Have a 1996 E250 Van which when I have A/C on and fan blowing. When accelerating on a hill all airflow diverts to defrost vents.If let up on gas the air comes back to the A/C vents. I have a vacuum gauge under dash and vacuum stays at 18 to 20 while this happens. What could be causing this?
Ford dealer where I live could not figure it out. Also a A/C n Heat shop couldn't either.