Why AMC’s Cars Used To Have A ‘Desert Only’ Mode

The vast majority of cars on the market have very similar controls. Indicator stalks, accelerator and brake pedals, and volume knobs all used to be fairly standard across the industry. What really lights up our dopamine receptors as enthusiasts are when we find something on a car we’ve never seen anywhere else. A great example comes to us from American Motors Corporation, who used to equip their cars with a special “Desert Only” setting.

If you’ve driven any modern off-roaders, you might be thinking this involved some kind of suspension tweak or differential lock. After all, everything from the Toyota RAV4 to the Porsche 911 Dakar has a “Sand” drive mode these days for tweaking traction control or other handling parameters.

In the case of vintage AMC products, though, it was unrelated to the vehicle’s driving performance entirely. Instead, it was all about the air con. No, I’m not kidding.

Only In The Desert, Promise

Just about every car comes with air conditioning these days, but it wasn’t always thus. Once upon a time, you were lucky enough to get some kind of ventilation system, or maybe a basic heater. By the early 1960s, around 20% of cars had air conditioning in the US.

By this point, Nash had set the standard that would become typical for automotive air-conditioning. The work started with the thermostatically-controlled “Weather Eye” heater from 1939. A bit over a decade later, the 1954 model year would see the introduction of the “All-Weather Eye” on the new Nash Ambassador, Statesman, and Rambler models. It combined air conditioning, heating, and ventilation into one combined unit. The system had a single control panel that brought all the controls together in one place on the dash, and the components of the system were all accessible under the hood. This was a big advance from earlier systems from other automakers, which often had separate air conditioning and heater systems entirely.

It was no coincidence that Nash made a big move here. The company had significant experience in vapor-compression refrigeration thanks to its merger with Kelvinator in 1937. The latter company is famous for fridges, which use effectively the same techniques for cooling as automotive air conditioners.

Also in 1954, Nash merged with Hudson to become the American Motors Corporation. Then the largest corporate merger in US history, it would see the new company eventually start releasing cars under the AMC name. It fought to maintain a position in the market against the brutal onslaught brought by the Big Three, innovating along the way. AMC would in fact become the first company to make air conditioning standard on an entire model line with the 1968 Ambassador. The underdogs at AMC even beat Lincoln and Cadillac to the punch with that move.

By the early 1970s, AMC began including the special Desert Only setting on its Weather Eye HVAC control panels. In this era, AMC had separate temperature controls for the air conditioning and heating. You could set the heater anywhere from OFF to the maximum WARM setting. As for the air conditioning, you’d use a slider or a knob to control temperature. At its lowest, the air conditioning compressor would be off. You could turn it colder, up until a certain point. Beyond that, the dial was marked DESERT ONLY. Basically, you could get the coldest possible temperature out of the air conditioning on this setting, but you were only supposed to use it in the desert.

To understand why, we need to understand the basics of vapor-compression refrigeration. It sounds complicated because it is, but I’ll give you a crash course based on AMC’s own technical manual from 1973. I’ll give you a full explanation in the next paragraph, but if that’s too much, breeze through and keep reading. I’ll provide a simpler one after that.

As per the diagrams above, the air conditioning unit consists of a closed loop full of refrigerant. In the case of AMC’s vintage designs, the systems used dichlorodifluoromethane, also known as R-12. In the loop, the compressor takes in refrigerant as a low-pressure gas and compresses it to a higher pressure. This causes it to heat up. It then passes to the condenser, which is essentially a big heat exchanger. The hot refrigerant gives up its heat to the atmosphere through the condenser, remaining at high pressure, but cooling to the point it becomes a liquid. From there, it passes to the expansion valve, where it goes from being a liquid under high pressure to a liquid at quite low pressure. This cools the refrigerant down significantly. It then passes through another heat exchanger, called the evaporator. Here, the cold liquid refrigerant sucks up heat from air passing over the evaporator. As the refrigerant sucks up heat from the air, it becomes a gas once more, while the air passing over the evaporator is cooled in turn. This generates the cold air flow desired from the air conditioner. Meanwhile, the gaseous refrigerant runs back to the compressor to be recompressed and reheated to continue the cycle.

If that’s too complex, all you need to know is this. In the air conditioner, the blower fan blows air over the evaporator to cool it down. This cold air is then sent out the vents to reduce the temperature in the cabin. The longer you run the compressor, the cooler the evaporator gets, and the colder the output from the air con. The temperature of the evaporator, and thus the output air, can be controlled by how long you run the compressor. If you leave it on all the time, it gets very cold. If you cycle it off and on to varying degrees, the evaporator, and thus the output, doesn’t get as cold.

Since the evaporator is cold, condensation tends to form on it during normal operation. The higher the humidity in the air, the more condensation forms. Similarly, the cooler the evaporator runs, the more condensation there is. At extremes, the condensation can even freeze onto the evaporator. This tends to block airflow through the evaporator, which cuts the flow of cold air to the cabin. The ice also tends to act as an insulating material, stopping the transfer of heat from the airflow to the refrigerant inside. Instead, the blower fan is just blowing over a big ice cube, which is not a very effective manner of cooling at all.

There’s one way around this, and it’s where the Desert Only mode comes in. We think of deserts as hot, and that’s often true. However, deserts are really defined by their lack of moisture. You can expect humidity to be very low in a desert climate. Thus, in such conditions, you can run the evaporator very cold with little to no risk of it freezing over. It’s not so much because the desert is hot, but because there’s little to no water in the air to freeze up the evaporator coils.

In Desert Only mode, the compressor is run continuously, making the evaporator very cold indeed. The air conditioner provides maximum cooling in this mode, but it will only work in dry conditions. If you try to use the Desert Only setting in more humid conditions, you’ll find the air conditioning loses its effectiveness in short order. Simply switching it back to a lower setting should allow it to recover in short order. Any ice buildup will clear quickly enough and the system can resume regular operation.

Why Don’t Cars Have Desert Only Settings Today?

There are a number of reasons behind that. For one thing, it’s generally poor practice to give the public buttons that mess things up if they’re used incorrectly. For example, if you set your AC to Desert Only mode in humid ol’ Florida, you’re probably going to wonder why it stops working all the time. Indeed, AMC’s own service manual instructed technicians to educate owners if they demanded a fix for their fully functional car after making this mistake.

There’s also something to be said about design choices. Many automakers simply didn’t feel the need to give customers air-conditioners that could go flat out and possibly freeze themselves over. Beyond that, technology has moved on. Modern computer-controlled air conditioners have temperature sensors on the evaporator coils that detect when they’re getting too cold. When this happens, the compressor can be shut off to let the evaporator warm back up to a temperature above freezing.

Indeed, AMC itself would abandon the Desert Only setting after just a few years. By the mid-1970s, it had started manufacturing HVAC controls with a single combined slider that went from cold to warm, as is familiar to us today. No more could you wind the coldness out to 11 and put your evaporator at risk of freezing over. More’s the pity.

Don’t worry if you’ve forgotten how air conditioners work by the time you’ve finished this article. I forget all the time and I took Heat Transfer & Thermodynamics in University (good course, by the by). What you do know, however, is how and why AMC built cars with a weird Desert Only setting in the early 1970s. That should give you plenty to bloviate about at your next classic car meet when you get sick of the endless blather about hemispherical combustion chambers from the Mopar set. It’s good to mix it up a little, you know?

Image credits: Nash, AMC, Hemmings, via eBay
Top graphic images: Hemmings