Corvette Air Conditioning - A/C Diagnostics: Keeping Your Cool, Part 2

James Berry Jun 1, 2011 0 Comment(s)
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Last month we discussed air-conditioning troubleshooting and how you can diagnose many of these problems yourself, including:

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&bull What tools are needed for diagnosing your A/C system
&bull How to hook up your A/C gauges
&bull Static pressure and what it means
&bull Normal running pressures
&bull The importance of having your system charged with the correct amount of refrigerant for proper performance
&bull The differences in gauge readings between R-12 and R134a
&bull Different leak detectors and how they should be used
&bull How to flush your A/C system
&bull The differences in compressor oils
&bull The need to have the old refrigerant reclaimed and the new refrigerant added by a certified A/C-repair shop

This month we will provide you with detailed troubleshooting diagnostics using the A/C-gauge set. This will help you to understand the role pressure plays in different parts of the system. Don't forget to wear safety equipment and protective eye wear. Remember: refrigerant can be under high pressure and is therefore potentially dangerous.

Gauge Readings: low side—60 to 90 psi, high side—60 to 90 psi

With the vehicle off, the gauges should read the same or be extremely close on both the low-pressure and high-pressure sides (see lead photo). This indicates that both sides are equalized. Static pressure is an indicator that there is some refrigerant in the system and that you can proceed with the A/C-performance test.

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 can deduce that the system is low, and that it will need to be recharged before proceeding with performance testing or diagnostics.

Assuming that's the case, you'll first need to recover the remaining refrigerant from the system. Put the system into a vacuum for approximately 20 minutes; this allows any existing moisture to be boiled out. Then recharge the system with the factory-specified amount of refrigerant (usually found on a label somewhere inside the engine compartment). Any oil that was lost should be re-added during the recharge process. Finally, check the A/C system's operation and performance.

While the system is being evacuated, it's a good idea to perform a visual inspection. The A/C system contains 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. As a result, they may be hard to detect with a visual inspection and may instead require the use of a leak-detection device after the system has been recharged.

Gauge Readings: low side—35 psi/high side—212 psi

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Symptoms: The air coming from the vents is approximately 32-40 degrees. No bubbles are present in the sight glass.

Conclusion: The system is working correctly.

A good rule of thumb is that pressures should be in the region of 35 psi on the low side and 212 psi on the high side at idle. A reading of 35 psi is equivalent to evaporator temperature, which is what you feel coming out of the vents in the car; this should be close to the freezing point of water.

With systems using R134a, the high-side pressure usually will equate to 2.2-2.5 times ambient temperature. That means on an 85-degree (F) day, you should see high-pressure gauge readings of between 187 and 212 psi.

Gauge Readings: low side—2 psi (low)/high side—102 psi (low)

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Symptoms: The air coming from the vents is only slightly cool or even warm. Bubbles or oily streaks are seen in the sight glass.

Conclusion: Insufficient refrigerant charge

Repair Procedure: When a low refrigerant charge is indicated, be sure to check for leaks. Leaks are the most common cause of automotive A/C problems. The vehicle's refrigerant will leak out and reduce the total pressure in the system, causing the low-pressure switch to disengage the compressor. This is a safety feature designed to protect the compressor from damage if it becomes low on refrigerant.

When looking for leaks, a good visual inspection of the entire system is a great first step. The system contains 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.

If you don't find a leak with a visual inspection, your next step is to recover the remaining refrigerant from the system. Put the system into a vacuum for approximately 20 minutes and recharge it to the factory specifications.

There are several methods you can use to detect small leaks. The first is an electronic leak detector, which can be used with all types of refrigerants. The second method is to use a fluorescent-dye leak detector. To find leaks using the fluorescent system, the dye must be installed in the vehicle's A/C system.

If a component or line is leaking or defective, you'll once again need to recover the remaining refrigerant from the system. Replace the defective component and the accumulator/dryer, then put the system into a vacuum for approximately 20 minutes to boil out any moisture. Finally, recharge the system with the factory specified amount of refrigerant. Any oil that was lost should be re-added during the recharge process. Check the A/C system's operation and performance.

Gauge Readings: low side—35 psi (normal) to 45 (slightly high)/high side—212 (normal) to 250 (slightly high)

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Symptoms: The air coming from the vents is only slightly cool or even warm. Bubbles may or may not be present in the sight glass.

Conclusion: Air or moisture may be present inside the A/C system.

Repair Procedure: When the system was recharged, it may not have put into a vacuum long enough to remove the air or boil out the moisture. It is also possible the accumulator/dryer was not replaced after the repair and has reached its moisture-absorbing capacity.

After any repair is performed, the air and moisture will need to be removed. To do this, you'll need to replace the accumulator/dryer and evacuate the A/C system, recovering any remaining refrigerant. Put the system into a vacuum for approximately 20 minutes to boil out any moisture, then recharge the system with the factory specified amount of refrigerant. Any oil that was lost should be re-added during the recharge process. Check the system's operation and performance.

Gauge Readings: low side—0 psi or vacuum (low)/high side—350+ psi (high)

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Symptoms: The air coming from the vents is only slightly cool.

Conclusion: The expansion valve could be stuck in the closed position, or the screen on the orifice tube could be clogged with debris. Check the expansion valve or orifice tube area for frosting. This area should be “sweating” condensation when the system is operating correctly.

Repair Procedure: Recover the refrigerant from the system, remove the expansion valve or orifice tube, and inspect for debris. If debris is present and the contamination is metallic, it could be coming from a failing compressor. If the contamination is black, it could be coming from deteriorating rubber hoses or desiccant from the accumulator/dryer.

