I have a 1978 928 #338 and I am working the HVAC system,
and the the center vent flap will not open,
the center vent pod is good, i tested it with my mytivac,
the system holds vacuum when the engine is off .
The lines appear to be hooked up to the back of the HVAC unit as the WSM per the second edition version ,
I did swap them to the early edition but still no joy.
Though i dont know when the early /late versions appeared.
Anyway does anyone have wisdom on what to do to fix this.
The HVAC head is screwed to the console from the back so the console must be removed before the HVAC head can be inspected .
Any help is appreciated

 

Stan, did you have a look at Jim's diagrams at 928 Classics? He reverse engineered everything and uncovered a number of mistakes in the WSM (well, the '78 Service info book) diagrams. His clarifications are listed here:

http://928classics.com/resources/category/hvac%20vacuum

If it's a US market #338, it's likely the 'late' 78 configuration, that's what worked for Minerva, US #107.

 

Hi Rob thanks i have been using these diagrams,
I believe this is the car they were performed on,
but the center flap wont open on any position of the lower or upper slider

 

The vacuum is either non-existant or weak at the flapper pod. Check how much vacuum it takes to open in with your mity-vac, then check vacuum measurement at the hose. if no vacuum getting to the hose and you're sure hoses are correctly positions you may have a leak in the slide valves on the HVAC unit...also these slide valves can be assembled improperly once taken apart if I remember correctly. weak vacuum can also come from a slow leak or momentary leak at a certain position of the slide valves.

If you want to play with 78s get used to pulling those consoles apart with the backwards screws in them !

 

Thanks Drew I will look into this

 

Stan, when I took apart the interior of my '79, I made a diagram of how the vacuum lines on the back of the HVAC were connected, that I included in the manual of additions I made to the car (wiring diagrams, additional fuses and wire colors, etc). Here's a grab of that diagram in case it helps. # pattern should correspond to approximate positions of the connection ports with notes on the side.

 

Hi Pete thanks,
do you think that brown could mean orange? your version is quite different from how the system is routed
Does your center vent operate?

 

i guess it could be orange and just old, or maybe someone replaced it at some point. Yes, all my vents operate. I think the center one only flows air when the bottom slide is in a position other than "OFF" and it might always be a fresh air vent, with flow adjusted by the **** to the left of it - clockwise to open and counterclockwise to close it off.

 

All diaphragm pods held vacuum. My guess is the center vent pod (orange hose) got disconnected somehow (like getting hooked on a manual slider).
Should be pretty simple. Otherwise, try the alternative config on my diagram which works good for auto center vents (like #338 has).