Rich W

Randall,

The valve is definitely dumping hot air at the wheel well, so maybe that's it. I can hear one of the differential flappers when I shut the car door (engine off), but not sure if I hear both.

Thanks for the help!

Randall G.

Hey Rich,

This could simply mean that the mixing flap is closed (to heated air), forcing the valve to open (dump air to wheel-well). So, your problem could still be with the mixing servo. Any chance of disconnecting the servo motor and manipulating (opening) it manually?

If the fan isn't turning on the left-side, I imagine that could also block the air-flow, forcing the valve to dump air into the wheel-well.

How was the air-flow on the right-side dump valve? Did you feel no air flow while in defrost mode, while you had lots on the left-side?

Getting closer...

Randall G.

One more thought. What does the differential pressure ("dump") valve do when you ask for heat only (not defrost)? Do you get heated air on the left-side of the cabin, and no air venting in the wheel-wells? If so, this would suggest the valve is okay--being a purely mechanical device, and not subject to any abstract electrical interfaces.

Of course, if you're getting heat on the left side when not in defrost mode, that should also suggest the mixing flap servo is okay. Unless there's something abstract going on electrically?

Adrian

Dear Rich,
When you shut the doors what you hear is the check valves for the air circulation system not the differential pressure valves.
These diff valves, left and right work very simply. The rear blower fan if activated applies a pressure to the valve flapper forcing it open. The amount of force is dependent upon where the mixing chamber valves are and fan speed selected. If the mixing chamber valves are in the full cold position the pressure from the front exceeds that from the rear holding the valves closed and the hot air is dumped overboard.
The fact that hot air is being dumped overboard with heat selected clearly indicates that a mixing chamber is in the full cold position.
Ciao,
Adrian
964C4

PS: Over the years I have heard these comments as you have posted many times before. Many an aircraft has been grounded again and again due to this issue.

Rich W

I believe I may have caused some confusion here.

The heat was being dumped overboard into the wheel wells with the heater control set to cold.

I am not having an issue with getting heat to the windscreen. My issue is the distribution of airflow between the two sides when the windscreen vents are selected. Selecting the defroster button causes more air flow (as expected) but does not change the relative distribution of the air flow, i.e., there is significantly more on the passenger side of the vehicle than there is on the right hand side of the vehicle. (LHD)

That is the problem I face. There is quite a bit of air noise behind the dash on the driver's side, but not a lot of air flow. When the regular dash vents are selected, the air distribution is very nearly equal, although the window vents get less air than the center vents.

I have removed the left hand squirrel cage fan cover and looked into the chamber. I saw the air conditioning condenser coils, but nothing past that. The center/windscreen vent is operating properly - it opens when it's supposed to, and shuts similarly.

It would appear there is some form of blockage or defect on the driver's side (other than the driver) which is reducing the airflow to the driver's side winscreen vent.

Any ideas?

Rich W

Correction to the above post:

I am not having an issue with getting heat to the windscreen. My issue is the distribution of airflow between the two sides when the windscreen vents are selected. Selecting the defroster button causes more air flow (as expected) but does not change the relative distribution of the air flow, i.e., there is significantly more on the passenger's side of the vehicle than there is on the driver's side of the vehicle. (LHD)

Sorry about that.

Randall G.

Hey Rich,

If you can't even get cool air to the windshield vents, that would suggest the differential pressure valve is okay.

As you suggest, the problem shouldn't be with the servo for selecting between the dash and windshield vents, as it's common to both left and right sides.

So...this leaves you with the mixing servo motor and the left mixing fan itself. Seems like one of 'em has to be your problem.

Since you get normal flow on the left side when air is directed to the dash vents, this would seem to suggest your left-front blower is okay.

Here are excerpts from my previously mentioned inop left defroster repair:







My mechanic's handwriting was really bad. But, you can barely make out the word "flap" in mixing flap motor. I thought he might have replaced the left-front blower motor, especially since the part number wasn't listed. But, $71.62 is too cheap for the left-front blower, and is consistent with the cost of a servo motor.

I knew much less about the car than I do now, and the HVAC system was really a mysterious black box to me back then. So, I didn't ask for (or didn't understand) a good explanation of why the left mixing flap servo fixed the problem.

