Part 1
How does it work:
The ATC II 74-78 is an automatic control system that takes the outside air or cabin air and either heats or cools it (or both) in order to establish the selected desired temperature inside the car’s cabin.
It accomplishes this task not with the benefit of magic but the automated control of the following:
Amount and temp of out door or recirculated cabin air
Blower speed to move that air and vary the volume.
Movement of the air in the system through either a cooling coil (evaporator carrying expanding refrigerant at very low temp) or heater core (tiny radiator filled with engine coolant at the temp set by the thermostat)
That treated air is then introduced to the cabin through a variety of outlets determined where best and quickest it can change the cabin air to the DRIVER DESIRED AND SELECTED TEMPERATURE.
I bold this last part because like any system it requires some sort of setting to work.
And...
If you’ve been reading along you’ll notice that it’s all about the movement and control of AIR.
You’ll also notice that it’s not about making the evaporator cold (that’s the compressor’s job) or putting heat into the heater core.
In a manually controlled HVAC (Heating Ventilation and Air Conditioning) system, it’s the driver who directly operates the dampers, fan speed and directs the cooling or heating of the air being introduced in the cabin.
Like the driver, the ATC II does its work with its own version of a head, arms, and senses to make the air go where it can best bring the cabin air to the requested temp.
It does this by taking the temperature of the of air inside the cabin, the temperature outside the cabin, and the requested temperature of the thermostat (as set by the driver) to make a combined signal to the amplifier.
The INPUT side of the system:
The thermostat on the control panel is a variable resistance potentiometer. And before you get flustered by the name, you already know what these sorts of devices do. In fact, you use them ten times a day.... every time you raise or lower the volume on your tv or radio, dim the lights in your living room, or turn your electric stove on; you’ve actually worked a variable resistance potentiometer... The thermostat, in the ATC II, moves a conductive contact along a pair of resistor strips that vary in resistance depending on the length of the strip. Moving the lever to the right, the coldest setting, connects the resistor strips at their shortest length and thus produces the least resistance. Move the lever to the left, and the result is the highest resistance available from the thermostat. But, the thermostat doesn’t work alone.
The other two sensors (inside and outside air) contribute a variable resistance to the thermostat’s signal based on temperature of their immediate environment. Resistance in electrical terms is measured in ohms but the that’s not important yet. What is important to understand that there is a direct relationships between the sensors and temperature of their surroundings they are measuring.
The warmer the tempersture surrounding the sensor the greater the resistance. Likewise, the cooler, the lower the resistance.
Of the three, the only one you can change is the thermostat. Together, all three send a combined signal to the amplifier.
The amplifier then takes that signal and does what?? Well, as the name implies it amplifies it and compares it to ....
The OUTPUT or action part of the system:
That’s where the servo comes in. The servo is a machine that is basically a motor driven switch gear which directs the movement of dampers by directing vacuum to different damper vacuum motors, varying the fan speed, directing the mixture of cold to warm air, AND based on the position of these switches it sends a signal back to the amplifier.
As for how the servo works.... Think of a large geared edge wheel that is turned with an electric motor that also has a series of electrical contact radial strips that align with a fixed series of electrical contacts. These contacts control the blower speed. The blower speed is dependent on which of the radial contacts come into contact with the fixed contact “brushes” for the blower.
Likewise, now imagine that in the center of that geared circle there are a series of vacuum lines and connecting passages mounted on either face of that center section. One set rotates with the wheel mentioned earlier and the other is fixed and has vacuum lines out to the different vacuum motors in the air handler (big black box with all the dampers, evaporator and heater cores ... more later). You can see that by rotating the wheel it changes which lines are connected to vacuum at any given positions. Important for us to know is that the “wheel” we’ve been talking about only moves about 1/4 round (90 degrees) in its total maximum travel.
The blend door actuator is actually just a lever arm mounted to that same wheel that stickers out of one end of the servo. This lever arm is then linked to a bell crank on the blend door with an adjustable connecting rod. The blend door can either (1) direct air solely from the evaporator or (2) force all the evaporator air through the heater core or...(3) mix the evaporator air with a variable amount of the heater core air.....Thus changing the temperature of the air to the outlets.
