Weird HVAC problem

[JRKRACE]

Here goes:

 

2004 GMC HD2500 Crew 4x4 6.0 gas, SLT.

Plow package so it has the manual temp adjustment...NOT DIGITAL.

 

When my A/C or Heat is on, the airflow pulses back and forth between the floor and the dash when it is set on the dash. Defrost and floor work fine. When I put it to the dash vent position, you can actually hear and see the duct control motor under the dash right near the gas pedal go back and forth. It never stops until you switch positions or turn it off. Any ideas?

 

Thank you in advance...

 

Jim

[JRKRACE]

Thanks anyway for the help......It turned out to be the dashboard controller.