does anyone know how the vacuum should change at the bypass valve while changing the temperature slider inside the car?

i have no heat in the car, the slider sounds like its moving the door in the hvac system. i do have vacuum pressure to open the flaps to change air direction in the vents.

the car sat for a long time and needed some tlc. so far, i've replaced a few hoses, the bypass valve, radiator, cap, t-stat and heater core.

 

Have you tried putting vacuum to the bypass valve directly?

 

i found by removing the vacuum line from the bypass valve i get heat in the car.

maybe there is a leak in the switch that is sending vacuum when it shouldn't....that is my next place to look.

 

Still looking for how to introduce myself on this forum. Not an F-body guy, but saw useful info from some great people!

The valves by default allow coolant flow, i.e. heat, without vacuum. GM typically only supplied vacuum to heater valves when the control head was set to 'MAX' for A/C, or the temp lever was slid all the way to cold. Fully automatic climate control was another story.

I've lost count of the number of GM heater valves I have replaced (multiple styles) because they stick open, closed or leak.

Jerry

 

thanks