Thanks for responding to rje post.The house has 1 ac unit, dual control.the upstairs gets very cool but downstairs air seems to be blowing out at the upstairs return.I think its ether the damper not working correctly or pressure bypass.What do you think.

I believe the damper is the problem as before you can manually move the damper if you disconnect the motor drive. This way you can test the theory and find the defective part. In the mean time after manual adjustment you will be cool upstairs. The bypass check/test would be your next choice for testing.