have 93 g20 van conversion with front and rear a/c & heat.
heater blows cold until I remove vacuum line from main heater control vale. plug it back in and the valve arm actuates and lose hot air. I can't seem to find a diagram for my actual van , and right now I can't afford to take to a shop.

 

Hey, have the same problem. Did you just leave the vacuum hose off? Did that work for ya?
Also, do you have rear heat? and did taking the vacuum hose off mess with the rear heat?
How did you get this situation solved, if ever...
Thanks!
I don't have the time to mess with a new valve...and it's damned cold out (-2) so I'm not willing to spend a couple hours outside.

 

It was a vacuum problem, but it was the control panel.
On the temp control there is a extra little vacuum line that wasn't actuating, in fact there is a little control with separate vacuum lines to it , and it just grenaded.
Had to go to wrecking yard to get new control panel, now it works great!
Hope this helps, good luck.

 

Thanks for the quick reply! I'll probably tear into it tomorrow night...

 

Both of the ventilation control vacuum switches are available new.

The 9-port switch is ACDelco 15-71871 and the 3-port switch is ACDelco 15-7975.

 

Both of the ventilation control vacuum switches are available new.

The 9-port switch is ACDelco 15-71871 and the 2-port switch is ACDelco 15-7975.