The Eagle

Hi, today I have noticed that when I press the aircon button, a squealing noise, more a hiss-sound, starts which I suspect is the Temp Sensor fan. It's coming from behind the dasboard, passenger side (LHD). The blower fans are blowing unheated air, but not cooled air.
When I turn the speedknob to 2,3 and 4 : the hiss/squealing sound gets louder, when I turn the speedknob to 0 I can still hear the squealing. When I turn off the A/C the hiss-sound stops after +/- 2 seconds. What may be the cause of this?
Another question; should there be cool air coming through the central vents?
Greetz.

Adrian

The problem is not the temp sensor fan. I suspect your mixing chamber servo valves are not moving to the full cold position. They may be stuck. This is quite common.
The hissing sound I think is the air passing through a jammed set of flapper valves. Which ones I am not totally sure. You need to check which if any of the five flapper driving servo valves in the luggage compartment at the rear are not working. Yes cool air (if the aircon works anywhere near properly) should be coming through the centre vents. What air is passed through the vents is controlled by the differential pressure flapper valves and the servo controlled flapper valves.
Ciao,
Adrian
911C4

The Eagle

Hi Adrian, thanks for your reply.
I've changed a servomotor last year. The one on the driver side which changes the valve cold/warm air. But I've been doing a search in this forum and as Herbie says in one of his posts:
"Starting from the far left (1) to the far right (5), they control:

1) Both foot vents; can be checked moving the "foot air" cursor on the ccunit and looking at the servo arm, or feeling air on both footvents. It's a proportional control.

2) Left temperature; can be checked moving the temperature **** from blue point to red point. This is for hot/cold air mixing.

3) Air recirculation; can be checked with the air recirc. button, looking at the servo moving, or feeling (or not, if recirc is on) air from the frontal external airscope. It is an on/off control.

4) Right side temperature: checking it as in 2)

5) Windshield vents; can be checked by moving the "^" cursor on the ccunit and looking servo moving or controlling airflow from upper vents. It's a proportional control."

I don't understand that there's absolutly any cold air comming in. If one servo motor is broken the cold air should come in from the side where the servo motor is still working.
Am I missing the point here

Adrian

I am afraid Herbie is not totally correct.
The valves are
Fresh air
defrost
footwell
left mixture
right mixture.
The actual vents are controlled by differential pressure changes in the distribution system.
However I can understand that no cold air is coming in if the fresh air valve is stuck shut.
I am afraid you are going to have to dig in and have a look.
However may I suggest you try this little experiment.
Turn the temp control to blue dot position
Turn blower speed to 4 is the squeal evident?
select airconditioning on what happens?

turn air con off
turn fan speed to 0
select defrost what happens?

Then do the air con test again.

Ciao,
Adrian
911C4

The Eagle

Hi Adrian,
I followed you instructions:
1) With the temp control to the blue dot position and the blower speed to 4: I hear no squeal or hiss sounds. When I select the A/C the hiss sound starts.
2) I turn off the A/C and I turn the fan to speed 0, I select defrost and I can feel that airvalves are closing (or doing something else) and air comes out the windows vents at high speed and air stops coming out the central vents AND I hear the hiss sound again... The sound comes from the footwell at the passenger side.
What does this say?
Also I'm not sure if the far right servo motor is doing anything(right when standing in front of the car). You're saying that it controles fresh air, is this one controlled by the air recirc. button? I can hear some valves closing when I push this button. Makes this any sense?
Greetz

Randall G.

Hey Eagle,

What happens when you select recirculation? I know the 993 and replacement (currently being sold by Porsche) 993-prefix 964 climate control unit automatically places the system in recirculation mode when the A/C is turned on. My older, original 964 CCU (part #964-659-047-00) doesn't automatically place the system in recirculation (as I recall). Since you have a '92, you likely have #964-659-047-01, which may do it automatically.

FWIW, I seem to remember following Herbie's guide for servo motor location before, and it being correct. Actually, the fresh air servo motor has to be in the center, as that's where the fresh air intake is physically located. It wouldn't make sense for its servo motor to be all the way to the left or right. I'm not sure if Adrian meant to show title/function or sequence/location with his list above.

Adrian

The locations are correct the functions are wrong. I should have made that clear, sorry. I have been so busy with my 911SC book I forgot all about it until now.
Maybe the footwell flapper valve has failed. Try moving the vent controls to activate the footwell servo which will then open the flapper valve. Slide the lower slider control backwards and forwards a few times.
To check the fresh air servo valve is a little difficult. The best way is to turn the air con off and heating off with temp control in blue dot position. Fan speed to 0. Start engine. You should have airflow through the centre vents. The left and right blower fans should be running a low speed. If this works do not drive off but turn the fan speed to 4. You should get blasted by air. Then in this configuration go for a drive. As the speed of the 964 increases the airflow should start to reduce.
Ciao,
Adrian
911C4

The Eagle

Hi Randall,

I guess you're right, I've been reading this article: <a href="http://www.nethereal.co.uk/964/heater_servos/index.htm" target="_blank">Heater servos</a>
and I'll be checking servo motor #3 tonight.
When I select the recirc. button, it closes a valve, but the temp. of the air stays the same and the hiss sound is still there...
In my car when I switch the A/C on, the system is not placed in recirculation automatically.
Greetz.

Randall G.

Hey Eagle,

Another way to check your recirc. servo--without removing the black cover--is to simply listen/feel for airflow at the air intake, base of the front windshield, center. As you turn the recirc. on and off, you should feel/hear a difference. It should be obvious.

