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Corvette Air Conditioning - A/C Diagnostics: Keeping Your Cool

James Berry May 1, 2011
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Since this column's inception, we've received a substantial number of questions concerning air-conditioning troubleshooting. Answering each one would take quite some time, but we will attempt to address most of them over the next few issues.

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Questions like "I added some refrigerant to my Corvette, but it won't get cold—what do you think is wrong?" are almost impossible to answer without further detailed information. In an effort to help take away the mystery of A/C repair, we'll cover how you can diagnose these problems yourself.

Diagnostics are the essential first step in A/C-system servicing. To get started you'll need an A/C manifold-gauge set, a thermometer, and a leak detector. The manifold gauges will allow you to obtain more information about what is taking place inside of the system.

The basic manifold-gauge set should have three hoses. The hoses are color coded to help indicate which service ports they're intended for. The hose intended for the low-pressure service port is blue, and the hose intended for the high-pressure port is red. The yellow middle hose can be attached to a refrigerant can or cylinder for charging. It can also be used when a vacuum pump is needed to evacuate air and moisture from the system. Your manifold-gauge set will have corresponding gauges and control knobs for both low and high pressure. The gauges and knobs are usually the same color as the hoses, to indicate which is which.

The low-side pressure gauge should be on the left side of the gauge set, just above where the blue hose attaches. The low-side gauge serves a dual purpose: It can be used to measure pressure or vacuum. The numbers around the outside of this gauge indicate pressure in pounds per square inch (psi), and the numbers near the bottom of the gauge (in red) indicate vacuum in inches of mercury (in Hg). The smaller scales near the center of the gauge list the temperature relationship of different refrigerants. Regardless of which refrigerant is being used, the scale designated as pounds per square inch is the one used to read system pressure when charging and diagnosing an A/C system.

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The high-side gauge should be on the right side of the gauge set, just above where the red hose attaches. The high-pressure gauge is used to measure the high pressure in the system. Remember, never open the high-side control knob when the vehicle's A/C compressor is running. This could result in the pressure from the high side causing the can of refrigerant to explode. When attaching or disconnecting the A/C gauges, the vehicle should always be off.

The initial reading we'll take is the static-pressure reading. With the vehicle off, attach the gauge hoses to the corresponding service ports. The gauges should read the same (or be extremely close) on both the low- and high-pressure sides (Image A). This reading indicates that both sides are equalized. Static pressure will not show you if your system is fully charged with refrigerant. Static pressure is only an indicator that there is some refrigerant in the system, and that you can proceed with your A/C-performance test.

The purpose of checking the static pressure is to ensure that there is sufficient pressure in the system to engage the low-pressure switch and operate the compressor. There also must be enough refrigerant in the system to perform leak testing if necessary. Temperature plays a big role in static-pressure readings. The higher the ambient temperature, the higher the static pressure will read. If static pressures are under 50 psi, you should consider that the system is low. The system will need to be recharged before proceeding with any accurate performance testing or diagnostics.

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The best way to ensure exactly how much refrigerant is in the system is to recover all of the refrigerant, introduce a vacuum, install A/C-leak dye, and then recharge the system to the factory-specified amount of refrigerant. Most vehicles will have a label under the hood that specifies what type and how much refrigerant that particular vehicle's A/C system should have installed (Image B).

With the gauges installed, start the vehicle, then turn the A/C on at the maximum recirculation position with the blower motor on high and the vehicle's doors closed. (Open the driver-side window just enough to allow you to reach into the car if the vehicle becomes locked.) Using these settings will help keep the engine and exhaust heat from causing abnormal heat load on the evaporator, and should give you a more accurate temperature reading. A thermometer should be positioned in the center vent, so temperature can be checked periodically during testing (Image C).

One of the most commonly asked questions regarding A/C systems is what the low- and high-pressure gauge readings should be when the engine is running and the system is on. Unfortunately, there is no single answer to this question, due to variables such as engine speed, airflow across the condenser, system design, blower speed, mode setting, and refrigerant type, all of which can cause variances in the high- and low-side readings.

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A good rule of thumb is that pressures should read in the region of 35 psi on the low side and 212 psi on the high side at idle (Image D). The reason approximately 35 psi is the optimal pressure on the low side is that it is equivalent to evaporator temperature, which is what you feel coming out of the vents; this should be close to the freezing point of water.

If you look on the low-side pressure gauge, there is a temperature scale next to your pressure scale. The number on that scale translates into evaporator temperature. Since moisture will collect on the evaporator, you want to try and keep the evaporator temperature just above the freezing point of water; this will prevent the moisture on the evaporator from freezing. If this pressure becomes too low, and the evaporator starts to freeze, the low-pressure switch will cycle the compressor off temporally to allow the evaporator to thaw.

The reason we use 212 psi as a base reading on the high-pressure side is that it is the average reading you will have at 85 degrees if everything is working properly. With systems using R-134a refrigerant, the high-side pressure usually will equate to 2.2 and 2.5 times ambient temperature. That means on an 85-degree day, you should expect to see high-pressure gauge readings between 187 and 212 psi. High-side pressure has a broader operating range than low-side pressure due to heat loads on the evaporator, airflow across the condenser, engine speed, and humidity. All of these factors can cause a higher reading on your high-pressure gauge. We'll see how that plays into problem solving when we use the gauges to diagnose specific issues.

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If you're still using R-12 refrigerant, the procedure is the same, but the math is a bit different. On an R-12 system, the high-side pressure usually will equate to 1.8 to 2.1 times ambient temperature. That means on that same 85-degree day, you would expect to see high-pressure gauge readings of between 153 and 178 psi.

