Mysteries of the A/C Heater box revealed

Help Support

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
This started as a thread under maintenance I decided to copy it here so more people could look it over, I hope this helps everyone when they need to rebuild or service this box behind the dashboard on 71-73 cars equiped with air conditioning.

Link to my photos of rebuilding a heater A/C box you may have to copy and paste the link to make it work right due to spaces. Box AC/


Reference this for position of levers


Make sure your octopus is in good shape and the rubber isn't falling apart.

The problem is the colors fad over time. Example Purple can be faded to white.


Black is the main vacuum line from the Vacuum canister in the engine bay.

This is the main feed for all vacuum motors. The Vacuum canister is mounted on the

passenger side of the engine bay apron. It acts as Vacuum storage. The climate control system is

one giant vacuum leak so the vacuum canister helps stabilize the system.

It has a larger input vacuum line, that comes from full manifold vacuum, and the outlet

is a smaller line that goes into the Cabin through the firewall.

you will notice on the firewall there are 2 vacuum lines one going into the cabin and a second going to the

coolant Bypass valve on the Heater core feeds coming off the water pump on the engine.


Blue controls the center vents/defroster vents deflector.

when you set the system off Hi/lo vent, this motor activates, closes the door that

feeds the center dash vents and drivers and passenger side vents, sending all upper air in the system to the

defroster under the windshield

view of the blue motor


Red/yellow controls the floor vent and sends air to the upper vents. red is the bottom connection, yellow top/side.

this is the only double vacuum motor and its attached to the center fan blower unit.

this vacuum motor is used when you adjust how much air to send to the defroster or floor. as you adjust the control slider between floor and defrost this vacuum motor moves the flapper.

view of the red/yellow


White controls the fresh air intake, its only on when the A/C is set to max power, when the A/C control is set

to max(recycle cabin air) the arm comes down under vacuum and lifts the flapper door under the fresh air

intake from the cowl. this chokes off the fresh air and opens up the side door to allow internal cabin air to

go back through the system for additional heating or cooling, this motor is in the upper position all the time

letting fresh air in, until the upper selector lever is set all the way to the left A/C MAX.

view of the white vacuum motor

view of valve


purple controls the lower heater bypass, and is tied to the coolant bypass valve in the engine bay,

through a valve on the front of the heater box, behind the glove box.

As you shift the lower A/C controls from Hot to Cold purple slowly chokes off the heater core allowing air to bypass

at the bottom of the main box once the A/C control lever is set fully to cold the cable arm hits the button

on the heater box valve which turns on vacuum to the coolant bypass valve in the engine compartment and shuts down

the heater core completely. As you move the hot/cold slider there is also a mechanically connection to a second upper flapper that again controls the amount of heat entering the system. when the system is fully cold, the upper door is closed the bottom door is open, the upper flapper arm makes contact with the valve and vacuum goes back out to the engine bay to turn off coolant to the heater core.

complete view of Purple system

the octopus is in the lower right, feeds up the right side into the vacuum motor, it splits to the left center of

picture and feeds the white valve, that is turned on when the arm hits the button on it. which then turns on the vacuum line

out to the engine bay and shuts down the coolant valve to the heater core.


now there is an electrical switch for the A/C compressor which is only turned on when the lower selector is set

to full cold. this is part of the main A/C controls at the bottom of the center dash.

so its possible to set the upper lever to A/C MAX, and the lower lever off COOL and have the system recycle

cabin air and heat it again without the A/C compressor on.

there is also a temperature sensor that runs through the evaporator to prevent the evaporator from icing up and

overloading the A/C compressor.

there is also a mechanical pressure valve that both a/c lines go through in the engine bay it usually has

an orange cap on it. this prevents damage from over pressurizing the system.

What things look like installed in the dash

here you can see the Purple lines and the white line above.

connections to the temperature sensor for the evaporator core.

blue line feed on the center fan unit. you can also see the black line main feed and the purple line going out to

the coolant bypass valve

view of the red line

view of the octopus

complete system laid out on floor.

Last edited by a moderator:
Me too, I HATE that stupid photobucket! Just put your images in your hardrive and copy/paste people!