Question about the heat/ac controls in my 71 LeSabre

[John_Maine]

I've only driven my LeSabre a couple hundred miles this year. On one of my first drives, I discovered that the heater/ac controls are not working. I can't shut off the heat unless I put the AC on and am getting nothing out the defroster in either defrost or de-ice mode. I'm thinking these are vacuum controlled or something because I've always been able to hear things switching around when I moved the controls. Any idea as to what's involved to fix it?

[NTX5467]

The first thing to do would be to check the vacuum line from the intake manifold vacuum source, usually a pretty small diameter vacuum line, to the vacuum reservoir on or near the firewall (usually a round sphere with two lines, one "in" and one "out").

Then follow the "out" vacuum line to where it disappears through the firewall, to see that it "gets there intact", i.e., not split or cracked. Other than this visual check, you'll need to have the engine running (in PARK, in a well-ventilated open area!!) at idle to possibly check for vacuum leaks and operation of the in-car items.

Once under the instrument panel, the vacuum source line should go to the back of the instrument panel's HVAC control. On the back of that control is a plastic or pot metal vacuum routing mechanism (about the size of a quarter in diameter). When the slide lever is moved, it rotates one side of the switch to put the source-supplied vacuum to the correct vacuum actuator(s). You might need to watch to see if and what is moving as you move the mode lever between detents, just to make sure what is working and when.

Usually, the default mode (for when the source vacuum is lost) is air to the heater and defroster outlets ONLY, no matter what mode is selected.

Either on the back of the control mechanism or somewhere in the cable linkage, there should be a switch to turn the a/c compressor on in the defrost mode to dehumidify the air before it hits the windshield.

Other than the dedicated vacuum rotary switch, there could be some of the vacuum outlets on the back of a larger, rectangular plastic switch that is part of the pushbutton part of the main switch. The blower motor switch is a separate item, usually.

Just some thoughts,

NTX5467

Edited September 12, 2010 by NTX5467 (see edit history)

[JohnD1956]

Does your car have Cruise control? And if so, does it work right? If not, pull the vacuum line off and plug it with a golf tee, then try the A/C .