Bottom Line
Air conditioning
Air conditioning has been in vehicles since the 1950s. The units are proven and reliable, requiring little service. But when they do fail, more often than not, repairs are best left to a dealer or specialty shop. They have the supplies, the tools, the training and the experience to get you back on the road quickly.

by Paul Abelson, technical editor

You can probably see from the photo accompanying this article that I’ve been around a few years. That lets me remember when many of the things we take for granted today were brand new, and not easily accepted, especially by truckers. Today, I don’t think there is a Class 8 truck built in the U.S. that doesn’t come with air conditioning. But back in the ’60s and ’70s, when A/C units were expensive options, macho truckers boasted they didn’t need it. They had the good old 2-30 system: two windows open at 30 miles an hour.

Of course, these same types resisted optional power steering, preferring the traditional “armstrong” system. The sons of these Neanderthals are around today, bad-mouthing drivers who prefer the newest automated transmissions over good old, traditional, double-clutching crash-boxes. We all know shifting is the truest mark of a professional, isn’t it?

But I digress. Back to air conditioning. By the late 1970s, the Teamsters got air conditioning into their master contract. There was benefit for both drivers and fleets. Air conditioners keep drivers refreshed in hot weather, de-humidify interiors and assist in defrosting the windshield and side glass. When the interior air is laden with humidity, condensation will form on the inside of windows. That’s why, even in winter, the air conditioner runs when the climate control system is set to defrost. Dry air lets moisture evaporate.

How does A/C work? By absorbing, moving and rejecting heat. In the March issue of Land Line, we finished our look at trailer refrigeration. The principles of operation are identical, based on the central concept that there is no such thing as cold. There is heat, and the possible conditions are that there is more heat or there is less heat. Just like a trailer’s refrigeration unit, an air conditioner absorbs heat from the cab and moves it to a radiator, which releases the heat to the air.

The principal parts of an air conditioning system are the compressor, the receiver/dryer, the condenser, the evaporator, the blower and, of course, the controls. There will be a duplicate evaporator and blower set-up in the sleeper cab, if the truck has one. The hardware is there to change the refrigerant from a gas to a liquid and back again. When the refrigerant goes from gas to liquid, it rejects heat, giving it up to the air through the radiator-like condenser. When it goes from liquid to gas, it absorbs heat, drawing it from the air through the fins of the evaporator. Air blown over the evaporator loses heat and feels cooler than the surrounding air. The air also loses its ability to hold moisture, which condenses on the evaporator’s fins and drips away through a tube. That’s why we find those drops (or pools) of water beneath our cabs after we shut down.

The compressor, driven by the engine through a fan belt, circulates the refrigerant. There is a pressure switch between the compressor (high pressure gas) and condenser (high pressure liquid) that acts as a safety valve, turning off current to the magnetic field that holds the compressor pulley to the compressor shaft. Without current flowing through the electric clutch, the compressor is off. After being compressed and cooled, the now-liquid refrigerant flows through the receiver/dryer, which removes any water and filters the refrigerant so it won’t damage the expansion valves. The valves control flow to each evaporator (cab and sleeper) and atomize the liquid so it can more easily absorb heat. Then the refrigerant becomes a (relatively) low pressure gas that flows back to the compressor to start the cycle again.

Here are the most common reasons A/C units fail

  1. Refrigerant leakage. There is a low-pressure switch that turns the air conditioner off to protect the compressor when refrigerant has been lost. If the unit will not cycle on, check the pressure. Refrigerant could be lost through damaged or loose fittings, or in older trucks, through cracked or porous hoses.

    If refrigerant has been lost, the law requires that the unit be checked and repaired, and any old refrigerant be suctioned from the entire system before new refrigerant is added. This is especially true for older systems using R-12 Freon, which contains chlorine. Newer units use R-134a, a fluorocarbon refrigerant. In the old days, we’d just add a can of Freon, but because the gas damages the ozone layer, full repairs must be made. Contrary to popular belief, it is not illegal to use Freon, but it must be used by a professional, taking proper precautions. Don’t ask your shop to “just fill it up” with Freon. Shops are required by law to properly process air conditioners. If they don’t, they risk very serious fines.
  2. Belt slippage. When the air conditioner works sporadically, varying its output below full cold, the fan belt driving the compressor might be slipping. If so, replace the belt and check the output of anything else that belt may be driving.
  3. Clutch failure. If the magnetic clutch won’t engage, the chances are that a pressure switch has blocked operation due to high or low system pressure. Clutches rarely fail.
  4. Worn out blower motor. If everything seems to be functioning under the hood, but you’re just not getting enough cold air blowing out the vents, you may have a bad blower motor or damaged assembly. Replace the blower.
  5. Blocked flow. If the system seems in good shape with the motor functioning well, but you’re not getting the airflow you should, your evaporator or ducts could be blocked with debris. You may have to disassemble part of your dash for access.

Paul Abelson is Land Line’s technical editor and freelances from his office in Lisle, IL.