On a 1987 corvette, I’ve already replaced the vacuum check valve controlling both the hvac and cruise control. That fixed my problem with air blowing from all vents, but only at coolest temp setting. In economy mode and with temperature set at 60 degrees, the air will blow from the front dash vents as it should. As you increase temperature settings, the airflow will gradually shift to all vents (defrost, dash vents, floor). At temp setting 90 degrees, air blows equally amongst all vents. Heat works fine blowing hot air. I’m assuming the blend door controls the cold to hot airflow from the heater core. Am I right on this and if so, any ideas why I’m getting correct airflow from the dash vents on cool setting but wrong airflow at temp setting 90 degrees?

 

Electronic controllers work great till they don't. How did it operate before your vacuum issue? Could you still have a vacuum leak? Down by the brake pedal there is a climate control module, could it have bad vacuum, bad electronic connections and cracked solder issues. The box can be opened up and checked out. I would also check the electronic contacts on the back of the dash control to see if they are clean and tight. Good place to start before you start replacing parts..

 

Before I replaced the vacuum check valve, the cruise control didn’t work and air blew from all vents regardless of where the temperature setting was. I was leaning to the control module you’re talking about too, but was hoping to rule it out. I’m not a flexible as I was in my youth and GM made those foot wells impossible to get to the underside of the dash without hurting yourself in the process. 😂

 

If you think your vacuum source is good then it must be in the controls module or controller. I'm guessing. Without a manual. Not sure if utube has anything that would help. ebay carries a few parts for that system if you want to swap parts out. I have decided if I was to work under corvette dashes it would be easiest to pull the seat out and place a blanket or coat on the floor to make it a more comfortable project. The older corvette owners these days buy newer cars so they don't need to be worked on, I'm not there yet.

 

There is a company called batee.com that deals with these hvac systems, they take your old controllers and rebuild them.