No AC/Heater control

[Pits4sprint]

I have a 96 Blazer 4wd that sat for a year while I rebuilt the engine. After getting it all back together and running great I found out that the AC/Heater selector switch isn’t working. It stays on defrost all the time. After checking all fuses, I pulled the switch and found that it is a vacuum switch with color coded lines. When separated (engine running) there isn’t any vacuum at any port, but when vacuum is applied to a port the airflow is redirected to vents (only had enough lung power to try 1 port). I heard that there is a vacuum pump which supplies the vacuum, but I can’t find it without pulling the whole dash. My Chilton’s isn’t any help at all. Thanks Rob

[magnum3456]

Originally Posted by Pits4sprint

I have a 96 Blazer 4wd that sat for a year while I rebuilt the engine. After getting it all back together and running great I found out that the AC/Heater selector switch isn’t working. It stays on defrost all the time. After checking all fuses, I pulled the switch and found that it is a vacuum switch with color coded lines. When separated (engine running) there isn’t any vacuum at any port, but when vacuum is applied to a port the airflow is redirected to vents (only had enough lung power to try 1 port). I heard that there is a vacuum pump which supplies the vacuum, but I can’t find it without pulling the whole dash. My Chilton’s isn’t any help at all. Thanks Rob

I have sorta been having the same problems...i replaced the little rubber maze looking thing inside the air control knob switch thing...it only helped a little. I wanna figure this out too...so ima keep checkin up on this one...

[old_master]

Vacuum to the control selector is supplied by the engine through a 1/8" diameter soft black plastic vacuum line that connects to the intake manifold. There is a tee, a vacuum check valve and a vacuum reservoir under the hood. From there it goes through the bulkhead, (firewall) to the back of the selector switch. If vacuum is not supplied to the selector switch, the HVAC system will default to the "defrost" position.

[mrfirebird]

i just fixed the kids saturday, it was a tee at the back top of the motor at the firewall. you could here it sucking air with the motor running.
just what the old master said

[Pits4sprint]

Thanks for the help! It turned out when I rebuilt the engine, I tucked the manifold adapter for the pcv valve / vacuum in the master brake cylinder lines. I thought I had lost it and the replacement didnt have the vacuum port. Hooked up the old adapter and everything works great now. It even cleared up the random misfire which I thought was due to junk in the fuel lines. Thanks again, Rob