

In the Porsche 993 the CCU controls the operation of a vacuum-based recirculation flap in the dashboard behind the CCU, see picture. This enables the HVAC system to draw air from inside the cabin, combined with a closed fresh air flap.

The vacuum flap operation is controlled by an electric valve placed alongside the servos in the luggage compartment, see picture.

Testing this flap is simple, just hit the Resirc button on the CCU.

The 964 has, as far as I know, a spring-loaded resirc flap with no actuator.

A vacuum leak can cause the flap to be flabby and close with a "whoomph" sound when the engine is revving and vacuum is reduced. This especially occur with CCU fans on maximum setting creating a slight vacuum in the mixing chambers, sucking the flabby resirc flap to closed position.

Finding vacuum leaks can be very difficult, but are often caused by disconnected lines. Maybe your stereo installer disconnected the rubber hose on the actuator in the dash?

Check that all rubber hoses are connected on both actuator in the dash and valve in the luggage compartment. You can suck on the vacuum line going from the valve to the actuator to test for mechanical operation of the flap.

Faulty vacuum lines and components in the engine compartment can also be the culprit for leaks. See the picture for an example, a damaged vacuum diaphragm in my engine compartment.

Here's a complete vacuum diagram of the 993 (PDF).

OBD1 interface

Home made OBD1 interface made from information from the Rennlist Porsche forums. All files for the Scantool software and schematics for constructing the Rennlist serial interface can be found in the zip file at my T-OBD page.

Most portable PC's are no longer equipped with a RS232 interface, I've enhanced this design with a more simple switch config and an USB-based connection, the T-OBD, orderable here.