Leaks are the most common cause of automotive A/C-system problems. Refrigerant leaks out and reduces the total pressure in the system, causing the low-pressure switch to disengage the compressor. This is a safety feature to protect the compressor from damage if the system becomes low on refrigerant. Prolonged low refrigerant can cause the air conditioning compressor to become damaged internally and spread small metal flakes through the entire system. This can cause components to become clogged with debris and require expensive repairs.

Most of this debris collects in the condenser, which can cause it to become partially blocked. This reduces cooling performance. Condensers changed to a more efficient design when manufacturers switched from R-12 refrigerant to R-134a. Unfortunately, we're seeing more condensers fail on today's vehicles due to the multi-path layout of these condensers.

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The multi-path design has much smaller passages than did the earlier R-12 units. These condensers can easily become contaminated with debris from a failed compressor or other component. If you have a failed compressor and find metal particles in your system, it's highly recommended that you replace your condenser.

If the debris passes through the condenser and enters the high-pressure line, it can plug the orifice tube or the expansion valve. Debris can also migrate backwards from the compressor through the suction hose, causing a blockage in the accumulator or receiver dryer.

If you open the system and find blackish sludge, it's most likely the result of moisture contamination. This sludge can damage the compressor and plug the orifice tube or expansion valve. The moisture-absorbing desiccant in the accumulator is designed to prevent this from happening, but the desiccant can only hold so much moisture, and once saturated, sludge will begin to form.

Flushing can help clean blockages by dislodging sludge and debris and purging it from the system. There are several ways to flush an A/C system. One method is to use a plastic bottle to insert the approved solvent into the system. Then, blow it through using dry air or nitrogen from a rubber-tipped blow gun at about 100 psi. Only flush the solvent through heat exchangers and free-flowing hoses. Always flush in the reverse direction of refrigerant flow to dislodge any material caught inside. After the system is flushed completely, blow each component dry in both directions. When sludge or debris is found in an A/C system, the orifice tube or expansion valve, accumulator, and condenser should always be replaced.

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Testing the refrigerant system for leaks is one of the most important tasks when troubleshooting automotive A/C systems. Keep in mind that it's common for systems that are in good condition to lose a small amount of refrigerant each year; this is considered normal.

When looking for leaks, a good visual inspection of the entire A/C system is a great first step. The system contains the oil necessary to lubricate the compressor. The presence of an oily film around fittings, lines, the compressor, or any components is a strong indication of a refrigerant leak. Most leaks are small and allow refrigerant to escape over long periods of time, so they may be hard to detect with a visual inspection. There are several methods you can use to detect these small leaks.

The first is an electronic leak detector, which can be used with all types of refrigerants. This is a handheld, battery-operated unit that electronically "sniffs" for leaks. Refrigerant is heavier than air, so you'll need to run the leak detector's test probes directly below any suspected leak areas, including all fittings, lines, and components. Try to avoid getting oil or residue on the end of the leak sniffer, as this can give you a false reading.

Some of the more common sources of leaks are service-port Schrader valves, compressor front seals, compressor center seals, and evaporators. When checking the evaporator, you can insert the electronic leak detector's probe into the evaporator drain. Most detectors have an alarm that will go off when it detects the presence of refrigerant. Some detector models may have flashing lights or a buzzer to alert you when they detect refrigerant.

The method of leak detection preferred by most automotive technicians involves a fluorescent dye (Image E). To find refrigerant leaks using the fluorescent tracer system, the fluorescent dye must be installed into the vehicle's A/C system. There are several ways this can be accomplished, with two methods being the most common. The first uses an adaptor that can be placed on the yellow hose of your gauge set; this allows the dye to be installed when charging the system. Another method is to use a pressurized can that contains dye and install it through the low-side filling port.

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With the dye installed, run the A/C system for several minutes to distribute the dye evenly. Then, use yellow, UV-enhancing glasses and a black light to examine the system. If there are any leaks present, the fluorescent dye will show up easily using the light and UV glasses (Image F).

Once you've found your leak and are ready to replace the faulty component—a process that requires the removal of the old refrigerant—you'll need to have that refrigerant reclaimed and the new refrigerant added by a certified A/C-repair shop. Federal law prohibits the venting of any type of refrigerant into the atmosphere, and this law is enforced with a steep fine or even imprisonment.

Vehicles should only be recharged with the same refrigerant that is already in the system, unless an older system is being retrofitted. When two different refrigerants are intermixed, it changes the operating pressure and cooling properties of both. There are a number of EPA-approved refrigerant products that can be used in place of R-12 in older-vehicle retrofits. R-134a replacement refrigerant has been on the market for a long time and has proven to be reliable. Keep in mind that when you change refrigerants, you must install a compressor oil that is compatible with your new refrigerant. The R-12 system uses a mineral oil, while the R-134a requires POE or PAG oil when it is retrofitted (Image G).

After the repair is performed, the air and moisture will need to be removed. For this you'll need to evacuate the A/C system. This requires putting the system into a vacuum for approximately 20 minutes, which allows any moisture in the system to be boiled out. Remember from chemistry class that water will boil in a vacuum. This is an important step that requires specialized equipment.

This month we focused on the questions we most often receive about A/C gauges and basic system operation. Next month we'll provide you with detailed troubleshooting diagnostics using the A/C-gauge set and other methods, and explain how refrigerant flows through the system. Until then, good luck and stay cool.



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