Hi guys!

Years ago, the great running A/C in my 1977 started leaking and quit working shortly after.
As (here in Germany) I could not find anyone to replace the compressor front/shaft seal (which appeared to be the "simple" culprit), I had to have the R4-compressor replaced. While at it, I also had the drier bottle replaced at the same time.
Unfortunately R12 was already banned and I got the recommendation to re-fill with R413A, which had the benifit to retain the seals, which would have been necessary to be replaced in case of a change to R143A.
I was not overwhelmed with the cooling properties of the R413 in comparison to the old R12, but at least I got cool air back.

Then, a little while ago, the heater core started leaking and had to be replaced.
With that repair done, the heating was nice, but I did not get much cool air anymore.
It took until recently that I started to look for the problem and a solution...

I noticed (fairly easy if it had not been necessary to take apart half to the dash to get there) that the control cable from the temp-lever was not in contact with the "bypass valve vacuum switch" on top of the outlet of the heater core housing with the lever fully left="full cool".
I found this "screwing device" in line with the control cable, by which the length of the cable can be adjusted/fine-tuned, and I turned it until the switch was depressed with temp-lever in the "full cool"-position and until the lever started to move right. I assume that's the optimum cable length.

Result: I got my cool air back, as that switch stops hot water getting circled through the heater core and the vented air being cooled by the A/C instead.

That is the case when I have the A/C in its "max"-position, but when I shift the lever to the "A/C norm", the cool air dies down quickly...

There is a nice youtube-vid where a guy shows the replacement of vacuum lines in his 69 Corvette: ...in which he also explains how the vacuum operated vent doors should work.
With his explanations I was already able to check my car's "exterior air vent"-door at the plenum/wiper compartment. It is open (one can see the squirrel cage when looking behind that mesh screen in the wiper compartment, verifying the "open"-position) and/but it remains open with the car running and at whatever position of the vent lever. That guy says that at the "A/C max"-position this door should close together with the one in the passenger footwell opening to allow for a closed loop: passenger footwell->plenum->squirrel cage->interior of the car and back to start.
As both valves are operated on one and the same vacuum-source, I guess I should probably check next, if the pass footwell door opens at the "A/C max"-position... If yes, then the exterior air vent-door must be "stuck somehow", if not I have a leak somewhere in there...

But generally I was asking myself is: is there some troubleshoot-procedure that I can follow, or how shall I start with my investigations?

Thanks for your help and guidance.

Greetings from Germany
Oliver

 

On my 77. All is working great. Yes the door above the blower closes on AC max creating recirculating air in the cabin. When that door closes, the one in the kick panel opens.
The issue I had was the factory did a very poor job of sealing the upper one. Some foam glued in place helped a great deal to seal that door and keep out unwanted outside hot air. I also did a minor rewire so I can control fan speed on max air, thus no need to switch it over to normal air which allows outside air in.
I live in tropical Queensland Australia. I disconnected the vacuum switch on top of the heater box completely and removed the heater hoses and installed pipe plugs.
And yes you need to pull at least the passenger side dash panel and right side console cover to access the vacuum lines. Even then, you can't get to all of them.

 

Thanks for your QUICK reply.

Interesting to learn your location in Australia. Heat was not all too common in Germany, and when I purchased my 77 some 30 years ago, having an A/C in a car was more the exception to the rule. That changed a lot over the last years (especially as - I would say - one sees the results of climate change with temps exceeding 35°C/100°F more often than in the past). Anyhow I always very much appreciated having an A/C in the Corvette and was missing it badly when it did not work properly...

Also thanks for the above diagram... I already found that one on the net which helped enormously understanding what the guy in the youtube-vid was talking about.
I was just wondering if there was some sort of trobleshoot-procedure, like "in the old days": check #1 - if "yes", check #2 - if "no", move on to #8... (Or like, "with the car shut down, this and that door/valve should be closed, and when starting the car it should open..." or similar...)
I have not been able to find anything like that.....
In addition to your diagram above, I also found another more "schematic" one including a table telling how each of the control's outlets should "behave" at any given position of the Heater / A/C Control Switch. That would be one thing to check out the control switch, but I am reluctant to pull a perfectly sealing connection just to try out something I might find out some other way, maybe...

