GM-GMC: 89 chev silverado, chev silverado, vacuum line

Question
I have a 1989 chev silverado which has a fine working a/c. However when the a/c is turned on, it blows through the defrost vents. Nothing will blow through the front vents. I changed out the heater/a/c control but it didnt fix. Any idea what could be the cause?

Answer
There may be a disconnection of vacuum to the control switch or from the control switch to the duct control vacuum actuator.

If you can pull the control switch back out with everything still connected, start the engine and see if you have vacuum at the hose that connects to the control switch.  If there is no vacuum, trace the line back out to the engine and look closely for splits or nicks in the plastic tubing that GM uses for this purpose.  The plastic gets brittle in the engine compartment from the alternate cold-hot-cold-hot cyling when the vehicle is operated.

If you have vaccuum at the control switch, you will need to follow the vaccum line from the switch to the duct control vacuum actuator.  Again, look for splits, nicks, kinks, or a disconnection at the end of the tubing.  This may require disassembling a good part of the dashboard.

Finally, check the actuator for failure.  A simple test is to get a piece of vacuum line or tubing that fits on the actuator and suck on the other end.  The actuator arm should move.  If not, the actuator is bad and will need to be replaced.