If you’re like most truck operators, you know it’s time for preventive maintenance on your truck’s heating, ventilation, and air-conditioning (HVAC) system when the springtime A/C tune-up coupon from the local truck shop hits your mailbox. Otherwise, it’s if-it-ain’t-broke-don’t-fix-it, which might be alright if you sleep in your own cozy bed at night or tool around town in the company pickup truck.

But trucking is not winter camping or an endurance trek across the desert. A few labor unions figured this out some time ago and drafted contracts that require the truck to be shut down if the cab temperature exceeds a certain range. Fleets north of 60 treat heaters like bona fide safety items and spec them in multiples, plumbing in auxiliary units so one blown heater won’t lead to lost time or toes.

HVAC preventive maintenance should be an all-season endeavor. As we head into the winter months, remember that your air conditioner has a big effect on whether your defroster can work at peak efficiency, says Gary Hansen, vice president of engineering at Red Dot Corp., which designs and manufactures HVAC units and replacement parts for trucks, buses, and heavy equipment.

In defrost mode, the air conditioner kicks in to remove excess moisture from the air, which helps quickly clear the wiper-stroke area and the door window glass.

“If you’re noticing a loss of air-conditioning performance now, while the weather is still warm, don’t wait until next year to fix it,” Hansen explains. “Your air conditioner doesn’t go dormant in the winter. When you go to run the defroster, you’ll engage the A/C and risk damaging the compressor.”

To improve the performance of your defroster — and to increase your margin of safety on a winter highway — make sure the A/C unit is functioning properly. Check to see that it’s fully charged with refrigerant at the start of the heating season. Engage the A/C and let it run it for five minutes once a month to circulate lubricant and make sure the condenser fan turns on and off. Clean debris off the evaporator fins. Inspect those hoses and clamps.

None of this requires A/C certification or cold, hard cash, just a little diligence and time.

There are other steps you can take to troubleshoot defroster and cab heater complaints. Most are quick, simple, and relatively inexpensive.

WATER VALVES. After sitting unused for a season, water valves can stick. Don’t try to force the issue: you risk stretching the actuator cable and damaging the valve. So go easy on the heater controls and check the valves to make sure they open and close completely. It’s easy to adjust the slack out of a cable, but it’s a job you can avoid with a little care.

BLOWER MOTORS. Blower motors get a workout in the winter. “On a cold morning, the motor goes from zero to full-speed in one swift turn of the knob,” Hansen says. “The motor should take no more than 30 minutes to remove and replace, so do it at the first sign of trouble.”

DUCTS. Turn on the defroster and run your hand under the dash, feeling for air leaks or lost connections. Fill holes in the ducts with putty or tape designed for heating systems.

FILTERS. Your heating system has at least one pleated paper or foam filter to capture dust, lint, carpet fibers, and other impurities that can clog the heat exchangers and reduce the efficiency of the heater system. Depending on the truck model, there will be one filter on the fresh-air inlet and another for recirculated air. The filter for the sleeper heating system is almost always for re-circulated air and it’s accessible either through the toolbox or under the bunk.

“Most filters are reusable. If the filter is made of paper, simply vacuum the dirt away,” Hansen explains. “If it’s a foam filter, wash it with warm water and dish soap.”

RECEIVER DRYER. OEMs are using less and less desiccant in the receiver dryers they install. On a heavy truck, for example, instead of the standard 8 or 15 cubic inches, the desiccant level may be 5 cu in. or less.

That leaves little margin for error should you decide to not replace the receiver dryer once a year or whenever the refrigeration system is opened up. Moisture isn’t something you want to invite into the system. Moisture and refrigerant combine to create hydrochloric or hydrofluoric acid, which attacks metal surfaces in the refrigerant circuit. So don’t chance it with a receiver dryer. Change it.

COOLING SYSTEM. Cold temperatures amplify problems with belts and hoses, which may already feel the strain of the aggressive fans and pulleys on big diesels today. Look for signs of leaks, like bits of crystallized antifreeze on the radiator tank tubes, water pump, and other places where a hose attaches. Tight clamps, or maybe the wrong kind, can cut into the hose surface. And have your cooling system checked for the proper glycol mix.