A/C System Performance

Ways to reduce costs, unexpected downtime for A/C related repairs

One reason air conditioning repairs tend to be costly is because there's less emphasis on preventive maintenance than there is with other systems on the vehicle. Often, the A/C gets attention only after the words "won't blow cold air" appear on a job ticket.

"Some maintenance managers think that if a technician isn't certified to work on air conditioning, or the shop doesn't have the service equipment, then A/C maintenance is out of their hands," says Frank Burrow, warranty and product support manager at Red Dot Corp. in Seattle, WA, a manufacturer of HVAC systems, components and replacement parts for commercial vehicles. "And it's true that once the system fails, a qualified A/C specialist should do the repair job. But you don't need special skills or equipment for the kind of preventive maintenance that can help you control expenses and minimize unplanned downtime throughout the year."

Burrow lists nine things maintenance managers can do to reduce costs and unexpected downtime for A/C-related repairs. Most require no more than a basic knowledge of the A/C system, just a year-round commitment to regular preventive maintenance.


There are three common varieties of canned refrigerant: R-134a with oil; R-134a with oil and sealant; and straight R-134a. Widely available, any one of these formulations can do more harm than good, Burrow warns.

For example, the oil mixed with R-134a may not match the compressor's original PAG (polyalkylene glycol) or ester oil spec. "If the compressor lunges, the manufacturer will analyze the oil for type and viscosity," he says. "If the oil is different from the original, the manufacturer will void the warranty."

Worse is R-134a with sealant because "sealant can gum up needle valves and orifice tubes, leading to costly repairs and downtime. It can also harm your shop's refrigerant recovery and recycling equipment."

Even a one-pound can of straight R-134a is "bad news," says Burrow. "First, you don't know if you need a pound of refrigerant or some other amount. Second, if your system is losing charge, you need to find out why."

Burrow advices against having cans of refrigerant in the shop. When the can is turned upside down, the refrigerant inside can change state from gas to liquid. If it's injected on the suction side of the system, the liquid will get into the compressor. "The pressure can blow the top right off," he notes. "That quick fix has turned into a compressor replacement."


One reason trucks, buses and other commercial vehicles repeatedly experience short A/C clutch or compressor life is low voltage at the clutch coil lead wire. On a 12-volt system, the A/C compressor clutch needs at least 11.5 volts to create the electromagnetic field required to engage the clutch pulley so refrigerant can flow. Without adequate voltage, the clutch hub will slip against the face of the pulley, generating friction. The resulting heat - in excess of 1,000 °F - will quickly melt the clutch bearing seal as well as the epoxy potting-compound that seals the coil.

"Poor ground wire connections are an obvious source of low voltage," observes Burrow. "But also consider that people are tapping into the electrical system to power marker lamps, coffee makers, microwave ovens, laptop computers - you name it, and hotel loads can steal the voltage an A/C clutch needs to do its job."

Most heavy duty alternators have an output of 70 to 85 amps. When the alternator is required to supply more than its capacity, the extra power comes from the batteries. As the batteries drain, power to components like the compressor clutch can drop from 13.5 volts to less than the required minimum of 11.5 volts. This causes the clutch to slip and eventually fail. So will its replacement.

"If lucky, the vehicle will lose only the air conditioning," he says. "A seized clutch and locked-up compressor also can snap a double-V or multi-groove belt that drives other important components, like a fan, water pump or alternator."

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