Similarly, you can test your A/C button for auto-recirculation by turning the A/C on/off, with the recirculation button off. If your CCU turns on the recirculation automatically, you will be able to feel/hear the change in air flow, just like you do for recirculation.

Perhaps your recirculation flap isn't closing fully, causing the hiss? It's supposed to be an on/off flap, not one that can be positioned anywhere in-between, like the footwells, center vents and windshield.

I bet you are busy with the book, Adrian. A few of us have since learned a good bit about the 964's HVAC system, but it was Adrian who first taught us the basics.

The Eagle

Hi Randall,
I've taken off the plastic cover which covers the servo motors.
I found out that they all work...
Fresh air is automatically closed when you turn off the car (the pin on the servo motor was at 12 o'clock) And reopens when you turn the ignition on (the pin on the servo was not visible anymore). I found out that when the car runs and you select A/C the fresh air valve closes but not completely, when I pushed the recirc. button, the fresh air valve was closed 100%.
But in all cases the hiss/squeal sound never stopped.
Could it be a bad fan located under the fuel tank which is making this noise?
Greetz

Adrian

There are no fans located under the fuel tank. Only the fuel pump down there. There are two fans located at the rear of the luggage compartment. The two big lumps on the left and right of the servo valves. However they should make a noise all the time not just when aircon is selected.
The servo may be working but maybe it is disconnected from the flappper valve.
Now the squeal could have nothing to do with any of this. You do not have the problem apart from with aircon. Maybe the problem is actually a leak in your aircon system. Either the 3-level pressure switch or the expansion valve. Both of these units are with the servo valves. Maybe you need to have a pressure check carried out on your aircon system. Engage the aircon and stick your ear into the area. You maybe able to feel the leak if there is one.
Ciao,
Adrian
911C4

Randall G.

Hey Eagle,

With respect to your fresh air servo not closing completely when the A/C is turned on, perhaps I spoke too soon when I said that it was only on/off? On my own car, with the updated (993 prefix) controller, it definitely closes the fresh air servo completely (same as using the recirc button) when the A/C is turned on. And, of course, there's no slider to control its position.

For the older model 964 CCU, the owner's manual says this with respect to A/C and the recirculation button:

"For maximum cooling set the fan switch (3) to position 4, close the windows, close the air distribution side side controls (4), turn the temperature selector (5) all the way to the left, and press the recirculation switch(2)."

Using my old CCU, I couldn't feel/hear any change in air flow at the fresh air intake when the A/C was turned on. But, I never checked the actual servo motor to see if it had closed partially.

If you read the 993 owner's manual, where it discusses maximum cooling, there is no mention of using the recirculation button (as it's done automatically).

Anyway, all of what I just wrote doesn't appear to have any bearing on your current problem, as you say you're getting the hiss in both recirc. (servo/fresh air flap fully closed) and with the A/C on.

Now, are you still only getting the hiss only with the A/C on--whether the recirc. button is on or off--or all the time? You initially stated it was only with the A/C on, but just wanted to be sure this is still the case.

If you're only getting the hiss with the A/C on, I would agree with Adrian, in that it may be a problem with your A/C system. If you have a leak, I imagine you would have other symptoms--i.e., lousy cooling.

Adrian, a question for you. When in recirculation mode (fresh air flap closed), A/C on (no heat requested) where does the interior fan draw air from? The owner's manual says "air will be drawn out of the passenger compartment through the rear seats." I don't see any ducting near the rear seats. I also looked under the dash, and could only find the footwell vents, no intake duct.

Now, if the heat was on and recirculation selected, I understand that air would be supplied via the rear blower. In this case, our recirculation mode isn't really a true recirculation mode, as outside air is being pumped by the rear blower across the heat exchangers. That is, there's no way to get air from inside the passenger compartment to pass across the heat source = heat exchangers.

As a reference, I provided a scan of the appropriate page of the owner's manual below. The reference to drawing air through the rear seats is near the bottom left corner of the page.

<img src="http://members.rennlist.com/rgranaas/Owners%20Manaul%20HVAC_125.jpg" alt=" - " />

The Eagle

Hi Randall,
thanks for all your information, it's great to read all this. The answer to your question:
</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica"> Now, are you still only getting the hiss only with the A/C on--whether the recirc. button is on or off--or all the time? You initially stated it was only with the A/C on, but just wanted to be sure this is still the case. </font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">I'm only getting the hiss-sound when the A/C is turned on. When I push the defrost button you can hear that sound too. This seems correct 'cause from your info it says the A/C compressor is switched on.
So like Adrian stated probably the problem is </font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Maybe the problem is actually a leak in your aircon system. Either the 3-level pressure switch or the expansion valve </font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">What do you think? And where are they located?
Greetz

Randall G.

Hi Eagle,

Well, if it only does it when running the A/C or with the defroster on (which runs the compressor), then it definitely sounds A/C system related.

I seem to remember someone else once complaining of their interior fan only squealing when running the A/C. Might be somewhere in the archives, if you search hard enough. May not be worth the effort, as I don't recall a fix being posted or the cause verified.

Back to your A/C. The first question to ask is how your cooling is. How is your A/C working? If you have a leak of significance, it's likely you've already lost enough charge to affect performance.

If your A/C is kinda' working, I think your next step would be to verify the charge.

If you've lost charge, an A/C shop can perform a vacuum test to check for leaks. If the system won't hold vacuum for an appreciable amount of time, you have a leak that must be located.

Best of luck, and keep us posted on what you find.

The Eagle

Hi Randall,
well I have made an appointment for june 23rd with my OPC. My A/C is working but there's almost no cooling.
I've done a search and I found out that the condenser coil and evaporator are located behind the dashboard, so maybe one of them is broken. What do you think?
Greetz