Wrench Tech – May 2022

There was a time when truckers and motorists drove through the heat and humidity of summer with windows down and road speed kept up as much as traffic would allow.

The flow of outside air across their bodies cooled them, even when ambient air was hotter than the 98.6 degrees that’s the typical body temperature. Technically, their bodies perspired, which is their self-cooling mechanism, and air flow evaporated perspiration from their skin. Today we’ve got strong mechanical air conditioning systems capable of almost refrigerating our bodies except for that temp-adjustment knob on the dash.

Air flow and evaporation are still among the physical principles employed by modern air conditioning systems. Even more basic is the irrepressible movement of heat toward something cooler, which in air conditioning systems is a chemical refrigerant that carries away the heat, notes Badger Truck Refrigeration, Eau Claire, Wis., in a 66-page manual on air conditioning systems posted online.

Thus the air is not cooled, as such. Rather, heat is removed from it. Air flows across the device, which accomplishes that heat exchange and therefore feels cool, and that’s what keeps us comfortable on hot days.

Of course, another principle is at work, too: the sudden depressurization of the refrigerant that causes the release of heat – a phenomenon that’s put to good use in air conditioning and refrigeration systems. Anything that interferes with those operations and the flow of refrigerant means your air conditioning “doesn’t work.”

Cabin air filter

Let’s start with air flow. An electric fan pushes air across the heat exchanger – the evaporator — near the firewall. Air enters the cab through dry filter element, part of the cab air filter. If that filter is too dirty to allow air to easily pass through, then air is slowed or stopped. That’s a common cause of an air conditioning failure, says TA & Petro’s maintenance blog, also online.

And it’s probably the first thing you or a technician should check because it’s usually easy to get to. Tilt the hood and go to the filter housing on the firewall, remove the element and check it over. Like any air filter, it doesn’t need to be clean as new. Dirt attaches to dirt, so some dirt on the element helps it work. But if it’s really cruddy, change it for a new one. Change the element regularly, per the truck manufacturer’s recommendations (often once a year), or more often if you run in dusty conditions.

Dirty heat exchangers

Air also moves across the evaporator and adjacent heater core. Even with filtered air, dirt can accumulate on their surfaces and within their fins. If the evaporator gets clogged, the system can freeze and damage the compressor. Air likewise moves through the condenser mounted ahead of the radiator behind the truck’s grille.

It’s conveniently bared when the hood is tilted, and you can quickly see if dirt and debris have built up and clogged its fins. You can also observe the engine fan turning on as the air conditioning compressor cycles, or not. If any of those heat exchangers are dirty, clean them carefully so you don’t damage them. In fact, check their condition during regular inspections.

Refrigerant charge?

Next comes refrigerant, which often is the first thing we think of: “It just needs a charge.” Maybe it does, because there are many connections and fittings in the air conditioning system and road and engine vibration tends to shake them loose, causing leaks.

Lack of refrigerant means it can’t carry heat away from the evaporator and hot air comes out of the vents inside. Often a technician will install a colored dye to trace where leaks are and then fix them before replenishing the refrigerant.

By the way, the refrigerant in a truck or car built since the mid-1990s is R-134a, not R-12, commonly called “Freon” (a name trademarked by a manufacturer). R-12 was banned by international treaty because it was a chlorofluorocarbon that was eating up the earth’s protective ozone layer. R-134a is a milder hydrofluorocarbon but is still a greenhouse gas that, like R-12, should never be vented into the atmosphere.

A competent technician knows how to capture and recycle refrigerant from an automotive air conditioning system. Molecules in R-134a are smaller than R-12 molecules, so any leak becomes more serious, and tight fittings and connectors become more important. Also, do not mix the two refrigerants because oils within them are not compatible.

New compressor?

The compressor can also fail, sometimes seizing, and where it seizes (clutch, bearings or internally) affects whether or not the pulley stops and the belt is burned and ruined. But replacing a belt is cheap compared to a complete compressor.

Watch for signs of impending failure, like squeaking or other noise coming from it, because it’s less costly and more convenient to repair or replace the compressor in a shop than on the road. Also, be sure water drips from the evaporator hose onto the pavement like it should. No dripping means something’s wrong.

When replacing a compressor, remember that a properly rebuilt or remanufactured unit can save a lot of money and work as well as a new one. Choose a brand-name product or one you know is from a quality workshop, but beware of counterfeits. That’s true of any of the system’s other components that help it work. Failures in them can be determined by a tech who knows his business.

Safety first

Among other things, that tech knows that part of a system runs at high pressure, up to 350 psi, and must be treated with care. Unless you’re a former air conditioning technician who’s now a trucker, or have been well taught on how to service an air conditioning system, it’s a good idea to leave the work in the hands of qualified and experienced people. Meanwhile, roll down your windows and cool off the old-fashioned way. LL