i have a 71 chevelle w/ac the controls in the dash seem to work fine but when i turn the fan on i can hear it running but nothing comes out of the vents and it wont change where it comes out at. any suggestions? is this ran off of vacuum. if so where would they be?

If there is a vacuum problem the system should go to full heat and into defrost mode. This is done for safety. If there is no air coming out of the defroster nozzles, then it may be that the blower is not working. This may be caused by the blower wheel not turning with the motor. Sometimes the hubs of blower wheels can fail.

There Is 2 Vac Lines Coming From The Fire Wall Inside The Engine Compartment On The Pass Side . Do You Know If These Are For The Blower Control Also ?

Those are to supply vacuum to the control panel. Vacuum is what works the little doors that change from dash to floor to defrost. As JWagner says, it should at least be blowing out the defrost vent if the fan is blowing and it's the vacuum that's messed up. Are the vac lines connected to something? If not, get that fixed.

ok thanks i will try that and see if that works

The vacuum line to the heater control unit has a check valve in it so that the heater system doesn't change mode upon loss of engine vacuum. If you don't include that check valve in that line you won't be satisfied with the heater system performance. Every time you step on the gas the system will default to defroster without that check valve.

Where Would The Check Valve Be Located?

It should go on the line coming thru the firewall on the passenger side of the car. On my car its a vacuum line maybe 6" long , then the check valve and finally a line to the vacuum source all of which is right beside the evaporator casing.

This is what that check valve looks like. It allows vacuum to the heater control system and holds the vacuum when your engine vacuum decreases. I am guessing that you can still buy one of these from your chevy dealer. In my particular case I have it routed to the three way vacuum fitting on the intake manifold, behind the Q-Jet, that supplies the vacuum modulater on your auto trans.

71 has a vacume ball instead of the check valve that a 72 has,vacume ball is on the side of a/c case