mountainmanjoe

weird. I would expect if the ignition switch was bad, that you would notice other things not working.
It also doesn't explain why the blower didn't work on high speed when activating the relay manually.

yeah vacuum leak.

trbsami

Ok... this is becoming a rascal: all fan speeds and temp selection work fine. All air still blows out the defrost vents. Vacuum leak somewhere I assumed and have been searching. Replaced (GM part) the 3 legged check valve in the vac line. Old part passed the blow/ suction test same as new, so that`s not it. Good vacuum from motor. There`s a canister under the battery box I assume is just a reservoir? to hold blend doors in place while the motor`s not running? Vacuum line makes it`s way into the firewall and becomes 2 separate lines: black and brown. With the truck running I get good vacuum on the black, but none on the brown line- am I supposed to? If so, could there be a problem at that passthrough point/ manifold thingy? Maybe the blend doors are stuck/ broken inside the plenum? Any ideas appreciated:-) It`s starting to get cold here in central Indiana...

mountainmanjoe

Yes. But also when there's insufficient vacuum in the intake manifold (ie open throttle, driving up hills etc.)

Brown should be a heater valve
I've posted a vacuum diagram here
All of these are possibilities.
You can rent a smoke machine that will give you positive confirmation of no leaks.

You can also use a hand operated vacuum pump directly on the control unit to makes sure it works.

Pink and blue hoses control the vent mode door

trbsami

Up and running complete! Thanks for your help! Turns out my neighborhood mechanic who replaced the ignition switch had gone through all my new stuff first instead of listening to me first, (boo)... then left the vacuum hoses disconnected from the control panel in the dash. I plugged them back in and viola- all good. Now searching for the best front brake rotors that will last more than 10K miles...