Dash Temperature Control: Hot or Cold, no inbetween

hikerman wrote:
You might install a manual water cut-off to your heater core. That may also give you colder a/c in the summer.
It helped my '04' Newmar WH chassis.

true sometimes when using AC only but OP is trying regulate the cabin air temp while in heater mode.sounds like the controll switch sends too much vacuum to the control module and its either closed or open and not staying part way