John2

I have a 78 coupe and lately when I select A/C I only get air out of the floor ducts...even when I select max ac... When I select defrost I get air flow on the windshield like I'm supposed to. Any help is greatly appreciated

Duke94

There a vacuum servo on the top of the heater box, yours must be rusted or broken. Could also be the vacuum valve in the center console that selects which way the door opens/closes. As I recall on my 78' you may be able to get your hand on top of the heater box to see if the servo moves, but maybe not.

John2

Thanks, I'll try that in the morning

hunt4cleanair

Quote:

I have a 78 coupe and lately when I select A/C I only get air out of the floor ducts...even when I select max ac... When I select defrost I get air flow on the windshield like I'm supposed to. Any help is greatly appreciated

From what you describe you are not getting cool air out of the center ducts...is that correct? And I'm assuming the air you receive is cool.

I went through this on my 78 this past winter while replacing a heater core. Vacuum operates the doors through an actuator. So you must discern whether
1. Vacuum is adequate
2. actuator is healthy (working)
3. Flaps or doors are operable.

Sounds like you've got vacuum if your defroster valve is working. There are five actuator valves...each operates a different door...sounds like the faulty one is the A/C valve located at the back of the distributor box...unless I'm missing something simple. Here they are:

Actuator Valve Location & Operation
Ø A/C Air Valve: This valve is located on the backside of the air distribution box and is operated when the selector lever selects A/C mode. This valve opens and closes the door that shuttles air to the heat ducts and defroster.
Ø Defroster Valve: located on the driver’s side and attached to the selector duct. The DFR control operates the door that shuttles hot air to the windshield when DEF is selected. When DEF is selected, it works in conjunction with the
Ø Recirculation Air Valve: The Recirc Air Valve is located at the top of the kick panel on the passenger’s side and works in conjunction with the outside air valve and the A/C valve. When MAX is selected, outside air is shutoff and interior air is re-circulated which aids cooling of the cabin. In all other positions (VENT, HEAT, etc.), the door remains closed and permits outside air into the cabin.
Ø Outside Air Valve: is located at the top of the passenger fender and can be seen with the hood up while viewing the opening at the passenger side of the windshield wiper tray. You’ll see the black hose with green stripe running to the actuator valve. This permits fresh air to enter and mix with heat or cool air processed by your air distribution system.
Ø Water Shut-Off Valve: This valve is located in the engine compartment and connects two pieces of heater hose together, eventually connecting to the top of the water pump. The water shut-off switch is located on the top of the air distributor box inside the vehicle and operates the water shut-off valve. The intent of the water shut-off valve is to turn off the flow of hot water from the engine to the heater core…thus eliminating heat from the hot water entering the air distribution box. The valve is shut-off when the selector is placed in A/C mode.

At this point and before going further...I would check to make sure each control function is doing what it is suppose to according to your manual. This will isolate the problem area. Oh and the A/C valve I don't think you can get to without pulling the ductwork but look up under the dash toward the floorboard and see if you can catch a glimpse of the A/C valve. It looks like the cruise control servo unit...you know what of those pie tins soldered together with a bladder inside. If you've got cool air and it is simply not coming through those center ducts...it is probably a faulty A/C valve...this is what you must diagnose...let us know.

John2

Thank you very much for the detailed information, I'm going to print it and go hunting. You were correct to assume that the air coming from the duct is cool.

Thanks again,

John

Neo Fender

"The (hot water) valve is shut-off when the selector is placed in A/C mode."

I have to respectfully disagree with you there. On my '79, the valve is controlled by a small diameter vacuum tube which enters the passenger side firewall. This tube is connected to a switch which is located above and forward of the glove box, below the dashboard. This switch is controlled by the temperature control via a cable to the switch. When the temperature level is moved toward "hot", the cable actuates a plunger on the switch which routes vacuum from the switch to open the hot water shutoff valve.

hunt4cleanair

Quote:

"The (hot water) valve is shut-off when the selector is placed in A/C mode."

I have to respectfully disagree with you there. When the temperature level is moved toward "hot", the cable actuates a plunger on the switch which routes vacuum from the switch to open the hot water shutoff valve.

I believe we're saying the same thing. I'm referencing A/C mode and for the sake of brevity...I intentionally dropped the detail. But specifically what happens is that the flow of coolant is stopped to the heater core (in a normally operating system) while in A/C MAX mode.

You are correct that there are two valves (actually three) in A/C MAX mode...the plenum actuator, the Bowden cable switch you describe and the hot water shut off valve located in the engine compartment. And maybe a fourth...it also shuts out fresh air (that is hot) that comes in through the passenger side kick panel actuator valve actuated door. Valves and switches...all of which are vacuum operated...bottom line...when A/C MAX is selected cool are comes through central console duct and prevents hot coolant from entering the heater core and contaminating the cool air.