The vacuum switch for the HVAC could cause this, and slapping the console can correct the issues sometimes. There is nothing electrical about the "mode" control...it's all vacuum operated. The only electrical is for the blower motor, and A/C compressor signal. You'll prolly find a loose/cracked hose, or you may be able to hear a vacuum leak right at the switch.

______________

Joel Adams
C3VR Lifetime Member #56    
My Link


(click for Texas-sized view!)             NCRS

"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

Joel is correct.  I've taken apart and replaced the AC/Heat control panel.  The two parts that are controlled together are the electrical switch for the AC compressor and the blower motor and the positioning of the blower vanes that route the air to Defrost/Floor/Upper Vents.  The vacuum switch can break.  It is a layered switch and can get in the wrong orientation to the lever and electrical switch.  The new panel electrical switch failed about a year after I replaced the entire panel.  I'm now using the old switch externally hanging out of the center console and can route the AC to any of the positions.  Kinda cool but, does not look good with the switch hanging out.
 
This was a temporary fix using cardboard to get the vacuum routed to the AC Normal position.  I did that until the replacement panel came. The color coded vacuum lines are at the rear (right) and you are looking at the panel from the bottom.   I hope this helps.

Keith
kwoody2011-07-29 11:51:58

______________

Nobody ever waves when I'm driving the Explorer.
K Woody

Or in my case, it was as simple as just reattaching the connector that had been pulled off by my car stereo people. I chased that vac leak all over the car.  I have all new vac lines now though....

______________

Finally got around to the vacuum leaks.  Fixed the lights by replacing the relays.  The Ac unit has a really bad leak as the vacuum is gone just a soon as your stop the engine (I have the headlight side plugged off and only looking at the HVAC side now.  I initially found a leak at teh control valve at under the center console.  I can move this around and get it to stop (no hissing sound) and everything seems to work ok.  Do I need to purcahse a complete new unit or do they make replacement parts to fix this leak?

If the control panel is bad you can buy just the vaccum assembly, but I would replace the whole unit (to me it's easier). Just make sure you get the mostly metal one like Mid-America sells. I bought the plastic one first and after about a year it got really screwed up. You had to jiggle the lever around to get it to work.   

______________
"Down on the Boardwalk...

(Click image to see a larger version}