Even though R-134a is relatively benign compared with R-12, federal regulations require technicians to follow procedures designed to prevent the refrigerant from escaping the atmosphere during maintenance.

Use the proper equipment and follow those procedures carefully to evacuate and capture refrigerant. It's not enough to add a can or two of R-134a to the unit. You're required by law to determine why refrigerant is low and correct any leaks you find — or face steep fines.

At each oil change, visually inspect the sight glass on the receiver-drier. Bubbles indicate a leak in the system. Another obvious sign of trouble is that the system doesn't cool adequately.

Wet floor mats indicate a clogged drain; vibration or unusual noise indicates debris or animal nests in the blower fan; reduced air flow comes from ice buildup on the evaporator; and a musty, mildew odor indicates mold growth in the evaporator box.

The American Trucking Association's Technology and Maintenance Council publishes recommended practices manual RP 418, “Heavy-Duty In-Cab R-134a Air Conditioning Systems.” It includes preventive maintenance, service procedures, diagnostics, and repair procedures.


Earlier I mentioned that A/C units can be delicate due to their relatively soft materials. With proper care, they'll withstand a lifetime of use, but a little abuse can lead to costly repairs. Make sure no heavy tools contact tubing or fins in either heat exchanger (condenser or evaporator). Just laying a tool against them can cause a dent or rupture, especially if the unit is still under pressure.

When tightening refrigerant lines, use a solid (non-ratcheting) wrench on both sides of the fitting. Avoid any excess torque on the tubing. Tube or line wrenches are best for flare nut fittings.

Never, ever discharge the system to the atmosphere by loosening a fitting, or removing a service valve. You not only risk large fines, but you can also lose any mechanics' licenses and accreditation your operation has obtained — plus your reputation and your job.

Finally, wear protective gear. Goggles or other eye protection are critical, as are work gloves designed for cold temperatures. A rapidly discharging component or refrigerant can draw a great deal of heat in seconds; if it touches skin you can suffer frostbite. Soak the area in cold water, then call 911.

With proper care, air conditioning provides years of trouble-free service, enhancing safety year-round and providing drivers and operators the comfort they need to stay productive in summer's heat. Just treat the system like any precision machine by following good inspection, maintenance, and safety practices.

— Paul Abelson ( is a former director of the Technology and Maintenance Council of the American Trucking Association, a board member of Truck Writers of North America, and active in the Society of Automotive Engineers.