HVAC control question

[BARRACUDA1968]

I'm going to dig in to my HVAC system and try and get everything working. Right now the air only comes out on the floor. I'd like to have it working properly.

Is there a common issue I need to look for? I'm assuming it's vacuum controlled and if so where is the source of the vacuum so I have a place to start?

Any tips would be appreciated.

 

[aminga]

sounds like no vacuum to the control head.

 

[BARRACUDA1968]

Is the vacuum supplied from the canister over by the stock A/F location?

 

[aminga]

There are 2 lines that go through the firewall by the ECM (yellow and purple). One is the signal to the heater control valve the other is the input to the heater head. It runs across the back of the firewall to the check valve with 3 ports on the firewall behind the coilpack. I'd start there and trace it to the heater head and see where there is no signal. It could also be the blend door actuator or the heater control or .....

 

[rev. 3:11]

Easy first check:
Inspect the check-valves (disc looking valves in-line) the can go bad & kill vac pressure;
easy replacement, although the newer ones I've seen are much wider than OEM; more a cylinder shape.
Just make sure you orient it properly when replacing.
Good time to check 'em all!

HTH!
D

 

[BARRACUDA1968]

Thanks, this gives me something to get started with.

 

[Nick Micale]

There can be many issues affecting the AC vacuum controls, and the basic and most important item is having vacuum to the control unit in the dash which has already been defined above.

The control head in the dash has a rotary vacuum switch that supplies "power" to actuators under the dash, and it can come apart and not do its job.

The default position of the main blend door is to the defrosters, and a vacuum actuator needs to move the door for air to come out the dash vents.

The actuator can be bad or sometimes the doors stick in place as the rubber gasket can be a glue to keep it from moving.

Sometimes a plastic retainer will break off a link or rod and a internal door may not move?

A simple broken plastic vac supply line can be broken, or one or more areas in the vacuum circuit can cause a malfunction of the AC/heater system.

Hope yours is a simple issue!