 

you have a vacuum operated bypass door and i am pretty sure a vacuum operated coolant shut-off to the heater core. that coolant shut off is not a 100% seal. add a shut-pff valve to the heater hose. there may be a few days when it is an inconvenience in the morning cuz of a fogged up windshield but your heater is now OFF. if the door that closes off air from the heater core is not quite closed it would be allowing air across a heater core with no hot water in it.

 

Agreed, don't unhook anything if you dont have to.
Do you have factory service manual?




Absolutely chocked full of information on how the system is designed to work and how to test.
If you don't have the FACTORY service manual. You don't have a manual.
You wouldn't bake a cake without a recipe would you?
Start with get the car running with the right hand kick panel removed and the screen above the blower motor removed.
Move your sector arm through the various positions. Does the air blow down on the floor when on heat? Does it blow up when on defrost? Does it blow out the center and side vents when on vent. On max air, does the upper door close? Lower door open? on reg. air. Upper door opens and lower door closes? If all this is working, don't fix what anit broke. When the temp control is swung all the way to cold. And this is the one thing you may need to unhook to check. Are you getting vacuum on the white vacuum line to the coolant shutoff valve?
If all these things check out. cool.
Next look at my Air-con upgrade thread.
"73-E77 Air-con upgrade"
I get truly cold air on 134 freon.

 

Oh, and Derek, it's a cable operated door. When it swings all the way shut it hits a vacuum switch that opens and sends vacuum to the coolant shut off valve, which is of course vacuum operated. And your correct, it's not a great part.
Also that door that closes off the heater core often does not seal off well.

 

COOL - many thanks for that! That'll be a good start...

And (aaarrgh!) yes, I DO have a factory manual - stupid me that I didn't check that first before going online... As always: it's so much easier asking, than finding a solution oneself... Embarrassing!

Thank you as well, Derek.

 

Good, you have the 77 service manual!
Yes, Section 1B you will find the correct info for the "Y" (Y=Corvette)
With the slider control lever in the COLD position it closes the air circulation path through the heater core.
Air still flows through the evaporator housing in ANY control switch positions.
The only difference between MAX A/C and NORM A/C is you are recirculating the cabin air on MAX A/C.
NORM A/C you are drawing in ambient air, and with the slider control lever in the (White to Red range) or HOT it opens the air circulation path that air also passing through your heater core.
"IF" the hot water shutoff valve in the engine compartment is leaking by that would explain your NORM A/C cooling problem.
Per-cooled air vs. trying to cool ambient air and expect to see similar results, not going to happen.

.

 

Fully understood - thanks, Joe...
So, the cabin air does only go by the A/C evaporator (and not the heater core) on A/C Max? (I will have to look up in my service manual if I can find a drawing before asking more "silly" questions...)

Again: thank you all. Helps a lot in getting me (my car) on track.

 

Not quite. On max air you get recirculating air. No fresh air is being drawn in from out side. On ALL other functions, the system pulls air from the wiper tray area. Heat, defrost, vent normal air all pull air from wiper tray area. Max air pulls air into that door in right side kick panel and recirculates the air within the cabin.
Air is pulled over heater core or not depending on how you swing the door with the temp lever/cable. However it is very common that the door doesn't seal very well. If the system is working correctly. vacuum goes to water shut off valve in heater hose and closes off the hot water to the core. at the same time of course the door is swung so air bypasses the heater core.
On normal air your pulling hot air from the wiper tray and trying to cool it. Make certain that the weather strip on your hood that meets the forward edge of the wiper tray area is in very good condition. Otherwise your pulling HOT engine compartment air in.
All this make sense?

 

Perfect sense... I just have not figured out the heater core/evaporator-thing - is it one and the same and circling either heated water or evaporating A/C-coolant trough it, or are there two "radiator-like" items in the heater box?

 

Please Note my edit of post #8, blue text

 

Since you have the 77 service manual,
Page 1B-19 figure 1B-24.
Selector Lever Position:
MAX:
Air Source:
Read NOTE following %

 

As 4-vettes said, when you select MAX A/C the re-circulation door (passenger kick panel) will (should) open.
Because you have selected MAX A/C your fan speed is automatically set to HI, as the re-circulation door opens you should hear a whooshing sound as it is opening.

