Many 964 drivers, myself included, have experienced problems with the heating and ventilation system. This is sometimes referred to on Rennlist and other mailing lists as the HVAC, climate control or air conditioning system. The system uses servos to open and close valves that control airflow to the cabin. These often fail and cause various problems but they can easily be removed and possibly repaired. This article shows how I removed and fixed my faulty servo.
Disclaimer, for what it's worth:
I'm not a qualified Porsche technician so you should take what I write with a
pinch of salt.
Symptomatic of many of these problems is a lack or excess of heat on one side of the cabin. My problem was a lack of heat on the driver's (LH) side. There was no warm air at the feet, face or screen vents. The passenger side worked perfectly with variable heat from all vents. So at least my passenger was comfortable, even if I wasn't. Another symptom of this problem was that in wet weather my screen would not demist properly because no amount of cold air was enough to drive off the condensation. Winter driving was definitely a chore.
I don't know nearly enough about the system to describe it completely and in
detail but this is an approximate description that will illustrate where the
servos fit in and what they're there for:
The air is blown from the engine compartment by the first of three fans forward
and into the cabin. (This fan's motor, also called the rear blower motor, often
fails - mine did - and is another common discussion point on Rennlist). A second
fan blows air into the cabin. Five servos actuate the valves that control how
air reaches the cabin. A third fan only operates when defrosting the windscreen.
Picture 1 illustrates which controls operate which components on a car which
does not have aircon fitted.
Picture 1 - Dash Controls
At this point I'm going to concentrate on the servo that failed on my 964. You will notice from the top right in Picture 1 that the temparature control knob actuates two servos, which I have called 2 and 4, and these control the valves for mixing warm and cold air on the driver's and passenger's side. These servos should operate in unison so that the same temperature is achieved on both sides. If one servo fails however, the valve is stuck in the position it was last in and that setting is now 'fixed' for that side of the car. This is why the thread titles on Rennlist are almost always something like 'half a heater' or 'driver's side heating not working'. It would be unlikely for both servos to fail at the same time.
Servos are simple devices (I found this out when I took mine apart) and essentially consist of :
 | a small electric motor that provides the motion |
| plastic gears that reduce the rapid motion of the motor such that the relatively heavy valve can be operated (slowly) |
| potentiometer circuitry that allows the valve to be fully open or closed or anywhere in between |
| In my case it was a small spring connection in the potentiometer circuity that failed. I'm sure that there are other reasons that these units fail but I haven't had them yet... |
To fix the servo, you need to remove it and take it apart. To do this, you
will need the following:
| Medium Philips screwdriver (preferably short handled) |
| Small flat blade screwdriver |
| Large flat blade screwdriver |
| 8mm spanner or socket |
| About an hour - maybe a bit longer - but once you see how it comes out it's a 5 minute job to put back |
First thing is to locate the servos. These live in the boot (trunk) behind the fuel tank below the line of the bottom of windscreen. A plastic cover secured by 2 screws runs across the width of the car and must be removed - see pictures 2 and 3.
 |
 |
| Picture 2 | Picture 3 |
Here's a bigger picture of the servos:
Picture 4
Now looking at servo 4 you can see that it is on the left hand side of the car (when facing forward) and this is the servo that controls the hot and cold air mixing on the left hand side of the cabin. My car is left hand drive and therefore servo 4 for me is the driver's side control. On right hand drive cars, servo 2 will control hot and cold air for the driver. Note that the arm is pointing to around 5 o'clock.
 |
Here's a close-up of
servo 4 and the rod that connects it to the mixing valve. As you can see from picture 5, the gaitor on the rod is compressed - the valve is closed. The dashboard temperature knob has been set to fully cold |
| Picture 5 | |
|
. |
|
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When set to hot, the output shaft rotates, bringing the arm round to about 9 o'clock (maximum) thereby opening the valve. The gaitor is no longer compressed as you can see in picture 6: |
| Picture 6 |
Here are another couple of pictures showing the fully closed and fully open positions
 |
 |
| Picture 7 - Closed (cold setting) | Picture 8 - Open (hot setting) |
Now comes the really hard bit. Pop the ball joint off using a flat blade screwdriver, pull off the 5 pin connector as shown in picture 9.
