A/C System Performance

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."

Burrow says there are three things to remember about reading voltage:

Re-create the full demand for voltage. "With the engine and air conditioner running, turn on lights, wipers, radios - anything that draws power. Then take a reading. Ideally, we want 13.5 volts but not less than 11.5 volts at the clutch coil."

Look inside the cab or sleeper for an aftermarket power inverter with 110-volt AC outlets, evidence that the driver is drawing even more power from the vehicle's electrical system.

Many clutches are grounded through the compressor casing, as indicated by a single wire on the power side of the clutch. A ground can be picked up on a bolt or bracket, but sometimes that bolt or bracket is the source of the bad ground due to corrosion or even resistance that comes from paint. With a case-grounded clutch, measure the ground at the compressor body.

"The clutch is the heart of the system and the number one warranty cost item in the A/C system," points out Burrow. "Perform a simple visual inspection every three months, looking for discoloration on the face of the clutch hub - a sign of heat; oil or dirt around the shaft seal - which signals a leak; proper alignment of the clutch pulley and crankshaft drive pulley; and glazing or cracking on the belts.


In this case, the dot is the moisture indicator on the receiver-dryer's sight glass.

The receiver-dryer removes moisture from the system and filters refrigerant. Whenever the engine oil is changed or a vehicle is in for scheduled maintenance, Burrow recommends checking the sight glass on the moisture indicator. A blue dot means the refrigerant is dry; pink, white or grey indicates acid or moisture in the system.

In general, a dryer should be replaced once a year or every time the A/C system is opened. Check with the vehicle's manufacturer for specific recommendations. Mark the installed date on the new dryer so it's easy to see.


"The mobile A/C market has some of the best all-makes parts options in the heavy-duty aftermarket to help reduce inventory and parts costs," he says. Examples include the Sanden FLX7 family of compressors, which enable "replacement of the majority of Sanden's OE compressors with just 15 models and five head adapters, and Aeroquip's E-Z Clip system - a fast, effective way to create hose fittings for any make of vehicle.

"Talk to your A/C parts suppliers about how they can help you service a broader range of vehicles with all-makes parts. Buy from a trusted source so you know you're getting a part that's an exact match for the fit and performance of the original."


"Your primary diagnostic tools are with you all the time: your hands," says Burrow. "Obviously, touch and feel aren't going to replace your service equipment. But they're a good first step in the troubleshooting process."

For example, a properly functioning receiver-dryer should be warm to the touch. If it's cold when the system is running, the dryer may have moisture or a restriction inside. It's normal for frosting to occur during the recovery process.

Grab a hose and palpate it, he suggests. Hoses tend to deteriorate from the inside out, shedding debris into the refrigerant or coolant. A spongy hose is a sign that it's weak and should be replaced. Feel the tension on the hose connections and make any necessary adjustments.


"Old rules of thumb say you should add so many ounces of oil when you replace a condenser, so many ounces when you replace an evaporator, etc.," says Red Dot's Burrow. "In fact, the only way to determine how much oil to put back into the system is to drain the oil from the old component, measure the amount you removed and replenish the system with the same amount of oil when you install the new component."

The compressor demands even more care when it comes to lubricants. Most replacement compressors come pre-charged with 5 to 10 ounces of oil depending on the make and model, he says. The type and amount should be clearly marked on the compressor's label. Don't rely on product literature or catalogs. The pre-charge means there should be no need to add oil when you install a new compressor.

Too much oil in a system can coat the inner walls of the evaporator and condenser coils and diminish their ability to absorb or dissipate heat. Because excess oil tends to collect at low points in the suction line, there is also the risk of slugging the oil into the compressor at start-up.

"When you add oil to a system, be precise. Replace only what you take out."


Dust, bugs, feathers and other debris collect on the face of condenser fins and tubes and act as a thermal barrier, so it's important to keep the condenser clean. "Trouble is, a high-pressure hose can damage fins and disrupt the airflow across the condenser. Remind your drivers and technicians to mind the fins."

Condensers also fail because of vibration. Hose connections can come loose and fatigue the condenser tubing adjacent to the fittings. A routine inspection will highlight the need to clean the condenser and comb the fins, and make sure the hose connections are securely clamped.


"Refrigerant recovery, recycling and recharging equipment will last a long time if they are properly maintained and calibrated," Burrow points out, and offers these two steps to take to improve service life:

Use a refrigerant identifier to detect blends and contaminated refrigerant that can harm the service equipment. "It's worth the expense if the identifier saves one expensive repair."

A recovery station has a compressor, hoses and filters that need attention after so many hours or jobs. "Make sure everyone knows the PM schedule and who is responsible for changing filters, calibrating scales and so on."


Training and product support should be part of the package when buying A/C replacement parts. "At Red Dot, we spend a lot of time in the field helping people specify and maintain heavy duty HVAC systems and components," says Burrow. "Chances are, we've seen the problem you're working hard to resolve and can help you handle it more efficiently and cost-effectively."