What I learned about 74-78 ATC II

73 T&C

Senior Member
FCBO Gold Member
Mar 19, 2013
Reaction score
Coral Gables, Florida


This is not my first attempt at repairing Automatic Temperature Control (ATC) systems. Having successfully beaten several Cadillac climate control systems into submission in the past, fear was not a determining factor in keeping me from digging in and trying to learn about and trying to bring back an ATC II.

In response to what was perhaps a casual suggestion, I thought it would be useful to both current and future members for me to create a specific record of my process in bringing the ATC II system in my 78 Chrysler New Yorker Brougham back into a functioning system.

Besides, after just getting mine to work, I will be taking it apart again to change out a leaky evaporator and possibly dodgy heater core in order to get my system back up and running reliably for the next few years....hopefully.

If your expecting this thread to be an extremely technical dissertation, it will not be. I’m thinking most people drawn here will just want to trouble shoot and repair their system. So that will be the focus, as it was mine at the time of this writing. After all, you don’t need to know the exact chemical composition of gun powder, the thermodynamic wonder of combustion or it’s exact burn rate to be able to effectively load, use or diagnose a problem with a firearm.

Likewise, if you want to learn about general manually controlled HVAC systems and their operation, repair etc. you need to come into this already knowing well how that works for this thread to be of use. After all, the ATC II, as the name implies, is about the automated control of both the heating and refrigerated air conditioning system.... so you don’t have to.

Before I began my repairs, and even before I actually had the car delivered, I began by doing research on this site, the web and factory service manuals that were relevant. Among the best information and source for needed parts was the Imperial club.

Online Imperial Club (OIC) for Imperial, Chrysler Imperial, and Chrysler New Yorker Brougham Enthusiasts

A fairly in depth description is available to anyone on this site which is a good way to start. In fact, I would read the article on the first version of the ATC II in the 69-73 earlier version of this system to understand not just it’s operation but the evolutionary advances made in the later 74-78 version to be described here. Likewise, for those so inclined, this specific article will greatly help you make sense of the actual Factory Service Manual description. It did for me.

What I will be describing over the next few postings will be: The basic operation of the system; Location and description of the components with pictures of both the parts and their location and; Finally, I will try to make a more useful trouble shooting guide to put down what I’ve learned in a useful series of checks and observations specific to the later version ATC II.

Because of the wealth of information in the enthusiastic community of owners, any and all relevant comments are welcome!
Last edited:
A badly needed thread!! :thumbsup:

Just a quick note that the ATC II in the 74-78 cars is different from the ATC II from 73 and back units.

Thanks. You are very correct!

They are operationally actually very similar Stan.

In fact, the amplifier for both is an exact plug and play replacement part for both. Just this fact alone helps keep the later ATC II systems viable for some key parts. The main difference lies in the servo component. Whereas the earlier version had the amplifier, servo and heater valve as an all-in-one component inside the engine compartment, the later 74-78 version relocated the more delicate servo and amplifier inside the cabin. Likewise, the earlier version, due to their Bakelite heater valves, were subject to internal leaks. These leaks would corrode the servo motors and subject the amplifiers to moisture and heat.

As I’m sure you know, the earlier system was used in similar year Mercedes Benz cars. As such, the carry over parts as well as their improved servos are still being made and refurbished.

Performance Analysis Co. refurbished my replacement amplifier and I am sending them my old one for them to fix as a spare.

Here is their home page.

Performance Analysis Co.
Last edited:
I don’t even have ATC II, (or even an A/C car) but this intrigued me as I was following your other thread.

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.


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.


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.
Last edited:
Javier, thank you so very much for all your hard work and efforts to give us a play-by-play on the system. I currently have one, and I’m fortunate to some degree that my system works as it should, however my heater core is shot, but since the system is all encompassed, I’m reading every word of your thread. And the detailed pictures that you’ve given in your other threads are very helpful as well, keep it come’en!! Thx again
Working with n it Stan! I’m working on it!!!!
My first drafts always suck.
Most of us suffer from that. You are doing something I've been wanting to get to for sometime now. I will follow along closely as you progress. Later on, with if you don't mind, I'd like to add my input to this same thread so it's all in one spot.
Most of us suffer from that. You are doing something I've been wanting to get to for sometime now. I will follow along closely as you progress. Later on, with if you don't mind, I'd like to add my input to this same thread so it's all in one spot.
Yes... please do.
and you're in Architecture?
You should see my first sketches... simply awful!!!! It’s the way of most creative work.

Thanks all for the encouragement and proofing.

My idea is that once we get all the input and text done we simply bump up all the finals to the top to create a useful guide for the future. This was always my intent. That way, we actually create something we can all be proud of and still share our girly pictures in the exhibits below it.

For now, it’s completely a work in progress. So please.... everyone’s input is more than welcome!!!
Please, don't encourage the children...:icon_fU:

View attachment 176323

Yesterday, after a particularly smarmy and juvenile remark from me, my 18 year old daughter Madeleine said;

“DAD!, you realize you’re really 12 years old, right?”

To which I answered

“Yep!” “All men are, and the Sooner you realize it the better!”
You should see my first sketches... simply awful!!!! It’s the way of most creative work.

Thanks all for the encouragement and proofing.

My idea is that once we get all the input and text done we simply bump up all the finals to the top to create a useful guide for the future. This was always my intent. That way, we actually create something we can all be proud of and still share our girly pictures in the exhibits below it.

For now, it’s completely a work in progress. So please.... everyone’s input is more than welcome!!!
I love this thread. But most of all, tell us where to go to get the darn thing fixed if it breaks!
Ok guys. Text for section 2 is now fully outlined. Your review check comments are needed.
Last edited: