Most cars today are equipped with (or at least have available as an option) some form of automatic climate control, where you set the desired temperature, press auto, and it does the rest.
Automatic climate control was rarely seen when I was growing up, and on the few occasions I did see one it always struck me as being a bit of black magic, the way they change the fan speed, temperature, and air direction all on their own. Residential heating and cooling systems, by contrast, are exceedingly simple, with just a basic thermostat to cycle the system on and off as needed.
Cadillac released Comfort Control, the world’s first fully automatic climate control system in 1964. This system is an amazing accomplishment and a reminder of how GM and Cadillac really once were the standard of the world. Let’s take a quick look at the development and workings of this system.
The obstacles to bringing Comfort Control to market were daunting. First up was the problem of airflow. Most early A/C systems looked similar to the diagram above. Other than a common blower, the heating and air conditioning systems were essentially separate, with the heat blowing out the floor and defroster, while the A/C blew out its own dedicated panel outlets. Remember how cars without A/C used to not come with panel vents? This is why. This approach also made it easy to add air conditioning to a car later: A/C was often available as both a factory- and dealer-installed option in its early years. (See here for a quick refresher on the history of automotive A/C).
Early attempts by GM to automate this setup performed poorly: Relying on a shutoff valve to control the hot water and a bypass valve to control the evaporator made the temperature of both difficult to modulate and slow to change. Humidity was also difficult to control with the heated air bypassing the A/C evaporator.
The insight that solved this was the idea of blending the hot and cold air together. All air first passed through the evaporator where it was cooled and dehumidified. Then warm are could be reintroduced using a blend door on the heater core if warmer air is called for. This blended air could then be sent to any outlet: Floor, panel, or defrost. Temperature changes in this setup are affected almost immediately since you no longer have to wait for the thermal mass of the evaporator or heater core to change. This is how virtually every modern automotive HVAC system, automatic or manual, works today.
With the airflow problem solved, GM now had to solve the problem of control. While modern climate control systems use microprocessors and electronic actuators to work their magic, the Cadillac Comfort Control is pure analog.
Temperature sensing is accomplished via three thermistors (thermal resistors) – one for outside temperature, one for inside, and one for duct temperature. The purpose of the duct temperature sensor is to compensate for variations in heat and air conditioning production due to variations in engine temperature and speed. However, these three thermistors are not treated as three separate inputs, as multi-channel control systems were expensive and complicated in the 1960s, and this system needed to be simple, inexpensive, and reliable. Instead, these three thermistors are wired in series into a resistor string (along with the rheostat on the temperature control dial), essentially summing their values into a single signal.
It is hard to believe that a large variety of behaviors (recirculate/fresh air blend, temperature blend door, outlet direction blend door, coolant shutoff valve, and blower speed) can be controlled by a single input, but you can see how they mapped the various output states to a single input in the function diagram above. High temperature readings on all the thermistors would push the function map all the way to the right, triggering various behaviors like switching the inlet from fresh air to recirculate and closing the coolant shutoff valve. Low readings from all the thermistors would correspondingly push the function map all the way to the other side. Intermediate temperatures and combinations of temperatures (e.g. high interior temperature and low exterior temperature) would push it somewhere to the middle.
So how was this “program” implemented without digital electronics? Simple. With a single two-transistor amplifier (to boost the signal), a single vacuum transducer (to convert the electrical signal into a vacuum signal), and a circuit board. You can literally see the blower speed control from the earlier diagram “hardcoded” on the PCB pictured above, while vacuum actuators and cams handled the airflow doors in a similar manner. Genius!
This single-channel analog control feedback system is the template that pretty much all subsequent electro-mechanical automatic climate control systems would follow, up until the introduction of digital systems in the 1980s.
Cadillac introduced Comfort Control in $1964 as a $495 ($4,100 in 2020) option. Amazingly, all this technology represented just a $21 increase over the $474 price of the manual 1963 A/C option. Cadillac felt confident enough in this system that Comfort Control was the only air conditioning system available in 1964 – no manual system was offered. In fact, Cadillac would only offer automatic climate control after 1964 (optional at first, and then standard starting in the 1970s). As near as I can tell, manual A/C wouldn’t appear again in a Cadillac until the 1981 Cimarron.
At first glance, the controls for this first generation Comfort Control system look deceptively simple with only two modes (three, if you count “Off”). No doubt this was at the behest of the marketers, who would have wanted the system to appear as simple and intelligent as possible. In actuality, adjusting the slider to the right Automatic detent would have boosted the fan speed, and the rightmost defrost marking was actually a deice setting (with all the air going through the defroster ducts), as explained in the brochure above. Later iterations of this system would make the operation of the various detents more clear with better labeling of the controls.
Comfort Control wouldn’t stay a Cadillac exclusive for long. Pontiac got automatic climate control the very next model year, in 1965. Or at least that is what the 1965 brochure says. The take rate in 1965 for this option must have been extremely low, as my Google image searches for a 1965 Pontiac so equipped came back empty.
Lincoln would introduce its first automatic temperature control in the 1966 Continental. It offered the same degree of manual control as the Cadillac Comfort Control system, but the operation was a little more discoverable due to the use of buttons instead of a slider. Unlike Cadillac, Lincoln continued to offer a manual air conditioner as a less expensive alternative to the fully automatic system and would continue to do so for many years.
Oldsmobile would get their own version of Cadillac’s automatic climate control in 1966, called Comfortron, initially available in the Toronado and Ninety-Eight. The Buick LeSabre, Wildcat, and Electra would get this system as an option starting in 1967.
Not to be outdone, Cadillac introduced their second generation automatic climate control system (no longer branded Comfort Control) in 1967, which offered more intuitive controls and a greater degree of manual overrides than the first generation Comfort Control.
Chrysler would finally join the automatic climate control ranks with the introduction of their Auto-Temp system in 1968. It was available on all Chrysler and Imperial models that year. Supposedly it was available late in the 1967 model year, but I was unable to find any 1967 models so equipped.
Much like manual air conditioning did in the late 50s, it didn’t take long for automatic climate control to trickle down from the higher brands to the lower brands. The Chevrolet Caprice got Comfortron in 1966, and the 1968 Thunderbird would be the first Ford branded car to get ATC (although it would be 1971 before it showed up in any Mercury). Still, ATC would be a rare sight in the entry-level brands for decades to come.
Chrysler introduced their second generation Auto-Temp II system in 1971. It replaced vacuum actuators with electromechanical servos, but otherwise still operated under the same principles of all the other single-channel analog systems.
Many European automakers relied on US automakers for their air conditioning systems in the 1960s and 1970s. Mercedes-Benz was no different, and they released their first ATC system in 1976, using components from a variety of US automakers. The controller was a Chrysler Auto-Temp II unit, while the compressor was a GM Harrison unit.
As late as 1979, every ATC system still operated under the same basic principles as the OG 1964 Cadillac Comfort Control introduced 15 years earlier. That would change in 1980, when Cadillac went digital in a big way, introducing the world’s first fully electronic automatic climate control system on the Eldorado and Seville. Modern automatic climate control had finally arrived.
So where did automotive air conditioning go from here? Check back tomorrow!