Aussie Alan 993

Today I sucessfully solved an A/C problem. Some dumbass had messed up the wiring on the servo motors for the fresh air/recirculate and the foot well valves in the trunk. Now the controls do what they are meant to.

While I was doing this I found a solenoid valve that has a vacuum hose connected to one side and the other side blocked off.

There are also 2 tubes, a black one and a white one. One of them was most probably connected to the blocked off side of the solenoid valve.

The questions are, What does this valve do and why would someone block it off? Which tube connects to it and what is the other for?

I would offer a picture but my camera is loaned to a friend right now.

Can any of you shed some light on this for me PLEASE....

Cheers

Alan

Randall G.

Does it look like this?

Aussie Alan 993

Thats the one,

Thanks Randall.

Do you know what the air bypass flaps do and what switches it on and off?

Or a better question may be, is it a common thing to disconnect it?

I gotta get me a good workshop manual!!!

Randall G.

Hey Alan,

As soon as you figure out how the bypass air flaps work, and how they fit within in the system, please let me know. LOL!

In all seriousness, I've yet to meet someone who can explain how the bypass air flaps work, and their placement within the 993's HVAC system. I give myself credit for having a pretty solid understanding of the 964/993's HVAC (the two systems being very similar), but the bypass air flaps are a mystery to me. Don't have 'em on a 964 (fortunately ).

There are two common vacuum-leak (loose connection, etc.) induced HVAC problems that 993s experience. At least these are two I've seen several times on this board:

-Loss of center vent flow at full throttle.
-Loss of center vent flow when running the A/C or in recirculation mode. Turning on the A/C automatically places the system in recirculation mode. At least it does on my car, so I'm assuming 993s are the same.

I suspect somehow or another the vacuum controlled bypass air-flap is at the root of this, but don't know exactly how. As I wrote above, the 964 doesn't use any vacuum controls for its HVAC, so we never see these vacuum-induced problems over on the 964 board.

I'm guessing the disconnected line on the solenoid goes to the bypass air-flap. And, if you reconnect it, you'll likely experience the problems described above. Rather than trying to find the source of the problem, the PO just disabled the solenoid. But, again, that's just a guess on my part.

Here's another interesting excerpt from the shop manual, describing the limited use of vacuum control in the 993's HVAC system:



Good luck, and please keep me posted on what you find out.

Aussie Alan 993

It took me a while but I worked it out. The Bypass Air Flaps are 2 horizontal flaps that open by way of a vacuum operated diaphram when the recirculate button is pushed to close off the fresh air intake.

They are the cabin air intake flaps.

The reason for this is so when on fresh air, it draws only fresh air, and when on recirculate it draws only cabin air.

Vacuum is supplied to it via the solenoid valve I discribed in the first thread of this post.

My bypass air flap mechanism was destroyed by a previous owner or doddgy service guy who I shall have flogged before hanging him up by the goolies with a barbed wire rope!

I have made a metal bracket to hold them open at all times, the logic is that it is better to have fresh air mixed with cabin air than to have no cabin air at all.

Also seeing as the AC CCU automatically selects recirculate when first switched on, it was most effective to force the flaps open rather than shut.

Other Rennlist posts have discribed a hissing sound and no air through the vents on AC. Bypass air flaps not opening, vacuum or mechanical fault.

To get a good look at the mechanism for the Bypass air flaps, remove the AC controls/CCU, look through the hole and there it is.

I have to say, if there are any Porsche guys reading this, as we suspect there are on the Rennlist Forums, vacuum operated Bypass Air Flaps in an otherwise all electric HVAC system is a dumb idea. Many other cars have the fresh air and the cabin air flaps on the same servo using a linkage to make them work in sequence with each other. GET YOUR ACT TOGETHER! There is more to great cars than great engines, suspensions and brakes.

Finally, a big thanks to Randall for the info he supplied, I would still be scratching my head if it was not for him.

Cheers

Aussie Alan

Randall G.

Hey Alan,

That's great news. And, thanks for the concise explanation of how the $%bypass%$ air flaps work.

Interestingly, on a 964, the cabin air intake flappers are controlled by differential pressure. As I recall--without having my reference handy--these flappers are spring loaded. They open via differential pressure when the fresh air intake (base of windshield) is shut-off (recirculation mode). On a 964, you can feel the draw of air at the interior intakes when you place the system in recirculation mode.

For some strange reason, Porsche decided to switch the cabin air intakes from differential pressure to vacuum operated on a 993. WTF?

So, when the fresh air intake is closed (recirculation mode), if the interior intakes (bypass air flaps) don't open (e.g., loss of vacuum), the interior fans have no source of air to draw on. You hear a hissing sound as the interior fans try to draw air through the closed bypass flappers. And, you get no air out the vents, because the interior fans have no air to pump. Sound 'bout right, Alan?

When you turn on the A/C, the system automatically swaps to recirc. mode. Again, no air out the vents if the bypass flaps don't open. But, if you turn up the temperature control **** (A/C still on), the mixer flaps reposition to allow heated air from the heat exchangers. Which gives the interior fans a source of air to draw upon, and air again flows through the center vents. Sound 'bout right here, too, Alan?

Randall G.

Oh, and why call 'em "bypass air flaps," if they're simply cabin air intakes? I was really thrown by that name, figuring they bypassed something or another (but what?). Apparently, they don't bypass anything. They just replace the fresh-air intake as the source of non-heated air for the interior fans.

Aussie Alan 993

Hear that Porsche Guys!!!

Randall the "GURU" has spoken out too...


"Thats what the G stands for from now on"

Aussie Alan 993

BTW, Right on both counts Randall, go to the top of the class.

I believe we got this all sussed out... patience is truly a virtue as is persistance.

It also explains the symptom I read about of having no air when on full throttle, there is no vacuum on full throttle, but I wonder if there is a simple fix with a check valve or a vacuum tank or both.

My '82 Accord used this sort of thing for the AC controls and it did not misbehave when on full throttle (I should know that's how I drove it all the time)

I'm sure it could be worked out if needed. Only not on my car seeing as the "Cabin Air Flaps" mechanism is AFU.


Alan (#1 Fan of the Guru)

ACEparts_com

I don't know if it's worth adding this after such a long time, but today I was getting cold air from one vent and hot from the others. Turned out to be the motor in the footwell controling the hot air flap.
Also discovered that my recirculate flaps and solenoid have been disconnected in exactly the same way as Alan's. Maybe this is to make the flaps work by differential pressure as Randall was stating?

Aussie Alan 993

Did you connect it up, and if so did it work.

Mine were butchered so even if the solenoid was connected, they would not give me "Only" fresh air. This not so bad.

It would be worse if I you could not shut off the fresh air, especially when you get caught behind a heap of crap with a smoky exhaust.

Good to see this old thread back on the front page...