If a component or line is found to be defective, replace the defective component and the accumulator/dryer. Flushing the air conditioning system can help clean blockages by dislodging sludge and debris and purging it from the system. After flushing, put the system into a vacuum for approximately 20 minutes to boil out any moisture. Then recharge the system with the factory-specified amount of refrigerant. Any oil that was lost should be re-added during the recharge process. Check the A/C system's operation and performance.

Gauge Readings: low side—55 psi (high)/high side—212 psi (normal)

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Symptoms: The air from the vents is only slightly cool or warm.

Conclusion: The expansion valve could be stuck in the open position, or the capillary tube might not be making proper contact with the evaporator.

Repair Procedure: If your vehicle is equipped with a capillary tube, check to ensure that it's making contact with the evaporator. If it is, recover the remaining refrigerant from the system, and replace the expansion valve and accumulator/dryer.

Flushing the A/C system can help clean blockages by dislodging sludge and debris and purging it from the system. After flushing, put the system into a vacuum for approximately 20 minutes to boil out any moisture. Then recharge the system with the factory-specified amount of refrigerant. Any oil that was lost should be re-added during the recharge process. Check the A/C system's operation and performance.

Gauge Readings: low side—0 psi or vacuum (low)/high side—212 to 250 psi (normal)

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Symptoms: The air from the vents is slightly cool. Look for frost or sweat on the high-side hoses. Remember, sweat is normal on the low-side hoses, but not the high-side ones.

Conclusion: There is likely a restriction in the high side of the system. Frost or sweat will be most prominent just after the point of restriction. The hose or line may be cool to the touch near the restriction.

Repair Procedure: If a component or line is found to be defective, replace the defective component and the accumulator/dryer. Flushing the A/C system can help clean blockages by dislodging sludge and debris and purging it from the system. After flushing, put the system into a vacuum for approximately 20 minutes to boil out any moisture. Then recharge the system with the factory-specified amount of refrigerant. Any oil that was lost should be re-added during the recharge process. Check the A/C system's operation and performance.

Gauge Readings: low side—55 to 65 psi (high)/high side—100 to 120 psi (low)

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Symptoms: The air from the vents is warm. The compressor may be noisy when it operates.

Conclusion: An internal component of the compressor has likely failed. Also check to make sure the drive belt is tight and not worn. A slipping drive belt could cause a false reading.

Repair Procedure: Recover the remaining refrigerant from the system and replace the compressor. You should always replace the expansion valve and accumulator/dryer when a compressor fails.

Flushing the A/C system can help clean blockages by dislodging sludge and debris and purging it from the system. After flushing, put the system into a vacuum for approximately 20 minutes to boil out any moisture. Then recharge the system with the factory-specified amount of refrigerant. Any oil that was lost should be re-added during the recharge process. Check the A/C system's operation and performance.

Gauge Readings: low side—55 to 65 psi (high)/high side—310 to 350 psi (high)

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Symptoms: The air from the vents is warm. The high pressure hoses are very hot.

Conclusion: The condenser is likely malfunctioning. It's also possible that there is an overcharge of refrigerant inside the system.

Repair Procedure: The best way to ensure you have the correct charge in the system is to recover the remaining refrigerant. Put the system into a vacuum for approximately 20 minutes to boil out any moisture, then recharge it with the factory-specified amount of refrigerant. Any oil that was lost should be re-added during the recharge process. Check the A/C system's operation and performance.

If both gauges still read high, verify that the engine-cooling system is full and operating correctly. Also verify that the engine-cooling fans are functioning properly. Electric cooling fans should continually run when the engine is on and the air conditioning is engaged.

Inspect the coil of the condenser and radiator for debris on the external fins. If debris is found, clean and retest the A/C system.

It's easy for modern condensers to become internally clogged with debris, reducing cooling performance. Condensers adopted a more efficient design when A/C systems switched from R-12 refrigerant to R134a. These newer condensers are more likely to fail, due to their multi-path configuration. If you have a failed compressor and find metal contamination in your system, it is highly recommended that you also replace your condenser.

If a condenser is found to be defective, replace it along with the expansion valve or orifice tube and the accumulator/dryer. Flushing the A/C system can help clean blockages by dislodging sludge and debris and purging it from the system. After flushing, put the system into a vacuum for approximately 20 minutes to boil out any moisture. Then recharge the system with the factory-specified amount of refrigerant. Any oil that was lost should be re-added during the recharge process. Check the A/C system's operation and performance.

Common A/C Terms and Diagnoses

Compressor-seal leak
Inspect the compressor, clutch, and surrounding areas for signs of oil and dirt. The clutch can sling oil onto surrounding areas if there is a leak.

Refrigerant-hose leak
Inspect the hoses for oily spots.

Evaporator drain oily
Look for oil near the evaporator drain.

Condenser-fan operation
If your vehicle is equipped with an electric cooling fan, verify that the fan runs continually when the engine is on and the A/C compressor is engaged.

Fan-clutch operation
Check the front of the engine-fan clutch for a buildup of grease on the thermostatic coil. This is an indicator that the fan clutch may be defective.

Is the compressor coming on?
Verify that none of the fuses are blown. If the fuses are good, check for 12 volts and a good ground at the compressor. If the proper voltage and ground are present and the compressor still won’t come, on suspect a compressor-coil problem.

Clutch-cycling switches
All A/C systems have some method to keep the evaporator from freezing up. These include a thermostatic switch (cycles the clutch when the evaporator temperature falls to a set point, usually just below 32 degrees), a pressure-cycling switch (usually mounts on the accumulator/dryer and cycles the compressor when the suction pressure drops to approximately 25 psi), and a variable-displacement compressor (changes its displacement in order to control evaporator temperature).

As you can see, diagnostics are an important first step in A/C-system servicing. Correctly diagnosing your problem early on will save you time and money in the long haul. Good luck, and stay cool.

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