plain fan

Could someon please explain to me how the differential pressure flapper valve works? I have Adrian's book and have read it repeatedly but I'm trying to trouble shoot a problem I've got on my 90 C2.

My question:
The amount of heat that enters into the car is dependent on the flapper valve which is controlled by the differing pressures between cold and hot air. When the cold air pressure is higher than the hot air pressure the flapper either allows a little warm air to pass, allows all hot air to pass or dumps all hot air into the wheel wells. Is this correct? I understand from the book that hot air pressure is created by the engine mounted fan blowing into through the heat exchangers but where does the cold air come from?

My problem:
Currently I have heat that seems to be radiating from the passenger side of the car. I don't have this problem on the driver side of the car. My CCU is set on the blue dot but heat is radiating from the heat tube and rear seat area. The heat does not come through the vents either in the dash or the footwell. I searched the archives and found where the hose between the mixing unit and the heating tube can cause a loss of pressure and allow heat to build up. I spent some time this afternoon replacing my hose (which did need it) but I still have the same problem. Right now I'm alsmost certain that my differential pressure flapper valve is stuck in the full heat position but since I don't fully understand where the cold air pressure comes from I am stuck.

Any help would be greatly appreciated!
Thank you.

plain fan

bump

JasonAndreas

In the photo below the red arrow points to the temperature mixing chamber door on the left hand side of the front trunk.
When the door is all the way down like the in photo only cold air is allowed to enter the cabin and air circulates following the path of the green arrows. As the door rises/opens warm air from the the rear is drawn (coming from below the door) into the chamber by the fan (and low pressure) and pushed into the cabin.



I'll try to take a photo tomorrow with the door open and with heat pseudo-colored arrows.

garrett376

Hey Jason, how do you open up the fan/climate control stuff in the front trunk? It seems encased in that black plastic cover...

JasonAndreas

Garrett,
The top and bottom plastic pieces that make up the blower chamber sections are held together with little metal clips (blue arrows) on the front and sides (maybe 5 or 6). Just grab them (they are shaped like the letter C) with needlenose pliers, twist them off and then you can pull the top half of the mixing chamber straight up.

Jeff Curtis

Jason, I received my new blower motor/fan assembly from Vertex last week. Is that pretty much all I have to do to remove the top cover to replace the fan motor pictured above?? Really, I've had little time to investigate, but just enough to purchase what I think is the right part.

Plain Fan, if you have heat radiating from the passenger side, I would suggest changing out the black plastic flapper valves that reside underneath your interior quarter-panel upholstery. These pieces are reasonably easy to remove, then you will see the black plastic flapper that is probably allowing heat in there.

On second thought, I haven't had those pieces (interior panels) in place for almost three years, I had the black plastic flappers removed and my GOD did the interior heat up!!

Once I replaced them with new (old ones were cracked/broken at the point where they bolt down) the heating system worked properly...until I had servo issues.

Anyway, check that...I believe I have a couple of new ones laying around if you need them, I accidentally bought TWO sets.

JasonAndreas

Yes, that is all you need to do to get to it but to actually remove it I had to pull the fusebox and the firewall. If you cut the squirrel cage on the fan you might be able to do it without removing everything. If you are using the Porsche/VW puller then you definitely have to remove everything cause it won't fit otherwise. For once the instructions in the workshop manuals actually cover pretty much everything for this except for the two torx head screws hold the fan in place. If you need them PM me.

Where are these located? Under the dash or when you say quarter-panel do you mean in the rear?

Jeff Curtis

Thanks Jason, I'll be sure to trouble you with more pesky questions once I get some time to tackle the job...but for now, HOW AND THE HECK DOES THIS THING ATTACH/DETACH?? There's some funky black plastic housing on the fan unit that appears like it snaps/twists in?? Suppose it'll be easy to figure out once I get the front cowl assembly apart - eh?

I'll checkout the workshop manuals I have on CD...and have yet to use...shame-shame.

Upon re-reading the original post, I've decided that I had my mind on something else. PlainFan, you say heat is emminating from the passenger VENT IN THE DASH?? If so, we're talking servo motor issues, I believe. They control the flaps in the mixing chambers, but I may have your troubles mixed up with the MANY that I have!

Jason, the black plastic "flapper" valves I was referring to are indeed underneath the factory interior quarter panel covers. They are cheap black plastic pieces with rubber "flaps" in them that let the compartment equalize when you close the doors, etc. - if you paint the interior and de-insulate the rear as I did, and NOT replace these, you will have heat like crazy if the hoses in the doorsills are removed...I neglected to remember that was MY case.

DarrylH

I think these are really his problem - sounds like they're not "flappin'" like they should! Where the heck are these, and how do you get to 'em? Are they user-serviceable (lubricate, free-up, ?) or if one goes south, as it sounds like has happened to "plain fan", do you just replace 'em?

JasonAndreas

Which thing?



Jeff,
Could you circle the general location in the photo below or take a picture with your hand pointing to it...

MB965

Jason,

The flapper valve is is located in the area where the front seatbelt retractor bolts on, which as Jeff says is under the rear interior side panel.
This valve lets air exhaust from the interior when the windows are closed.
The air goes through the chassis and exits in the side of the front wheelwell, under the liner. Took awhile to figure out what that hole was for.

I removed my valves when I installed a rollbar and do not notice any difference in the HVAC system operation, worked good before and after.

Mike
'94 Turbo 3.6

JasonAndreas

After all these years I think I've finally discovered how the bastards got into my car during the great mouse infestation!



They ate part of the door and the plastic fence...



Thank You Jeff & Mike!!! I had no idea this existed and for once a replacement is not that expensive, part #964-572-043-01 retails for only $15USD.

Jeff Curtis

Aaaaaah, those are the pieces I speak of.

...and THANK YOU for your tips as well - again, I may just get to tackle this project here soon. I have A LOT on the table this weekend with building a shop on my property, the dog needs a bath, lawn needs mowed, landscaping needs mulched and weeded...and last, but certainly not least - the 964 is STILL in the trailer from last weekend's race (with brake dust and rubber marks all over it - I detest cleaning my Fikses!!) and needs a good cleaning...MUST I GO ON!??

Ah screw it, maybe we'll just go ride the Harley.

MB965

Jason,

Interesting that with your carpeted interior side panels (RS?) I cannot tell how the cabin air gets to the flapper valve. It looks like the the valve is surrounded by carpeting.
On my car the side panel is a vinyl covered fiberboard and forms a pocket behind the the panel and has a slot opening at the top, which can be seen looking from the outside of the car through the rear side window.

If you have an ongoing problem with rodents wanting to nest in your car, you might want to consider installing a stainless steel screen over the opening in the chassis in the front wheelwell. Otherwise they will still be able to get into the car.

Mike

plain fan

Thanks to everyone for responding. I've been very busy and haven't even had an opportunity to check the board until tonight. I will pull my interior quarter panel and look at my flappers. How do I tell if they need to be replaced? Is there a visible "flapper" piece that will be missing/stuck/etc upon visual inspection?
This heat doesn't come up through any of the vents, it just seems to radiate from the passenger side door sill and rear quarter panel area. I replaced the passenger side hose last week since it was older and was not properly secured on the metal outlet.
Thanks again,
Grady (aka plain fan)