Keep Your Cool


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Index Sensors and Controls
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The most critical issue in air conditioner maintenance is keeping the system full of refrigerant, which in newer trucks is R-134a. The task is so important because the system operates under high pressure, making it prone to leaks. Over time, leaks can create problems much more severe than simply needing a refrigerant refill.

Even a slight loss of refrigerant will make the system pressures unstable, causing the clutch to start cycling several times a minute. Short-cycling is a big problem because the clutch has to slip a bit to bring the compressor up to speed without shock. Slippage creates heat. As a result, the clutch’s wear and operating temperature depend mostly on how frequently it has to engage. So the result of short-cycling is often heat, wear and premature clutch failure. The heat will often destroy the compressor by attacking the compressor shaft bearings.

As the charge leaks more, pressure drops so much on the low-pressure side of the system (where the evaporator is) that a vacuum is created. This draws in humid, outside air, adding moisture to the system. Under these conditions, the system’s desiccant dryer will quickly saturate, resulting in severe corrosion problems and then component failures – unless you catch the problem in time. So, keeping the system fully charged is the most critical part of keeping it in good operating condition.

It’s very difficult to keep the system sealed, says Gary Wilson, sales and marketing manager for Index Sensors & Controls, a manufacturer of air conditioning protection and monitoring products. The problem is maintaining pressure, typically well over 50 psi except in winter, even if you’re not running the air conditioner. “Seals will deteriorate over time, particularly if the air conditioning hasn’t been used for a number of months,” Wilson says. “If lubricant isn’t contacting the seals, they can warp and start leaking.”

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So, like most systems on your truck, the air conditioner thrives on regular, year-round operation. Running the unit circulates the oil, lubricating the shaft seal on the compressor and all the other seals in the system, helping them to hold the refrigerant in. Oil is also good for the hoses.

The compressor shaft seal is the most vulnerable spot for leakage, but routinely circulating the oil will help, says Steve Bonnen, product engineer at Bergstrom, the company that makes Kysor air conditioning products. He adds that such operation may help remove moisture that may have crept into the system by circulating it through the desiccant in the dryer.

The oil is pumped through the compressor with a shaft-driven oil pump and splashes onto pistons and bearings. Shutting the compressor down for long periods allows the oil to drain off the moving parts. The result can be a compressor that will drag enough to make the belt slip till oil is circulated. And, as Wilson says, another result can be a system that has lost most of its refrigerant charge because the shaft seal and other seals in the system have dried out and leaked.

Carl Garrod, Kenworth service training manager, says, “On Kenworth trucks, as with many of our competitors’ vehicles, the air conditioning operates any time the controls are put in defrost mode and the system pressure is above 26 psi. So, most of the time during the winter months the A/C system runs and circulates the oil to prevent seals from drying out. But, if a driver doesn’t use defrost, then it’s a good idea to periodically run the A/C or defrost to circulate the oil.”

Bonnen says that on some vehicles the compressor will run for a short time every time you start the engine. That will help, but, especially if your truck does not have this design feature, you should run the unit every week or two. He recommends that in winter you do this after driving long enough to get the cab and engine compartment warm so the unit will run for a while. Make sure the A/C switch is on for five minutes or so.

A certified technician should check your system every year or two, even if you don’t suspect a problem, says Bonnen. By measuring the operating pressures with a special gauge set and possibly checking subcooling, the tech can make sure it’s fully charged – and protected.

If you can maintain the necessary refrigerant charge, you will prevent most of the serious air conditioning system problems. You’ll not only save yourself that disappointing feeling of a hot cab on the first warm day, you’re likely to save money and hours of downtime when it comes to repairs.


In the days of R-12 refrigerant, the sight glass on top of the receiver-dryer would look clear when the system was right, indicating liquid refrigerant with no vapor bubbles. But you can’t really check the system charge this way any more because R-134a refrigerant uses a different type of oil, polyalkaline glycol.

“PAG oil does not fully emulsify the way the mineral oil used with R12 would in the past,” says Steve Bonnen, product engineer at Bergstrom. The result is that the sight glass looks slightly cloudy even if the system is fully charged, except at very cool outside temperatures. So how can you know if the system is OK?

CHECK FOR OIL FILM. Refrigerant boils away as it leaks, leaving no evidence, but the oil leaves a thin film on hoses or connection fittings likely to collect dust. First, check around the crankshaft seal. Follow up with checks of every connection.

