Mike Elgin

I have an 88 S4 that has an intermitten clicking sound coming from the center front vent. It is a single thump or click about every 30 seconds or so. I was also having trouble getting really cold air from the AC this summer so this may be part of the problem. Heat blows fine. Anyone had this problem or know what is causing this sound. Thanks in advance.

Gretch

is it a "click" or a "thump". My "Thump" comes from the flap that directs ventilation. I suspect a vacume leak.

Mike Elgin

I guess I would describe the sound as more of a mid range thump sound. It is definitely not a high pitched click and sounds like some sort of valve or flap trying to shut and seal. If it is a vacuum problem, what is the fix? Thanks!

WallyP

I have sent you a text file on the HVAC system that should answer your questions.

Mike Elgin

When I actually isolate the leaks, what should be replaced?

Mike Elgin

When I actually isolate the leaks, what should be replaced?

Dan Bise

Wally, I'm drawing a blank on which vacuum actuator pod has the two stages, the comb flap, I think. If the upper stage vacuum hose on the pod is disconnected, it can result in the cycling effect you are experiencing.

What I also found is that it this cycling can overwork the control head and start some nice burning smells that will cause you to have that puzzled/shocked/horrified look on your face 928 owners sometimes get.

It's real easy to disconnect one of these lines while replacing the center console, if you've been doing work in there recently. Don't ask how I know. :-)

(Sorry for the guesswork. I'm not next to my manuals right now.)

Dan Bise
Pasadena, CA
'83S

Garth

My 93 makes a thumping noise from inside the vent area when I put the temp selector anywhere but in the far left position and it is cold outside. I assumed that it was just the noise or the flaps mixing hot & cold air to get the right temperature as selected. Is there another possible problem?

Wally, would it be possible to get that email as well?

garthhadley@hotmail.com

Thanks,
Garth

User 4221

Dan,

The two stage unit controls the center vent... open and close. This was mainly in earlier cars of 80-86 I think.

I have noticed that the vacuum solenoids have a lot of "chatter" when the cabin temp. reaches the set temp. on the controller. I'm assuming the unit is regulating to maintain temp.

Anyone else see this?

Scott

Ron_H

Dan:
I am one of those 928 owners that occasionally detect a strange burning smell; if is actually more of an "exhaust" smell that comes and goes. I haven't been able to isolate the typical conditions under which it can be expected to occur, and it rarely lasts more than a few seconds. I simply stinks up the interior of the car, I open a window, and then it goes away. I haven't gotten drowsy when it occurs, so I ruled out an exhaust leak; I suspect that an exhaust leak would occur constantly. Also, it never occurs when I have anyone around to detect it; only 5 minutes after I hace left the shop !! People are beginning to think I'm a bit odd, because they smell nothing. Maybe I should visit a nose doctor....?? Any suggestions?