Sorry for adding yet another diverter valve question, but I am having trouble finding an answer.
I have an 85 Trans Am, factory 305/carb w/AC. The heater core was bypassed when I got it so I had nothing to use for reference on the car.
The harness from the firewall only has one vacuum line in it and it's for vacuum supply to the control unit.
The control unit doesn't appear to have any way to control vacuum to a diverter valve in the engine compartment, it does have a cable that controls a flap/blend door in the air box.
Do early third gens not use the vacuum controlled diverter valve? I don't see how one would function as this car is set up.

 

Most of these cars, esp the earlier ones, didn't have that. Coolant circulated in the heater core full-time. Can't speak to the matter of 85 Firebird particularly. The air doors took care of all temperature control.

 

No Diverter valve used on the 85 TPI,..... that's why the HVAC harness only has 1 vacuum line.

 

Thanks for confirming! My 88 has one so I was pretty confused trying to figure out what was going on with this car.