Alan

Tails -
Thanks for that - I will need to digest some more - but you do definitely have some new information to me there...

Esp on the micro-switch operation. thanks!

I'm feeling like I really need a disassmbled HVAC system to look at and understand better the airflow paths too...

Anyway I do have some work into a description of the HVAC system - this is one part that needs to be closed out for that document to be ready (its part of an ~400 page document though - so there are some other delays...)

I have already sent this HVAC section out for some review to an expert here on rennlist - awaiting feedback still... I may also send to WallyP as another expert on this. I do understand most features of the system quite well but there is less info on the exact config of the mixing & comb flaps.

Alan

Tails

Alan,

I completed an overhaul of my HVAC system earlier this year and cleaned up some wiring problems to the compressor clutch make by the PO and I also carried out repairs and adjustments as necessary. Fortunately it is now back to spec electrically and with the vacuum lines tight, however, as some stage I will have to change to R 134A to get it back to full cooling capacity.

I have read Wally P's overview of the system included with Jim’s CDs, as well as the WSMs and all other information I could get hold of. In addition I have brought my own knowledge to bear having been a refrigeration engineer during part of my past life.

With regards to the air distribution system the fundamental section that I mention in my post, I believe is one of the keys to the system, especially in understanding what is meant with a flap being in the open or closed position (ie set (opened) to fresh air). I have found quite a few schematic drawings showing end elevation cross sections of the air distribution box and they are all different in some way. The one showing the best view, closest to reality in my view, is on page 31 of “928 HVAC pub 1985” in Jim’s CDs 2.15 titled “Air Conditioner (1980 to 1983) – Flap/air Flow Layout”. You have to take into account that the c /d vacuum controller, which activated the cd1 central nozzle flap was removed with its associated solenoid valve (reducing the number from 6 to 5) in 1984 and the flap was made manually operated fitted with the micro switch.
If you read the pages following page 31 they describe, after translation into grammatically correct English with consistent terminology and alphanumeric part descriptors you will be able to ascertain what was changed in MY85 and how the air and flap system is designed to work. I believe that the system remains basically consistent with the above untill the end of production of the 928s in 1995.

If you want me to assist in reviewing some of or all of the HVAC sections I would be only too willing to do so.

Tails 1990 928S4 Auto

Alan

Tails - Yes happy to have you look at it too - PM me an email address and I will send you the .pdf

Alan

merchauser

BUMP
I need to repair my diaphragm for the comb flap, but with so many other obligations, I just don't have the time. (perhaps in a few months.)
in the meantime, is there a way to temporarily keep that flap open? can I unsnap the arm and do something to keep it open for a while?

FredR

You can wedge it open with a carefully crafted piece of cardboard jammed between the end tooth and the wall of the chamber

dr bob

The arm you want to get to takes the same effort to access for securing as it does for repairing. Plus it's pretty fragile, so you don't want to twice risk damaging it. With the diaphragm leaking, you'll need to go in to block the vacuum hose going to the diaphragm. So... same labor to disable or fix. Difference is the cost of the diaphragm in the actuator can. I think Roger is sourcing a silicone rubber MB part for that now, so a relatively lifetime part vs one that only lasts 20 years or so.

OK, but block off the vacuum line going to the actuator.


I'd just fix the darn thing, and get the other exposed actuators while the console is slid back.

merchauser

urghhhhh…..sounds like a no win situation here. I guess I will just leave it alone for the time being. I just finished motor mounts/oil pan,
and a host of WYAIT (mostly endless hours of cleaning...well worth it, but a real PITA) and would just like to enjoy driving for a little while.
last time I did the dash diaphragms (previous car) I ended up with a few weeks of other WYAIT duties that keep the car down nearly 3 weeks.

my AC works great, but I am not getting the full volume of ice cold air with that darn comb flap in the way.

FredR

Bob,

Has anyone ever figured out exactly why that comb flap is there? The only reason I could think of was as a diversion device that under certain conditions like demisting it helps more air get to the windscreen rather than the driver as it were.

As I recall the problem with that actuator is it is a "dash out" job to fix it. Of course I also tend to forget that in my environment the only system I need is the ac - the rest of the features I do not need or use thus why I have no heater core fitted and my recirc flap is in a fixed position following failure of that actuator [ironically the comb flap still works on the GTS]. I had the comb flap issue on my late 90S4 just after acquisition some 20 years ago. If I had the car in the UK I would be in real trouble but then the heater would be installed and I would have the comb flap in the down position if the actuator had failed- the joys of 928 ownership!

I pulled the dash out on my late [wrecked] 90S4 when pirating for spares and my reaction at the time was I might never be able to get it all back in there if I had to. Thus my current mindset is I will fix mine up when I really have to- not before. I could have put a new actuator on the recric flap last year when I had the unit out but the seals on the flap were well and truly shot and the only way to fix that is with open heart surgery as per Herman's approach so I simply sealed it with some Gorilla tape as a stop gap. If I want fresh air I open the window - on the plus side it tends to curtail certain anti-social tendencies!

dr bob

I did mine with dash in. I did not replace the defrost actuator at that time; that came later as part of another dash-out adventure. Console came out for the comb flap of course. My '89 is the last non-airbag year, so I could get to the comb flap actuator/lever connection through the glovebox AC hole. Reminder to others that this is in my LHD US-spec car too. The Most Fun parts of the project were figuring out how the accordian panel release levers at the gear lever worked, and of course the delicate brain surgery done with a mirror and small tools through the 3/4" opening, without breaking the tabs on the lever. Rest of it was a piece of cake.

Tony

My guess is it helps better mix the hot air from the bottom of the hvac core with the cold air from the top. Pull the center vent and look at the discharge points...I have the hot air exit at the bottom of the vent sealed off with aluminum tape and the comb flap fully open as to not reduce airflow..