An improperly charged air *conditioner is a bigger risk *than most technicians realize. Industry service experts said inaccurate charging is a common mistake that limits cooling, threatens air conditioner life and may even impair driveability. Plus, charging the system accurately the first time truly is the shortest path to an effective and profitable repair job, they said.
An air conditioner can be undercharged, overcharged or charged to specification. Many pros attribute overcharging to the misconception that the more refrigerant you put into the system, the colder its discharge air becomes.
Air conditioning fundamentals show the fallacy of this notion. Air conditioners cool by removing heat and the crux of heat removal is evaporation. For instance, a swimmer feels chilled when a passing breeze evaporates the water and removes heat from his skin.
The evaporator is the air conditioner's heat exchanger below the dashboard. As the component's name suggests, the evaporator is where refrigerant turns from a liquid into a gas. As the liquid refrigerant entering the evaporator turns into a gas, it removes heat from the vehicle's interior. When an air conditioner is properly charged, nearly all the refrigerant evaporates as it passes through the evaporator.
But there remains a tiny amount of liquid (unevaporated) refrigerant that flows out of the evaporator and back to the compressor. Most techs know refrigerant oil must circulate throughout the refrigerant circuit. But few understand that the trace of liquid refrigerant exiting the evaporator performs the critical chore of carrying refrigerant oil back to the compressor.
In an overcharged system, excessive liquid refrigerant flows out of the evaporator. The greater the overcharge, the greater the threat to the air conditioner. A relatively slight overcharge is wasteful but may not pose a serious risk. More severe overcharging impedes cooling and may ruin the compressor.
Overcharging increases operating pressure of the entire refrigeration circuit, particularly the high-side or liquid-side of the system. Not only does this hamper the air conditioner's cooling performance, it also increases the strain on the compressor and compressor clutch. Ultimately, this additional strain shortens the life of the compressor and/or compressor clutch.
The penalty for overcharging an air conditioner is greater than ever for two big reasons. First, the price of a replacement compressor alone can be extremely expensive.
More-affordable remanufactured compressors may not be available for the application.
Plus, a compressor clutch that succumbs to the strain of an overcharge usually costs the consumer a new compressor, too. When labor rates were lower, air conditioners were simpler and compressors were more accessible, replacing a failed compressor clutch was a common, cost-effective repair. But higher labor rates, increased labor times and/or limited parts availability means a failed clutch almost always dictates replacing the entire compressor/clutch assembly.
Air conditioning specialists warned that overcharge conditions may fool technicians by causing misleading symptoms. For example, refrigerant naturally attracts moisture through any leak or imperfection in the system's plumbing or seals. Desiccant inside the receiver-drier or accumulator protects the refrigeration circuit from sludge and corrosion by absorbing this moisture.
Many techs have seen intermittent cooling problems caused by saturated receiver-driers. The excessive moisture inside the system freezes inside the expansion valve (TXV), shutting down the system. When the TXV thaws again, refrigerant starts flowing and the system blows cold air again.
Experience shows overcharging can cause the same intermittent symptom. If a technician overcharges an air conditioner first thing in the morning or on a relatively mild day, the system may cool fine until the heat load increases. Then a combination of too much refrigerant and high ambient temperatures can increase high-side refrigerant pressures enough to activate the high-side shutoff switch. This switch shuts off the compressor until high-side pressure falls back within its normal range. As a result, warm air blows out of the ducts until the pressure settles down again-then the cycle repeats itself. The system behaves just like one with excessive moisture in it, but the root cause is excessive refrigerant.
Remember some key distinctions here. On a TXV-equipped system, frost forms on the TXV inlet as moisture freezes. Also, a moisture-laden refrigeration system may cool fairly well until the TXV freezes, then ceases cooling altogether.
On the other hand, overcharged
air conditioners usually cool poorly all the time and an overcharge won't cause frost to form on the TXV inlet.
Millions of units have an accumulator on the low-pressure side of the refrigeration system. Service pros caution that overcharge conditions may not be as obvious on such systems because the typically larger volume of their accumulator helps them endure the overcharge.
Because receiver-driers usually hold considerably smaller volumes than do accumulators, such systems are more vulnerable to the ill effects of overcharging.
