That annoying "clickity click" from your AC
 

Hey guys.

I haven't posted on here in awhile- I feel like a Porsche-Hypochondriac so the more time I spend reading posts the more I notice little issues that I have to completely dismantle my car to chase and fix. I am sure you all know what I mean.

Anyway, I felt like I should probably post about this because I searched and searched for a resolution and haven't found one. I am talking about that "click... clicky click... click click click" that emanates from below the dashboard when the AC is on. I know what it is now, and I know what to do about it.

My experience is on a 924S, so the pre 85.5 dash and air-con setup.

Behind the AC controls is outside air blower and heat exchanger (for the heater) assembly. In the driver's footwell, and behind the glovebox there are two actuators (referred to as "vacuum unit" drivers side - 944 573 053 00, passengers side 477 820 545") which control the position of the fresh air damper.

They are plumbed to the solenoid valve (944 573 165 00 for LL, 928 573 692 02 for RL), which is plumbed through the firewall into the vacuum system under the hood (there is a Y right on the other side of the firewall that goes towards the brake booster and the vacuum canister). The solenoid valve is located all the way to the left on the passenger side under the dash, and all of this is extremely simple to access if you remove the glovebox assembly, which is very easy- 5 screws total, remove the two at the top, the two on the bottom for the door and the one center screw in the middle of the bottom after the door is removed, unplug the wires and pull out.

Anyway, that clicking. I don't have the workshop manual for the HVAC system, but I have pieced together what I think is going on from what information I could find. If you have the full operational explanation, please, we'd like to hear it. But from what I can tell, the solenoid is controlled by a temperature switch that regulates recirculated air vs fresh air, so the solenoid is going to open and close the two actuators by opening and closing the vacuum based on the cabin air temperature. Why? Well, because it's a Porsche.

THE IMPORTANT PART. ---> The clicking was the repeated opening and closing of the fresh air damper via the actuator on the drivers side footwell (944 573 053 00) leaking. It's sort of a snapped-together body with a really old rubber diaphragm- its time for a replacement. I would suspect that this is the weakest link in this setup. As a vacuum is pulled on the actuator and it begins to move, the diaphragm expands, exposing its cracks and rips and losing the pressure differential and therefor slamming back closed again.

For the interim, while I see if I can justify paying for the german replacement part... I removed the snap ring from the actuator arm and zip-tied it to hold the door closed. I don't want fresh air, only recirc, you may differ- that's up to you. The easiest way to tell what position you have set this arm in is to turn the AC off and run the fresh air blower. As you move the arm, when you close it obviously you will feel no air through the vents.

I also cut about a 10" piece of silicon vacuum hose and deleted the Tee between the solenoid and the two actuators, and ran the vacuum line from the solenoid directly to the passenger actuator. I am going to let that one do it's thing for now, but if it clicks, I may change my mind.

Anyway, I hope this helps. It doesn't take major surgery to the dash to get to all this stuff, just the glovebox removal.



I figured all this out via the 924S PET, Illustration 813-45 pg 234.

I actually went ahead and removed the actuator (~$55 per our host, shipped from Germany) and I did confirm that it was bad, and for the reason I described. I admit I am not that smart- I do commercial HVAC and this is a common issue for old pneumatic actuators, and they exhibit the same symptoms.

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As soon as little vacuum gets pulled, the diaphragm exposes this tear and the vacuum is lost, so both solenoids go CLICK CLICK.

Nice to know information. Bookmarked.
I will listen carefully...
Thanks.

John

Awesome. I have experienced this in my '86 as well and wondered what the cause was. Have a feeling mine is bad as well and causing it to stay on recirc instead of fresh air.

I’m curious if I am suffering with the same issue but with an unpleasant side effect. When I drive my 87 944 NA and I open the sunroof or windows I get a very strong odor engine bay odor/rich running fumes in my cabin. If the cabin is sealed up, no fumes. I can run the blower with no fumes. AC was disconnected from previous owner.

Your thoughts would be appreciated.

My '84 does the same thing with the sun roof tilted up while sitting still or decelerating while driving. I don't notice it when I'm driving at a steady speed. I think that it is exhaust fumes, not engine compartment fumes, because it has that sulfurous exhaust smell to it.
I think that there is a low pressure area at the rear of the car that captures some of the exhaust while driving and then, when decelerating, that exhaust cloud that is trailing the car is then drawn into the rear of the open sunroof. I can think of no other reason why it behaves in this manner.
I have a new hatch seal in place and the fumes do not enter the cabin with the sunroof closed, only when it is open, so I have ruled out the hatch seal as a source.

Most likely cause is a gap in the seal between the body and the rear hatch. It is at the low pressure area in the back of the car, allowing exhaust to be pulled into the cabin. Fix that and you should be able to lift your sunroof without further problem.