 

As I noted earlier, but worth mentioning again in case it was missed. I did a minor rewire so on max air I have full control of fan speed. Minor, maybe. But on max air, factory wiring, you can only have high speed fan. To drop the fan speed means selecting the normal air. Which introduces the outside air, Unlike most modern cars. if you just want to turn down the fan without taking it out of recirculating air you need to get creative..
To me in tropical Queensland. This was the correct answer.
In Germany, maybe not so much. But who knows?

 

Oh, and to be clear, the heater core and the evaporator are 2 totally separate things. the door swung by the cable determines which core, or both if halfway, the air passes.

 

no to heater-evap being the same piece. only on home heat pumps. the ac evap has refrigerant in it. the heater core has hot coolant in it. they are right next to each other. even when heater sealed off completely there is hot coolant circulating through it. that is why most of these cars get some kind of ball valve to completely stop hot coolant from getting to the passenger side of the firewall during AC season. every bit of heat you remove or block from where you are sitting makes AC work better.

 

I disagree,
Air flow passes through the evaporator housing no matter what position the control switch is in or COLD HOT lever position.
Both case inlet and case outlet housings are secured together and then attache to the inner duct assembly housing.
3 different housings, air flows through all.
There are no manual or vacuum operated device directing air flow to the inner Duct assembly.
Case Inlet, Fan blower housing
Case Outlet, Outer evaporator housing
Inner Duct, Assembly shown above

 

Thanks for your comments/details provided - especially the photos! That gives me a far better picture.

I also checked out the extensive section in the service manual. (Interesting to see that nothing of all that has found its way into the repair manuals of Clymer, Chilton, Haynes or the 68-82 Shop Manual published by MBI.)

In the Service Manual I also found a hint to check for the operation of the heater/evap-door (that must be the door operated by the temp-selector-cable) on pg. 1B-34: by moving the temp-selector from cold to hot and back quickly a few times, one should hear the door hitting at both ends - if not that leads to an adjustment of the door. Another good thing for me to check...

Btw: the photos show what I saw when had the dash off and found the "Heater Hot Water Shutoff" vac-switch - the yellow arrow in photo #2 points to - not being touched by the lever at the end of the cable (the first thing that needed to be corrected). Until then I always had hot water flowing through the heater core, and even with the door closed/selected for A/C the system had to fight the nearby hot air in/from the heater core...

Another check from the Service Manual is for the blower - I know it does blow with A/C Max, but think there is an issue with all other modes. I tend to recall that after having the blower run in Max for a while, it does run in other modes as well, but am currently not too sure, whether the blower kicks in from modes other than A/C Max... (@ 4-vettes: interesting to learn your mod about the blower, but I guess much more of value in your hot environment. Ad then I'm the NCRS-type of guy... )

So, I will take the car out of the garage and start checking:
1) heater/evap door operation
2) if the kick panel door is closed - and will open with A/C on Max
3) if the door shutting of outside air and allowing for a re-circling of pre-cooled air does close at A/C Max
4) if/where the wind blows at any selected mode

 

Okay, the results:
1) yes - I hear the door hitting at least at the lever's "cold" position - which should mean that at "cold" no warm air is added (right?) - not sure if I hear it hitting "hot"...
2a) closed yes - but b) it does not open at A/C Max. I pulled the vac tube at the valve and don't hear any "hissing" or feel any "suction" at the tube
3) no - outside air door remains open, kick panel door remains closed

I can confirm that the blower blows at A/C Max, but little wind only on other modes (I forgot to check if the squirrel cage remains on/high - which it is on A/C Max, as I can see through the faulty open outside air door...)

4) with only little blower performance, I think I felt air coming out of the defrost and the other vents at their respective modes - but that needs to be verified with the blower problem fixed.

Anyhow, I obviously do have a vac-problem (at least) at the green and brown tube, which I need to find a remedy for. (Removal of the dash necessary again... )
Any hint, where to start?

But a good point is that I do have cool air and it can only get better, as/when the difference between A/C Max or Norm only is the position of the doors in check #3...