you might want to pull the fan out of the housing and get in there and pill out all the leaves and possibly mouse nest remnants. stick shop vac hose in and suck out whatever you can.

 

Let's start with:
Do you have a vacuum signal at plenum fresh air intake actuator in the MAX A/C position.
1. You will need to remove the (2) 3/8" hex head screws.
2. Then remove the plastic trash screen.
3. Now you can see where the Green Vacuum Hose connects to a coupling part of the fresh air door.
The actual vacuum actuator is on the back side of door so door would need to be removed to access it.
The short Black vacuum hose connects to the other side of the coupling and then to the actuator.
4. My fresh air door never worked.
So one day I removed the fresh air intake door just to test actuator.
This is what I found...
My guess it left the factory pinched.

 

Both: thank you for your posts.

I just did some more checking (after -aaargh- dismantling the dash again)...

1) The squirrel cage is running (on the same speed) irrespective of the blower switch's postion (off - m1 - m2 - high) - is that how it should be? But how comes, that with A/C Max there is a strong blow, but not much left in all other modes? I would say there's the squirrel cage and "some other blower", correct?

2) footwell and outside air doors/valves, both do work perfectly well - I pulled the vacuum "switch"/plug from the center console's HVAC-selector, sucked at the green port (until I could hear the footwell door opening) and plugged it: footwell door was (and stayed) open, outside air door was (and stayed) closed. Good!

3) I sucked at the black port (the vacuum source) with no result...
I pulled the black vac tube from the T at the master cylinder (where the vac comes from): at the T there is some slight but constant vacuum (I easily could pull my blocking finger off the T but felt some "nibbling").
I sucked the black tube as long as I wanted but did not feel any resistance... I blowed into that tube with the same result. That leads me to think that there's some leak in the black tube.
I dipped the black tube under water stretch by stretch (from the T to as far as possible) and blowed in - no bubbles = no leak. But this was only possible on a very short stretch of the black tube..... There's a lot of black tube left, but to get there I would not only have had to dismantle the dash, but also pull everything behind it apart - that's too much for today!


But what I also was able to confirm, is:

a) the hot water shut-off-switch starts to be pushed in where the white mark meets the blue (cold) on the temperature panel (great)

b) I DO feel a difference in where the air comes from depending on mode-settings VENT, HTR or DEFR. (Not much though, due to little blower performance...)
That would (at least partly) torpedo my assumption about the leaking black tube - as all these settings (the valves and respective doors' positions) are being regulated by vac coming from the black tube, right???
AND the hot water shut-off is also triggered by the black vac tube and should also not have THAT much of an effect then!?!?

I'm a bit lost - and shied off by the outlook of pullig the whole dash apart, I must admit.


On your posts, Derek and Joe:
wow, that I do call a pinched tube, Joe!
Pulling the fan and clean that area would be a bit much at this time, Derek, but I clearly see your point. My car has been 100% garaged for the last 30 years (since I have it) - at least I cannot see any dirt in the area the squirrel cage runs in...

 

Are you going to be online for a few minuets?

 

Is your 77 a early build or a late build?
Anti Theft key in fender or door?

 

Yup, online...

Late - alarm is in door...

 

your blower resistor is shot. and high is supposed to bypass the resistor. i think you feel more flow cuz the maz door is working. try hot wiring the blower to straight 12 volts.

 

Okay, thanks for your analysis!
Where do I find that resistor and how does a dummie like me "hotwire" the blower?

(Guess I read about that checking procedure in the Service Manual...)

 

Here is the Late 77 A/C drawing, I changed a couple wire colors to match a Late 77.
Make note of the function control switch...
Brown wire circuit #50 is from fuse panel
Circuit #50 also jumpers directly to LO at the blower resistor.
With Ignition Switch ON your fan WILL run on lo speed, this is designed that way so fresh air enters the cabin.

 

That brown wire circuit #50 at the blower resistor now passes through the blower resistor and through a normally closed set of contacts of the HIGH SPEED BLOWER RELAY and then to blower motor.
If you go back to my photo on post #22 you can see the High Speed Blower Relay.

 

Ah, that one in pic 1. - I see...

Thanks, guys - I will dig into that but will have to go off now...
Thanks for your support! Much appreciated!

 

I would disagree at this point with derekderek about a bad blower resistor, only because you can see there is a Common circuit out of the resistor to the N/C contacts of the High Speed Blower Relay.
And you do have the Lo speed of fan motor with ignition switch on...

