KEEPING COOL

| July 15, 2009

How to maintain your A/C system to avoid poor cooling


Cab air conditioners use R134a, a refrigerant that’s always under pressure. Even at 32 degrees F, the refrigerant is already under more than 30 psi, making leakage a constant threat. If refrigerant leaks out, the liquid refrigerant level in the evaporator will drop, and the cooling capacity of the system will drop drastically. Low refrigerant does more than reduce performance – it soon leads to damage.

The refrigerant isn’t the only important fluid inside the system. There also is a synthetic oil that lubricates the compressor’s moving parts even at low temperatures. Both fluids absorb water from the humidity in the air outside unless the system is tight.

An abnormally low pressure level because of a leak will allow moisture to be drawn in. Running low enough on refrigerant can even cause a vacuum in the system, which brings in even more moisture. The moisture produces acid, which causes corrosion. You could even get metal shavings that block narrow passages.

Also, when refrigerant leaks, oil leaks too. It’s carried around the system by the R134a. Oil acts as a coolant for the compressor, and a low level can cause the compressor to overheat. So do routine inspection and maintenance yourself, along with having at least an annual check by a pro. This means some time and money, but will surely save you on repairs and help guarantee reliability. Follow these steps to prevent leaks and avoid expensive repairs.

Run the system, even in winter. This keeps the compressor shaft seal lubricated and stops leaks. The compressor runs when the defroster is on. Frank Burrow, warranty and product support manager at Red Dot Corporation, an A/C component manufacturer, recommends making sure you turn the system on for a few minutes at least once a month if you don’t need to engage defrost. Some service managers prefer once a week. If you run most of the time in an extremely cold climate, remember to do this whenever you get into a warmer area.

Regularly inspect hoses, connections and other components at every oil change. Burrow says, “Look for dirt buildup at all connections, on condenser fins and along all hoses.” Any leak of refrigerant will be accompanied by the oil, which circulates around the entire system, and an oil film always attracts dirt. It may even pay to look for that dirt as often as once a week during your walk-around inspection. At the first sign of streaks of dirt, have the offending component replaced, or a leaking connection taken apart, resealed with a new O-ring and properly tightened. Since you may not catch every leak visually, it also pays to have the system inspected by a licensed refrigeration mechanic at least once a year. The tech will measure system pressures, which reveals problems.

Check the appearance of the colored dot on the sight glass. This glass is located on top of a small black cylinder called the receiver-dryer 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. The sight glass is a diagnostic aid to check for moisture in the receiver-dryer, which functions like the air dryer on your braking system. It has a colored dot inside the sight glass that changes color if the desiccant has become saturated with moisture, a sure sign of trouble. It’s normally blue or aqua when the system is dry and changes to pink or white when the dryer becomes saturated.

Inspect compressor mounts and belt-tensioning systems. Look at the belts themselves and carefully check condenser mounting bolts and brackets. Vibration is one of the system’s enemies.

Watch the clutch engage. It should grab in a second or two, without much slippage. Slow or partial clutch engagement may indicate an electrical or mechanical problem that could result in unnecessary and often non-warrantable component failures.

Recharge yearly. Have the system evacuated, the oil inspected for color and the system recharged with the proper amount of refrigerant at least once a year under heavy use. That sounds excessive, but after 120,000-150,000 miles, the system may have run for 1,000 hours or more.

Don’t fix it yourself. When performance drops, don’t buy a can of refrigerant and put it in. You’ll still have a leak, and aftermarket refrigerant may contain incompatible oil or no oil. Either could damage your compressor and void the warranty. Also, with no way to gauge how much to put in, you could easily cause excess pressure or even a liquid lock in the compressor, ruining a major component.

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