Keep your cool

Every truck owner has faced flipping on the cab air conditioner the first warm day of spring and feeling little or no cool air. Following a few basic maintenance steps can help prevent that.

Just as tires are nearly indestructible if you keep them full of air, the same principle applies to A/C. Low refrigerant reduces performance and, at worst, leads to damage.

Cab air conditioners use R134a, a refrigerant that boils under atmospheric pressure at about minus 26 degrees F. So even at a chilly 32 degrees F., it’s already under more than 30 psi, and on a hot day the pressure can exceed 100 psi even when the system is off. When the system is operating properly, the evaporator, which is the cooling coil inside the dash, is nearly full of boiling refrigerant, removing heat from cab air.

Even the tiniest imperfection in a hose or sealed connection will allow the refrigerant to leak out. If that happens, cooling occurs only in the lower part of the coil that still has liquid, and the system’s cooling capacity drops drastically.

Refrigerant isn’t the only important fluid inside the system. There is also a light, synthetic oil that lubricates the compressor’s moving parts even at low temperatures. Both the refrigerant and the oil are “hygroscopic.” This means they attract water, which is contained in outside air as humidity.

When the system is fully charged, it has enough pressure, even in the low-pressure (about 30 psi) side where the cooling is done, to keep moisture from entering. But, as the system leaks and pressure falls, moisture is drawn in because of the powerful attraction to the A/C fluids. Running low on refrigerant can even create a vacuum, so the result of a low charge often is a significant amount of moisture being drawn into the system.

When the system is fully charged, it’s free of anything that could corrode the metal parts, which are chosen mainly for their light weight and ability to transfer heat. But when moisture reacts with the oil and refrigerant, it plays havoc with the light metals (like aluminum) inside the system. The acidic reactions can produce metal shavings that block narrow passages.

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Also, the system’s refrigerant oil not only lubricates the compressor, but cools it. When refrigerant leaks, so does the oil because it’s carried around the system by the R134a. A low oil level can eventually damage the compressor because of overheating.

Keeping the system free of leaks and charged with refrigerant and oil will do more than help keep your morale up on the first hot day. It will also eliminate expensive repairs. Two preventive maintenance practices will help keep your system up to snuff.

1. Run the system often
The first step in leak prevention is simple – keep the system lubricated by running it. Air conditioners have compressors with a shaft seal that works effectively only when coated with oil. When running, the compressor oil is splashed onto the seal by the oil pump and the motion of the internal parts. But if the compressor sits for weeks, the oil drains off the seal, and the sealing effect may be largely lost. More than one trucker who hasn’t used his air conditioner in weeks has cranked it up to find that some refrigerant has leaked.

Most manufacturers minimize this problem by having the controls engage the compressor when you hit the defrost button. This works because getting moisture out of the cab air greatly enhances the heater’s defroster function. This feature works well except in areas of the country where there may be little or no precipitation for weeks at a time in winter, or in frigid areas where the temperature or pressure controls on the system may cause the compressor to stay cycled off even when you hit the defrost button.

Frank Burrow, warranty and product support manager at Red Dot Corp., an A/C component manufacturer, recommends turning the system on for a few minutes at least once a month if you don’t need to engage defrost. Ron Whitley, director of technology and support at Cooper Kenworth in Durham, N.C., thinks weekly running of the system is ideal.

2. Do regular inspections
The next step is periodic inspection, say at every oil change. Burrow recommends, “Look for dirt buildup at all connections, on condenser fins and along all hoses.” Dirt collects where the oil in the system and refrigerant leak. Mike Kalkoske, Kenworth quality services manager, says it may even pay to look for that dirt as often as once a week during your walkaround inspection.

At the first signs of dirt, ask a technician to replace the soiled component, such as a hose or condenser, or have him repair a leaking connection and reseal it with a new O-ring, properly tightened.

Since you may not see every leak, have the system inspected by a licensed refrigeration mechanic at least once a year. The technician will check operating pressures on both the low-pressure side, where the cooling is done, and the high-pressure side, where refrigerant gas coming out of the compressor is condensed into a liquid.

Another check is the appearance of a colored dot on the sight glass. This glass is located on top of a small, black cylinder – the receiver-drier – that’s in the thin piping leading from the bottom of the condenser on the front of the truck to the evaporator inside the cab.
Burrow says the sight glass, which with earlier refrigerants could help you detect a leak, is now used only to check for moisture in the receiver-drier. A colored dot inside the sight glass functions like litmus paper, Burrow says. It’s normally blue or aqua, changing to pink or white when the drier desiccant becomes saturated.

In that case, the system has a leak that needs to be fixed, requires a new drier, and must be evacuated and recharged. After that’s done, the technician installs a vacuum pump to remove moisture. The final step is to install new or recycled refrigerant and oil. Doing this will often forestall the need for further repairs, like replacing the expansion valve or other expensive, parts, or flushing parts that contain metal debris.

“Vibration is one of the biggest air conditioner problems,” Whitley says. This means periodic inspection of compressor mounts and belt tensioning systems, along with the belts, and carefully checking condenser mounting bolts and brackets “rather than waiting for them to fall off.”

Whitley and David Tow, a service engineer with Daimler Trucks, recommend watching the clutch when the system is running to make sure it’s engaging properly. Tow says, “It should do so quickly and without slippage. Slow or partial clutch engagement may indicate an electrical or mechanical problem that could result in unnecessary and often non-warrantable component failures.”

Whitley says the ideal procedure, especially for a truck with team drivers, is to have the system evacuated, the oil color inspected and the system recharged with the proper amount of refrigerant at least once a year. That sounds excessive, but doing such maintenance every 120,000 to 150,000 miles is reasonable considering how many hours the air conditioner may have run.

Doing routine inspection and maintenance frequently yourself, along with paying for an annual check by a pro, will surely save you on repairs in the long run. It also guarantees you won’t get surprised with hours in an overheated cab.

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