Constant airflow to the center vent / defrost
11-02-2013, 01:30 PM
#1
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Constant airflow to the center vent / defrost
Hi all,
I have an issue in my 92 968, I always have air coming out the center vent or defrost. This is a minor annoyance
With the servo motor cover off the HVAC unit I can see all three servos operate but the uppermost one only moves with the temperature control ****. I think it is left side #2 here:
http://www.clarks-garage.com/shop-manual/hvac-01.htm
I can hear a motor whir when I operate the upper slider and feel a slight airflow change but nothing dramatic. I pulled the center vent and the flap in there is a bit sloppy and moves slightly with the temperature control and/or the drost button but otherwise does not adjust. I can see the control linkage (the stamped metal) go through a full range of motion if I futz with the controls but that flap never seems to completely close to prevent airflow to the center vent.
My current theory is that if you do not want hot air in your eyes you are supposed to switch the center vent to defrost and replace the missing crumbled insulation on the center vent assembly but this seems a bit crude. Keeping the top slider to the left seems to reduce the air a bit, but there is still a lot. Maybe there is more crumbled foam that is an issue here.
Any hints? Experience and advice welcome.
-Joel.
I have an issue in my 92 968, I always have air coming out the center vent or defrost. This is a minor annoyance
With the servo motor cover off the HVAC unit I can see all three servos operate but the uppermost one only moves with the temperature control ****. I think it is left side #2 here:
http://www.clarks-garage.com/shop-manual/hvac-01.htm
I can hear a motor whir when I operate the upper slider and feel a slight airflow change but nothing dramatic. I pulled the center vent and the flap in there is a bit sloppy and moves slightly with the temperature control and/or the drost button but otherwise does not adjust. I can see the control linkage (the stamped metal) go through a full range of motion if I futz with the controls but that flap never seems to completely close to prevent airflow to the center vent.
My current theory is that if you do not want hot air in your eyes you are supposed to switch the center vent to defrost and replace the missing crumbled insulation on the center vent assembly but this seems a bit crude. Keeping the top slider to the left seems to reduce the air a bit, but there is still a lot. Maybe there is more crumbled foam that is an issue here.
Any hints? Experience and advice welcome.
-Joel.
11-02-2013, 01:39 PM
#2
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Location: one thousand, five hundred miles north of Ft. Lauderdale for the summer.
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I can only get any airflow from the defrost vents if I locate the up/down slider to the up position, and then push the high speed fan button to the 'on/blast furnace' position (the square button adjacent to the rear window defrost).... otherwise, I get no flow to the windshield via the 1-2-3-4 fan speed control **** whatsoever...
I can however control the flow of the blast furnace function to come out the lower vents by locating the up/down slider to the down position... and the blast furnace flow rate is about equal to position '4' on the fan speed control ****.
the temperature control **** works fine. but flow to the heater core has always been a problem at idle -
regardless of whether or not my coolant is topped off.
been that way since I purchased the car, and no clue what to do to fix this issue.
German engineering is bafflng to say the least,
but my car turned 20 last month (manufacture date). 
11-02-2013, 01:44 PM
#3
Rainman
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I've noticed this too, I don't believe the fan is running always but it could be a result of air pressure at the cowl versus in the cabin, the air flows in towards lower pressure...
For a cheap seal on the flap try the weatherstrip foam stuff from an Ace hardware/similar store. Self-adhesive, black foam strip in variety of widths in a roll.
For a cheap seal on the flap try the weatherstrip foam stuff from an Ace hardware/similar store. Self-adhesive, black foam strip in variety of widths in a roll.
11-02-2013, 02:00 PM
#4
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Yes I had some gray stuff on hand that is of good quality. This issue is noticeable with the car not moving, it looks like either I am missing more foam, have incomplete flap movement, or there is always some pretty significant flow to the center vent/defrost plenum.
11-02-2013, 03:22 PM
#5
I have a similar issue as well. But, mine isn't a problem when in idle or standing still, its when moving. It feels like someone put the heater **** at max and broke it off. I have been trying to solve the problem for years now, still no idea whats causing it. Only way to get ride of it is to turn on the AC. Even closed the heater valve completely and it did nothing.
I heard it could be the foam too, as I tried everything else.
I heard it could be the foam too, as I tried everything else.
11-02-2013, 04:32 PM
#6
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Have the same problem as well and to add that my passenger if I have one the foot well where their feets are gets extremely hot how can I fix that?? Is it the heat shield from the exhaust system or what
11-03-2013, 08:18 PM
#7
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If you get heat all the time it's possible your water valve is allowing hot water into the heater core even when closed, I had this condition in a 928 (same valve.) Apparently normal operation but lots of heat even when the valve was closed. I replaced it with one of the non-OEM alternatives I prefer (Ford Escort, Audi 5000 fitment.)
As far as the foam gasket replacement goes this seems to have worked very well in my case and with the defrost slider on the left and the center vent flipped to defrost I do not get heat in my face when all I want is heat in the footwells. Much better.
What I did was pull the center vent / cluster surround and put a strip of 1/4" insulation foam tape in it where the center vent flap is supposed to close off. The flap now seals against the foam and does not leak hot air out the center vent to annoy me.
As far as the foam gasket replacement goes this seems to have worked very well in my case and with the defrost slider on the left and the center vent flipped to defrost I do not get heat in my face when all I want is heat in the footwells. Much better.
What I did was pull the center vent / cluster surround and put a strip of 1/4" insulation foam tape in it where the center vent flap is supposed to close off. The flap now seals against the foam and does not leak hot air out the center vent to annoy me.