71 GMC Factory Air Problem

[Eddie H.]

Originally Posted by Wrenchbender Ret

There is a box under the dash that has a diverter flap in it. These are nortorious for going bad. The valve comes loose & won't let the air go through the core.

Yep, that's another common problem with these systems. The blend air door, directs air through the heater or evaporater cores depending on the temperature control position.