Hey guys, need some professional/expert advice.  This morning when I started my 1963 Cadillac and turned the heat on, all it would do is blow out cool air from the lower (floor) vent. If I switched it to air conditioning, the compressor would kick on, but it would still only blow out cool air (not air conditioned) from the lower vent.  In other words both the heater and air conditioner settings do not seem to be functional.  If I slide the lower temperature control adjustment from cool to hot or from hot to cool, it makes no difference in the temperature of the air coming out.  Is this a vacuum issue of some kind and if so where would the likely culprit be?  Is there something else I could check as well?  It worked fine last time I drove the car. Thanks, Doug

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Sounds like a vacuum issue. The selector mode door is not working to switch from floor to vent, and the heater control valve is not working. Check that the vacuum lines under the dash are connected. Look at the NSS, parking brake dash pot, AC/heater controls, and the firewall electrical connector in the engine bay.

Thanks, Russ.  I briefly checked the NSS connections this morning before I had to go out of town and did not see any loose connections, but I will check the other areas tomorrow when I get back.  I know the vacuum check valve near the NSS is bad as I usually have to manually (from dash control) move the lever from cold to warm and back to get the selector mode door to open/close when the air conditioning is turned on, but this did not work this morning in either the heat or air conditioning settings and I could get no warm or refrigerated air out of any vents, just the blower air out of the floor vent or defroster vents. 

You mention that your vacuum check valve is bad. That could very well be the culprit here. Its an easy and cheap remedy. Just buy a vacuum check valve and fit above the original. Takes about a minute and will eliminate that problem.

The temperature valve (door ... see Fig 13-18 pg 13-20 in Shop Manual) is controlled by a Bowden cable from the dash control panel.  (see note bottom left page 13-18 in Shop Manual).
The diverter valve (mode door) that directs air up to the AC dash vents or down to the floor heat vents is controlled by the tan striped vacuum hose. Check that the Bowden cable on the temperature door has not slipped off. Also check the tan striped vacuum hose to the mode door actuator. Regardless if check valve is operational, if system is tight and you have constant vacuum manifold applied to the system the mode door should operate correctly.

Thank you, Jason!

Just to add, check the mode door actuator. Just disconnect the tan striped hose and connect another hose with vacuum pump and see if it holds vacuum. I see these busted in a lot of the parts cars I have brought in. Also, check the hose tip and fitting. Cut a but off as Tony suggested but first make sure actuator is good. That and checking the Bowden cable to temp  door are the 2 main things I would check first.
As far as the Check Valve, you can replace with a T until you can get another check valve or just keep the one in place if it is not leaking. The only thing this really does is keep vacuum to the ac/heat system when you have very low engine vacuum (e.g. Wide Open Throttle). It can cause the mode door to temporarily switch from AC vents down to Floor Vents, but as soon as engine stabilizes and you have vacuum, it will return to correct vents, assuming the mode door actuator is good. The temp door is mechanical.. completely controlled by that Bowden Cable. If temp does not change, I would see of the cable has popped off at the control head or at the temp door.
For reference on the check valve you might want to check out this article on our Help Page:

and this video for testing:


You might want to cut off the tip of all your vacuum lines and reinstall them as long as they are not so brittle they would split if you do that
If brittle due to age they need to be replaced
That could correct your issue- maybe
Check the Mode vacuum valve for a vacuum failure located behind the black ducts under the dash for the heat and AC to flow thru
Follow the vacuum hose to it to find it and see if it hold a vacuum as it should
The Mode valve moves the door inside the black duct that directs the air flow from the floor to the AC vents
Any air leaking into vacuum system can cause the whole system to not work correctly
You can plug a vacuum hose with a screw to seal the system until you can correct the air leak you may locate
All vacuum results from the back of the intake manifold and goes into the cabin from a vacuum hose located there


Thank you for this information!  Doug

When working on trying to come up with a solution for the vacuum check valve in my 63' the top of the right vacuum hose came off and I am not sure where it was attached. I have looked all over and I even removed the dash pad to see if I could find where it came off, but cannot locate.  I have attached a photo of the vacuum line I am talking about. I am sure one of you will be able to help me out. Thanks in advance, Doug


Never mind regarding the vacuum hose connection that comes out of the right side of the vacuum check valve.  I found  where it had came off, unfortunately it was a bummer to get to, but I finally got it on. I was smart enough to use new vacuum hose so it will have a tighter fit than the old hose did. Now I still need to figure out Russ' makeshift vacuum check valve that he designed.  Doug 



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