 

To OP:
You asked about getting all fan speeds when MAX A/C is selected.
In 1979 the circuits changed allowing ALL fan speeds to be available in MAX A/C position.
4-vettes said he did the circuit changes needed to do this.
TEST:
Now that the OP has the colored diagram for a Late 77 post #29 can the OP or anyone else figure out what changes are needed?
The solution I found requires no wire cutting, but is associated with the Function Control Switch.
If you were to purchase a replacement Function Control Switch for a 1977-1980 it would be GM #9348045 so all the Function Control Switches are the same.
The drawing I made below includes both the Late 1977 and 1979 Fan Switch and Function Control Switch circuits.
What would others do to get ALL fan speed available in MAX A/C on a 1977....
Notice, some circuit numbers differ between the 77 & 79 don't let that confuse you...
Tip:
Circuit #50 Brown Wire is your common switched +12vdc to the Function Control Switch.

 

Hey guys - just another update:
- over the weekend I had a look at the resistor, cleaned its contacts just a bit and found the blower working in all stages, now, just as advertised! Great!
- not so much air is coming out of the vents (except on AC Max), which may well be due to the doors not working properly.

My ordered Mityvac vac-pump/tester was delivered on Sat, so as a next step I now plan to check for proper vacuum at the center console's HVAC-"switch"/plug.
I think I recall reading that the system works with some 8 inches of Hg, correct?

...My GUESS is that at some recent dismantling of the dash the vac-source-hose may "just" have slipped off from a tee, that being the reason that the vac just does not "arrive" at the switch. But for that to check/verify and EASILY fix, I will have to pull the dash.......

I will let you know what the above mentioned quick test of the vac from "switch"/plug to the servoes will bring.

 

Good to see you are willing to spend the money for a very valuable tool for your C3
Lots of variables with the 8" rule...
Here is how I would perform a vacuum test just to see what my system is actually producing.
In the engine compartment disconnect the smaller of the 2 vacuum hoses on the check valve.
Attach vacuum gauge and record. Use that value as your benchmark.
Take a reading at the black hose at the Hot Water Shutoff valve on top of the heater box, record
If you have a tight system the reading should be the same at Port #1 (black) hose of the HVAC vacuum switch connector.
Remember, that same vacuum line at the check valve is also feeding the Headlight Switch and is used as the signal circuit to the headlight relays.
Generally, I use the same test for any actuator as they do for the Cruise Control Servo.
Apply 14" of vacuum, if the actuator leaks down more than 5" of vacuum in one minute it is considered defective.

 

Thanks for the checking-procedure. Very valuable!

One thing: my headlight actuators do work a-okay - shouldn't that not be the case if my system would have a leak?

 

The 8" rule applies here,
I only brought up the headlight switch because it is part of the vacuum circuit.
Yes, The headlight relay(s) should only require 5" or more to shift the relay spool...
Let's use this as an example...
Your engine is producing 17" of vacuum at the check valve...good...
You have 10" of vacuum at the headlight relay actuator with the headlight switch pulled, loose of 7" of vacuum somewhere...
But the headlight actuators are still working a-okay...
Is the lose due to a faulty HVAC device?
Is the lose due to a headlight vacuum circuit device?
See where this is leading???
Use the benchmark at the check valve for your HVAC controls for now.

 

Yes - thanks for your explanations... Very educative!

I still don't look forward to pulling the dashboard (I am not even sure if I will start to tackle this before next year - and during winter I won't be able to, as my garage does not allow for fully opening the doors), but I very much appreciate the learning curve from all this...

 

If you pull the map pocket on the passenger side you should be able to get to the Hot Water Vacuum valve to get a reading without removing the center cluster, just a thought...

 

But the map pocket is screwed to (from) the rear of the passenger dashboard...!?

(Just) Loosening the center is not THAT much of a hassle (this I did a few times in the past weeks), but pulling the radio plus disconnecting the connectors of the center gauges is - and then the ducting.....
Anyhow, whenever I will dive into that, I will do it once and right then. I will also perform your suggested test then!
I also thought about getting a new hose-set, just in case I find anything in need of replacement (if it's not ony a slipped connection). I don't have to wait for getting the parts from overseas, then (remember: I'm located in Germany)...
Taking off the side panel from the transmission tunnel to get to the HVAC-switch/-connector to test the vacuum from there is piece of cake, though, and next in line for me to check out.

Let me repeat that I extremely appreciate your comments and help!