JohnCh

I need help diagnosing a problem with my car's fresh air ventilation system. The flap that controls the flow of fresh air into the cabin was stuck in the closed position, so I pulled the harness plug from the servo that controls it and applied an external 12v source to pins 4 (+) and 5 (-). The servo responded and opened the flap, so I know the motor isn't bad. However, when I reattached the harness and turned on the ignition, the servo immediately closed the flap regardless of whether the recirculation button was in or out. Does this mean the CCU is bad, or is it still possible that the problem lies within the guts of the servo? If so, how do I determine which is at fault? I have multi-meter, and I’m not afraid to use it – I just don’t know what to check. I've done a search, but haven't had luck finding something that covers this specific problem. Any ideas?

Thanks,
John

Warpig

I am not that familiar with the HVAC system in these cars yet. But from a pure troubleshooting standpoint, if you can manually operate the servo mechanism with an external power source and the thing closes when plugged in. I would try to unplug the same wires from the CCU end of the circuit, leaving the servo and the wiring at the servo hooked up. If it still closes it would sound like a wiring problem, if not I would guess the CCU is at fault in some way.

Nick

ToreB

The servo's consist of a motor with gearbox and a potentiometer. I've seen several examples of burnt potentiometers inside these things. If the pot send incorrect position info due to a fault to the CCU the result is the one you are observing, sadly a very common fault in 993 and 964.
You have to change your servo, it is not possible to mend the potentiometer circuit.
It's a little narrow in there when changing the fresh air servo, but it's easier if you set your CCU controls to Resirc and connect the cable to the servo before mounting it. The servo arm then moves to a position that makes it easier to slip into place. You have to reset the Resirc button to be able to connect the pushrod after the servo is in place.
Regards,
Tore
Here's a pic of a typical servo fault, I'll bet your's look the same:

Body Roll

When you say the flap is stuck in the closed position, do you mean the fresh air position or recirc?

Anyone know how is the recirc flap supposed to operate? When I looked at this in my car, the only time the flap would go into recirc mode was when both the AC and the recirc buttons were pressed and the temperature control **** was set to fully cold.

You might try that setting to see if you get the flap to change positions.

JohnCh

Thanks folks. A little more background. The servo that controls airflow between the dash vents and the defroster vents (upper slider control on the CCU) is also dead. That didn’t respond when I applied an external power source so I took it apart and it looked similar to the picture ToreB provided above. I tried to clean it, but no luck, so I ordered a new one yesterday morning. Unfortunately Porsche is out of stock in the US and Germany so I am still awaiting an ETA.

The servo for the fresh air flap is a different story. What I’m trying to understand is how it operates. There are 5 wires going into it. Pin 4 is the 12V+ and pin 5 is the ground. I’d love to find out what pins 1-3 are for and determine if there is a way to check the corresponding wires coming out of the CCU to confirm that the CCU is still sending the correct signals and it is indeed the servo at fault. That servo is about $170, so I’d rather not spend the money on a new one only to plug it in and find out that the CCU is at fault.

The flap automatically moves to the recirc (air flow closed) position when it is hooked into the wiring harness and the key is turned. Applying an external 12V supply will open the flap to the fresh air position and it will stay there provided I don’t reconnect the harness.

Thanks,
John

ToreB

Hi there John.
Pin 1, 2 and 3 is the potentiometer connections. +5 V on pin 2, GND on pin 1 and the centre tap on pin 3. Output on pin 3 is position dependent.
See complete HVAC schematics here.
If the pot centre tap output either are constant GND or +5V (or any other fixed voltage inbetween) the CCU will try to change the position if it's needed, but will give up and stop the motion if the servo pot doesn't respond in a proper way. It is therefore difficult to test whether the CCU opamp outputs are OK before you change the servo.
However, you could plug the fresh air servo harness plug into one of the other servos to test the CCU operation.
Servo faults are very common, CCU fault is more scarce. If the latter is the case it will probably be fully possible to repair the CCU, see my CCU repair site.
Bets regards,
Tore

JohnCh

Thanks Tore, that's precisely the information I was looking for. I had initially planned to plug in the new defroster servo when it arrives to check the CCU, but given that might not be here for a few months I had hoped a simple multimeter test would suffice. Looks like I'll need to remove the third, and only working servo, and test the CCU with that.

Edited to add:
I just looked at Tor's CCU repair site...Wow! Really well done and full of great information. This should be on p-car.com.

-John

ToreB

By the way, you have two additional servo's behind both footwell covers controlling the hot air flow from the engine. It may be easier to use one of them.
I may be even possible to stretch the fresh air servo harness/cable to plug it in one of the other without removing the servo mechanism. Anyway, good luck John, I can assist in finding any faults on your CCU if necessary. Been there done that. :-)

An OBD Scantool setup will also make it possible to run the diagnosis tests on all CCU servos, cycling all flaps.
Tore

JohnCh

I already tried to stretch the harness and use the plug for the footwell/dash servo, but it's about 1/4" short. I am afraid to try and stretch it any further and potentially create new problems (I'm a magnet for Murphy's Law). I'll look into the other servos you mention and see if they are any easier to remove for the test. Thanks again for the help!

-John

DIS

Hi

My fresh air servo is not functional, and I get error 24, as well as not rotation of the servo when I press the Recirculate air button.

I have tried to remove the servo, but since it is stuck with the while lever pointing forward, I cannot remove it. Tried to apply 12 to terminals 4 & 5 to rotate the arm but reaction (as expected). I hate to break the arm, as I do have a spare servo (without arm) to replace the defective one (or maybe it is fixable).

Anyone with any suggestions?

Thanks

Don S.

ToreB

I've been able to get my fresh air servo out with the arm in the forward position, but that required a lot of wiggling and swearing. I got it in again as well, but I slightly bent down the edge of the metal frame below the servo.
Cheers,
Tore

Stealth 993

I have a spare servo, you can play with. You can pick it up from me at work in Bellevue, across from the Porsche dealership.

I had the same thing going on with a servo. I took it apart, cleaned it, repositioned it, then it worked.

DIS

Hi and thanks to all

Problem fixed. Servo removed, opened, cleaned and tested. It worked.
Back installed into the car, and it worked. Keep you fingers crossed that it will work for the next 21 years as it has for the past 21!

Thanks