94' Blazer a/c heater blower problem

I have a 94 Blazer that has a problem with air distribution. The air will come out of the vents where it's supposed to, then it will automatically change to defrost and/or down low. It seems to have a mind of it's own. It also seems to depend on the rpms of the engine. I'm assuming this is a vaccum issue??? Help would be appreciated.

Vacuum leak.

Check all vacuum hose going to and from the engine.

Has your engine been idling higher than normal or rough?

Vacuum leak.

Check all vacuum hose going to and from the engine.

Has your engine been idling higher than normal or rough?

No, there is no indication of a vacuum leak from the way the engine runs. It doesn't seem to be sucking air. Could it be the vacuum canister under the hood?

I just cured the same problem in mine. For the hell of it, I got the check valve, all of the hoses changed, and after plugging the hardline that runs the climate controls in the cab interior, checked it for leak-down with a brake bleeding tool. Once I determined that it was not the hardline, with the engine off but ignition on, I did my patent best to keep the climate controls "pumped down". Turns out I had a leak I could not hear with the engine running, and it would cool the windshield and my boots when turned to the "max" position. As soon as I turned the knob to "max" the vacuum gage in my hands would drop!

The solution in my case was in the end, being a rotary knob control. Picked one up after ordering it overnight for $30. Sometimes the grease and whatnot packed into the rotary switch just gets dry and caked with dust and all sorts of crud and start leaking. Swapping it out after disassembling the bezels on the driver's side cluster and radio was a cinch. I put a light dab of vacuum grease around all of the little nipples sticking up out of the back of the switch before plugging the spaghetti of vacuum hoses back into it. I was in and out even with fiddling and replacing all of the vacuum lines under the hood within an hour.

EDIT: Checking the vacuum "ball" reserve under the hood is easy, again using your brake bleeding tool. It would definitely do 2 things for you. 1) You'll find out if it;s cracked and leaking 2) There are some rare cases where it's filled with water or oil. Use the brake tools' capture cup in that case so you do not mess up the pump with trash that *may* be in the ball. The check valve I mentioned is a little round plastic part, usually held to the top of the cruise control servo motor with a piece of electrician's tape. That's a quick, easy $3 to get.

The vacuum ball lines do often go bad. Mine did.