Cadillac Repair: Heater control 68 Cadillac, vacuum tank, vacuum motors

Question
Recently I got my Caddy out of storage and the heat/defrost isn't working. None of the options on the control are working. I think its regulated by vacuum, but don't know anthing about it. Thanks, Bill

Answer
Hi Bill,I never had the oppertunity to work on one this old but you are probably correct in that the mode doors are controlled by vacuum. Do you have all the fan speeds and does the air just come out at the windshield with the engine running? If so then you will need to trace the vacuum lines from the engine to the vacuum tank...it is usually mounted on the firewall or wheelhouse under the hood and might be a plastic tank or a big can that looks like fruit or vegetables are sold in...to the back of the selector switch...these can leak vacuum...to the individual vacuum motors that control the doors. A vacuum gage works real well to help determine the problem area. With the engine running test for vacuum at the intake manifold to see what your engine is creating. Around 16 or higher would be a normal number. Then trace the hose circuits and by watching the gage you would know where the problem is due to less or no vacuum than your first reading. If I had to guess I would say the selector switch behind the control panel is bad and with good hearing you might be able to hear a hissing sound from there as the lever is moved. You could also have several cracked or mouse chewn hoses. Hope that helps. Bill