(Also of importance, you may have already noticed, is that all the air from the blower goes through the evaporator first and always. More about that in the notes below)
Finally, imagine that along the edge of that geared wheel is another of those variable resistant potentiometers that varies an electrical signal back to the amplifier based on the position of the wheel. It is that potentiometer that creates the feedback signal from the servo back to the amplifier based on the position of that famous “wheel”
The AMPLIFIER or controlling BALANCE director:
The amplifier takes the input signal from the thermostat and sensors and the feedback signal from the servo and compares them. If the difference is positive it directs the servo to go one way, if negative, it direct the servo in the other, to get the two signals to match. In the balancing process, the movement of the servo’s “Wheel”; (1) directs and controls the voltage of current to the blower (2) directs the placement of vacuum to control damper motors in the air handler box to move and direct air inside the air box and to the cabin....and (3) directs the blend door to create an temperature that will close the loop created by the thermostat and it’s sensors on the input side of the amplifier and the signal sent to it by the relative position of the servo on the other.
And that’s it!!!
Well, any way, in brief, that is how the ATC II system works. Simple, no?
Of course, there’s a little more to it than that: like the outside air damper, the differential temp control built into the system, air handler box specifics, defrost air, control button settings vacuum and electrical switchgear etc etc.
...... but the above are the basics you need to know to get started and help make the rest make sense.
For the rest look at the notes below.
And remember, if one man can make it, another can figure it out and fix it.
Notes:
Differential temperature.
The ATC II does not process air to the desired temperature,....... until balance is reached between the input and feedback signals. And even then, only to within a few degrees.
In between, starting from the moment you set the thermostat level, the system will automatically set the best way to get there within it’s operating parameters. The ATC II, in good working condition, should be capable of producing processed air in a temperature range from 32 - 180 degrees. Therefore, if it’s 189 degrees in your car in Florida (that would be me) and you have the thermostat at 65 you will not be getting 65 degrees out of the upper vents until the cabin reaches 65. Prior to that you will be getting the coolest air at the maximum volume that the system can produce. That is the operating temperature differential at work.
As the cabin gets closer to the selected temperature, the blower fan speed will begin to drop and other dampers will come into play like outside air/recirculating air.
Outside air/recirculated air damper
The outside air/recirculating air damper allows the feed air into the air handler box from either inside or outside the cabin. In the later 74-78 ATC II, it is actually a delegate damper box outside the main air handler box. And, although directed by vacuum from the servo, in the ATC II, this damper also actuates a vacuum control switch which back feeds the servos switch gear to set the amount it will open. This range is roughly 0% outside air, 20% outside air or 100% outside air. This allows the system to provide maximum cooling and heating when the operating temperature differential (OTD) is greatest. As soon as the OTD is within ~15 or so degrees the servo will begin adjusting and the damper will allow for its default setting of 20% outside air to begin and introduce outside air into the air handler for two reasons. (1) keep the cabin positively pressured and (2) keep the cabin air fresh and oxygenated.
Air handler box or airbox general and specifics
Generally, the airbox works like the air conditioner in your house. Just like the one in your closet, it’s a closed box with a fan forcing air through a cooling coil and some sort of heating element or coil. Inside the air handler box is located the evaporator coil and heater core. It works on the principal of chilled and reheated air. It cools and, in so doing, dehumidifies the incoming air to as close to freezing as possible and then reheats it (which further dehumidifies it by the way) to a differential temperature set by the servo. Even under full heat mode the air always goes through the evaporator first. And if the temperature of the incoming air is more than the mid 30 degrees it will be chilled by the charged evaporator and the A/C compressor working.
Specifically, the air handler box also has openings to direct air to the upper cooling vents in the dash and just below, the heater floor outlets or up to the windsheild outlets when in defrost mode to warm and demise the glass. The direction of the processed air is directed to the proper servo selected outlet by damper doors operated by vacuum motors. In the next section we will be looking at each of these in greater detail. So, hang on or go there now if you can’t wait.
Defrost and control panel settings.
The settings on the ATC II control panel has the following settings: OFF, Vent, Auto Lo, Auto Hi, Defrost Lo, Defrost Hi.
Vent operates the same as auto with the exception that the A/C compressor is left off. (In GM parlance it is known as “ECON”).
The difference between Lo and Hi in the above is the strips which are energized in the servo for control of the blower fan.
The “Auto” selection will allow the ATC II will provide heat or air conditioning within its full operating capacity.
Now.... the cheater..... Defrost.
The defrost setting directly provide vacuum to the defrost air control damper and overrides the thermostat directing the servo to go to full heat. It is an outlier setting in that it does not use the ATC II’s automatic control to do its job other than select the fan speed.
Next up. the parts that make up the ATC II.
Note: the above thread has benefitted from all the comments and support of many members.