 |
It may not be possible to do this if the servo is set on full cold and the arm is directly in front of the connector. If this is your situation, remove the arm by undoing the bolt that holds it to the output shaft using an 8mm spanner. It may help to mark the position of the arm on the shaft although it's straightforward to reset the position on reassembly. It might be worthwhile checking the position anyway (see later). |
| Picture 9 | |
 |
Next, undo the two screws that are visible in picture 6. You won't be able to see them unless your eyes are on the sides of your head like a bird's. Just feel for the screwheads with your fingers. Try and remove the screws if you can. Don't worry if you can't - just be careful when you take the servo out not to lose them (or the two spring washers) in the bowels of the car. Now lift the servo up and out and go back into the comfort of your kitchen to do the rest. See how hard that was?
|
| Picture 10 |
This is what the servo looks like out of the car:
 |
 |
| Picture 11 | Picture 12 |
Remove the brass coloured mounting plate by unscrewing the three screws and this is what you're left with:
 |
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| Picture 13 | Picture 14 |
 |
 |
| Picture 15 | Picture 16 |
Remove the 8mm nut and washer and pull off the arm (picture 15). I marked the shaft and arm beforehand to ease reassembly.
Using a small flat blade screwdriver or similar, unclip the white cover from the black case. Be careful not to damage the circuit board (picture 16).
Carefully withdraw the output shaft and cover:
 |
| Picture 17 |
The two parts now separated look like this:
 |
 |
| Picture 18 | Picture 19 |
You can see in pictures 18 and 19 how the fine spring connections (19) line up with the potentiometer circuitry (18) but I'm not going to explain how all that works because..er.. I don't know. What I do know is that those connections are supposed to be made and if they don't then your servo doesn't work and you (or your passengers) get cold feet, the screen mists up etc etc.
Clean up the connections as best you can with electrical contact cleaner and bend the springs back carefully. Guage how much you need to do this by comparing resistance of the spring (periodically replace the output shaft and gear) and by comparing the positions of each of the spring fingers. Try and bend the spring at the point of the original crease.
Your circuit board may also have connection errors - it may be worth checking these too if the spring contact fix doesn't sort your problem out.
 |
Carefully lift out the circuit board. There will be some resistance as the terminals unclip (see picture left). The motor will now be visible. You can simply remove the remaining components if you wish. Try not to wipe off the white grease if you do. Applying 12v to the motor terminals will confirm whether it's OK or not. If you don't have a 12v car battery in your kitchen you can use a 9v 1604 battery. It's also possible to apply 12v to terminals 4 and 5 (they're marked) whilst the servo is in the car to achieve the same result. I'd like to think you can get replacement motors but I haven't heard anyone say they got one. It's probably easier to replace the unit as a whole anyway. At least you know you won't get cold (or too hot). Check the connections arrowed in picture 20 are good. |
| Picture 20 |
Put everything back the way it came out ensuring as far as you can that the
connections are good. If you tested the motor directly and it worked, now check
that applying the current to pins 4 and 5 causes the motor to operate also. If
it doesn't, you still have a bad connection somewhere.
Now refit your working (or replacement) servo in the car.
For servos 2 and 4, adjustment is as follows:
Apply 12v to terminals 4 & 5 until the servo reaches its end position
Close the mixing valve by pushing the linkage
The linkage should be in line with the arm
If you've dismantled the servo you 'll have to realign the arm as above
Adjustment can also be achieved by screwing the ball joint in and out along its
adjusting thread
These threads on the Rennlist Forums were really helpful to me when fixing my problem
http://forums.rennlist.com/cgi-bin/rennforums/ultimatebb.cgi?ubb=get_topic;f=4;t=002830
http://forums.rennlist.com/cgi-bin/rennforums/ultimatebb.cgi?ubb=get_topic;f=4;t=001405
http://forums.rennlist.com/cgi-bin/rennforums/ultimatebb.cgi?ubb=get_topic;f=4;t=001721
Thanks also to the following Rennlist people (in no particular order) who contributed to the above and other threads:
| Taj |
| Herbie |
| Adrian |
| Randall G |
| Jeff Curtis |
Please let me know if I made any mistakes in this article and thanks for
reading it.
Roly Baldwin
9th January 2003
All pictures from the following model:
Porsche 911 (964) Carrera 4 , Model Year 1991, Chassis Number: WPOZZZ96ZMS430584