In an earlier post, I took a stab at theorizing how the left-mixing flap servo might cause the problem you're having. Actually, it was my best effort at explaining my own repair in '97. Adrian, does my theory sound plausible to you? Or, can you offer a theory on how the left-mixing servo might cause the left-defrost to be inop?

Seems like you struggled with servo #5 (the left mixing flap servo) earlier in this thread, so perhaps it deserves a revisit?

We'll figure this thing out yet...hopefully.

Rich W

Further investigation this evening revealed the following:

1. The air coming out the center vents is not as balanced as previously thought. The driver's side flow is noticeably stronger than the passenger's side flow (opposite of the windscreen vent).

2. Depressing the defroster had no effect, other than to increase the airflow.

3. I disconnected the passenger side fan, then ran the series of tests again - same results, except for the lack of increased airflow when the defrost button was depressed (expected result).

I'm beginning to think there is some obstruction in or around the AC condenser coils on the driver's side. Unfortunately, I can't get an inspection mirror into the cavity from the cabin.

Randall G.

>1. The air coming out the center vents is not as balanced as previously thought. The driver's side flow is noticeably stronger than the passenger's side flow

This is quite common, Rich. My own car does this, as well as many other 964s and 993s.

Adrian

Dear Rich,
Have you considered that the fault may lie with the actual mixing chamber flapper valve itself. I know you had problems with mechanical connections and hooking the arm up around the wrong way. I wonder if you have managed somehow to disturb the valve.
I am afraid I do not understand your comment about AC condenser coils. They are under the left front fender. The evaporator in the luggage compartment is in the fresh air flow system not the heating system air flow system. Heated airflow comes from the rear. Fresh air or air con air from the front.
If you select full cold what is the air distribution like then?
Ciao,
Adrian
964C4

PS: Dear Randall I agree entirely but added in my post that maybe the actual mixing chamber valve itself is now the problem and not the servo.

Rich W

Perhaps the coils I see are the evaporator?

Adrian, could you provide relative locations of the various valves, and also the correct terminology (which you are undoubtedly using, but I'm trying to make sure we're speaking about the same components)?

BTW, I observed the operation of #5 Servo and it's associated valve when the cover for the left fan was removed. It appeared to operate correctly, raising the door to allow heat to enter the chamber when heat was selected.

Adrian

Dear Rich,
I do not use numbers I use their names.
Are you 100% sure that the heat is getting through. How about having the engine running and see if you are really getting any heat through the mixing chamber valve. Will at least dry out the luggage compartment.
The only two servos you need to work with are the ones with the long arms disappearing into the blower fan assembly. These are the mixing chamber servos. I think somebody called them 2 and 4 before I am not sure. I just know them by their function and recognise them by the actuating arms.
You MUST have heat selected of course. Having blue dot selected means no heat.
Ciao,
Adrian
964C4

PS: Have you considered you have a leaking, damaged or disconnected feeder pipe to the mixing chambers.

Rich W

Adrian,

Understand no numbers. I'm just trying to make sure I understand the location and function of each device you mention.

Absolutely positive I'm getting heat to the mixing chamber. When the temperature is set to maximum heat, the air coming out is VERY hot.

I have verified the operation of the mixing chamber servo on the driver's side by observation with the fan cover removed. The servo and mixing chamber valve functioned properly. The fan was operating, and blowing air.

Randall G.

Sorry Rich. I added to the confusion, calling the #5 servo (per Roly's convention) the left mixing flap servo. It's actually #4, per Roly's convention. But, I think you knew what I meant.

Speaking of Roly, I just took a closer look at Roly's pictures on the first page of this thread. Wonderful, wonderful, stuff. I want my own spare HVAC unit to play with. (X-mas idea for next year? )

Roly's picture of "#1" (center/windshield vent) shows the flapper being one piece, common to both the left and right-hand sides. So, as we've already concluded, your problem can't lie there.

Sooo...where does that leave you? Missing duct (as Adrian has suggested)? Left-mixing servo? Left mixing fan? An obstruction?

Well, Rich, I'm outta' ideas for the time being. Perhaps rechecking everything will show up something you didn't notice the first time? Or, maybe Adrian's latest thoughts will lead somewhere.

Rich W

Yeah, I'm running out of ideas, too.

I checked the "paper" ductwork was intact. I just don't know... and may not know until I tear it all done to replace the evaporator (in my spare time - HA!)

Thanks for all your input Randall - it's good to have someone to bounce ideas around.