There is a small vacuum hose that runs along the front of the throttle body.Usually gets brittle and brakes .This runs to the passenger side of the firewall this goes to the heater box.without vacuum the valve under the dash can not switch.I hope this helps
SOURCE: The air from the airconditioner and heater only will blow out the
Sounds like a vacuum leak. Check the vac supply on the intake manifold. Passenger side front of intake.
SOURCE: heater blower does not come out vents
i had the same issue and it was the vacuum hose that goes over the alternator to the ac unit
SOURCE: defrost vent door wont open
sounds like you have a vaccum leak or a blockage in the blend door area of the HVAC system most of the doors are controlled by vacuum if not by vacuum on of the servo motors that control the blend door is not functuning properly. Hopr this is helpfull
SOURCE: a/c and heater only blow air out of defrost
either a vacumn pipe is broken or the ends split so its not passing a vacumn to open the flaps,where they are and how to get to them i cant help because i have never worked on one as they are not here in spain.check the engine end first though .Look for some small rubber pipes running from the inlet manifold
SOURCE: GMC 2003 truck heater/ac not consistent.
check for any vacumm hose leaks that comes from the engine to the ac box, check for vacumm actuator proper operation.
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