Overcharged air conditioners can cause rough idle or stalling symptoms that trick technicians into suspecting engine or engine computer trouble. Some cars have adjustable throttle-opening devices that open the throttle blade farther to stabilize idle rpm when the air conditioner is turned on.
However, powering an overcharged air conditioner may strain the engine so much the throttle opener can't maintain normal ``air-on'' idle speed. Technicians who encounter difficulty adjusting ``air-on'' throttle opener rpm should suspect an over-charged air conditioner.
Most fuel-injected engines have an electric air valve that controls idle speed by regulating the volume of air entering the engine. In some instances, an overcharged air conditioner has triggered erroneous idle air valve fault codes by forcing the air valve to operate out of its normal range for an extended period of time.
An air conditioner that cools inadequately is a traditional symptom of an undercharged refrigeration system. Also, pressures on both sides of the system will be lower than normal and the compressor clutch may cycle on and off very rapidly.
Refrigerant oil is the compressor's life-blood lubricant. Earlier, we explained that traces of liquid refrigerant leaving the evaporator carry oil back to the compressor. But if every droplet of refrigerant totally evaporates due to an undercharge, oil circulation stops and the compressor eventually fails.
A leak is the only reason a refrigeration system should be low-unless someone already serviced the system and didn't charge it accurately. Adding refrigerant to a leaky system may temporarily restore normal cooling, but this approach is false economy. The expensive refrigerant you install will only leak out again.
Furthermore, ``topping off'' the system with fresh refrigerant won't stop moisture from entering it or correct any moisture-related damage that has already occurred inside it.
Refrigerant, like brake fluid, readily absorbs moisture. Mixing moisture with refrigerant creates corrosive acids that destroy the system. Mixing moisture with refrigerant oil creates sludge that can restrict or clog tiny orifices inside the system.
Explain the truth to cost-conscious consumers: Prolonging a proper leak repair only prolongs the damage to the system, resulting in higher repair bills later on.
Sources said proper procedure is essential for accurate air conditioner testing. Always warm up the engine completely and run the air conditioner for at least 5-10 minutes before taking temperature or pressure readings.
When checking for an overcharge, remember the ``100-plus'' rule for estimating normal high-side refrigerant pressure for a given ambient temperature. Adding 100 to the ambient temperature should approximate normal high-side pressure.
Some techs may confuse high pressures caused by overcharging with those caused by inadequate air flow through the condenser.
When the root cause of the problem is inadequate air flow, spraying the condenser with cold water will cause a sudden, dramatic drop in high-side pressures.
Air conditioning specialists repeatedly emphasized that fixing a
``problem'' air conditioner often entails no more than recovering its existing refrigerant, evacuating the system and then installing the specified charge!
Today, installing the specified charge accurately is easier than ever. Thanks to modern equipment such as the electronic charging scale shown on the front page of this service supplement, techs can just key the correct charge into the machine's control panel. When this amount of refrigerant has been dispensed, the machine shuts off, preventing an overcharge.
Many technicians bemoan the fact that most domestic air conditioners no longer have a sightglass. But Ward Atkinson, an air conditioning engineer at Sun Test Engineering in Scottsdale, Ariz., cautioned against relying too heavily on sightglass readings. Many techs ignore specifications and force refrigerant into a system until all bubbles in its sightglass disappear. In reality, normal operation of an accurately charged system may create an occasional bubble or two in the sightglass.
Consequently, the ``clear-
sightglass'' approach often leads to overcharging, Mr. Atkinson explained.
What's more, technicians usually can use their fingertips for a reliable charge check on popular orifice-tube systems, Mr. Atkinson added. Before doing this, bring the engine up to operating temperature, be sure the compressor clutch is engaged and the blower is on high. Either open the doors and windows or set the vent to the outside air position.
With the engine idling, feel the evaporator inlet tube and the top of the accumulator. Both should feel the same-ice-cold. The system is undercharged when the accumulator is distinctly warmer than the evaporator inlet tube, he said.
Another good check is to close the car doors and slip your fingers into a dashboard air outlet. Monitor the discharge air with the engine idling in gear for a minute or two, then accelerate suddenly.
The refrigerant charge is usually good if the air remains cold throughout this test. The system is usually undercharged when the air warms up for a moment, then turns cold again, Mr. Atkinson said.