CHECK TEMPERATURE DROP. Crank the system up with the blower on one of the higher speeds and the temperature setting as low as it will go. While the system stabilizes, use a small thermometer to measure the cab temperature and make note of it. Then, after the A/C has run for a few minutes and the compressor is cycling, use the thermometer to measure the air coming out of the louvers in the dash nearest to the evaporator. If the compressor doesn’t cycle, pick a slower blower speed.

Mack says cycling cab air through the system should result in a drop of 30 degrees. If it’s 70 degrees in the cab, for example, air at the louvers should be 40. Bonnen says a 25-30 degree drop is typical of most systems. Kenworth’s Carl Garrod notes that high humidity will normally decrease the temperature drop.

USE A TECHNICIAN. Refrigerant leakage normally produces less temperature drop and rapid clutch cycling. If you get those symptoms, have a certified technician find and fix the leak. If the system has leaked only slightly, he can then check the refrigerant charge by measuring system pressures and subcooling – which relates to the temperature of the refrigerant leaving the bottom of the condenser. If the system has leaked a lot, he will remove the refrigerant and replace with the specified weight for the system.

Don’t buy charging kits and attempt to do this yourself. You can too easily contaminate the system or overcharge it. If you want to do this yourself, Bonnen recommends taking classes at a technical school and becoming certified to handle refrigerant.


In addition to inspecting for signs of refrigerant leakage, other maintenance practices will help head off problems with your air conditioning system. Two critical areas are the clutch and the evaporator.

CLUTCH. Have someone turn the air conditioner on as you watch the compressor start. The clutch should engage with just a second or two of slippage. If it slips longer, even after the compressor has warmed up and started to cycle, it may be worn, which would give it an excessive gap when the compressor is off.

If you have electrical instruments, you may want to check for proper voltage and amperage draw. Carl Garrod of Kenworth says the clutch should be getting at least 11.5 volts – less can cause it to slip and wear. Amperage draw varies, but Garrod pegs it at 3.6 to 4.2 for compressors on most Kenworths.

Steve Bonnen of Bergstrom says clutch replacement is often best done with the system discharged and the compressor removed from the truck, so don’t attempt to repair a bad clutch. He also says it’s pointless to replace a bad clutch only unless you’re sure the slippage has been minimal because the heat most likely will have destroyed the compressor shaft bearings. Replace both clutch and compressor.

EVAPORATOR. Make sure fins and ducts are clean. Replace or clean any air filters. (Filters that can be cleaned will have instructions printed on them.) These are normally located in the fresh air intake, which is in the engine compartment or in the evaporator case. Greg Kolodziej, HVAC maintenance and development engineer at International Truck and Engine Corp., says High Performance series trucks have a fresh air filter in the system. These filters don’t get dirty very fast in highway service, but require frequent attention in off-road applications. “When the filter is clogged, the operator may notice less cooling and airflow,” he says.

Do a visual check once or twice a year for highway trucks and at least every month in dusty off-road conditions. If your truck has a filter, it will be somewhere in the ducts leading to the evaporator. The filter on High Performance trucks is located in the evaporator module, accessible on the forward side of the dash in the engine compartment. Look into the air intake duct. If the filter is dirty, remove the grate, pull the filter out and clean it. Replace it after five or six cleanings.

There is a hose that allows drainage of the water that the evaporator wrings from the air. Make sure it is open and not kinked, as well as properly connected. Bonnen says that where the evaporator blower draws the air through the evaporator rather than pushing it, there is often a check valve at the bottom of the drain hose. Make sure it’s still tightly in the hose. If the valve falls out, water comes out of the louvers on high blower speeds. Here are other system maintenance tips:

  • Check the routing of all hoses to ensure there is no chaffing. Where hoses rub, add ties or clamps to isolate them. Have seriously chaffed hoses replaced (the refrigerant must be removed).
  • The receiver-dryer sight glass may have a moisture button that changes color if moisture saturates the desiccant. The dryer may have a label explaining what to look for. If the dryer needs replacement, use a certified technician because removal of the refrigerant is required.
  • Check that the evaporator blower operates on all speeds. Repair wiring or replace the motor or switch if it fails to work properly.
  • Check compressor belt tension and engine fan belt tension. Adjust if necessary. Replace frayed belts.
  • If you have an on-off engine fan, it should run at least sometimes when the A/C is running and the truck isn’t moving. If it never runs except in response to high engine coolant temperature, and A/C performance has deteriorated, the problem may be that the fan does not come on in response to a switch that reacts to condenser pressure. A technician can test the switch and replace it if necessary.
  • Check the condition of the fins on the condenser and clean them with a low-pressure water spray, if necessary. Bent fins will raise condenser pressure and cause performance deterioration. You can use a fin comb to straighten bent fins.