Thursday, February 23, 2017 Detailed Auto Topics
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Detailed Topics

They refer to the gas in an air conditioner as refrigerant. Over time, refrigerant may leak, and low refrigerant is one cause for an air conditioner to stop cooling. Many other things also cause the system to not cool. Without diagnosing the real problem, adding more refrigerant may destroy the compressor and worse.

What to do when an air conditioner stops cooling

When an air conditioner stops working or does not cools well, we need to learn the problem. Finding the actual cause is not difficult. What we need are a few tools and a little knowledge. To check the refrigerant process, we use a pair of gauges. Understanding refrigerant gauges is straight forward, with a few tips.

Our vehicle air conditioner has two main parts. All problems will be with one or the other, as described in part one of this series. One part is the electrical system and the other is a refrigerant system. These must work together to cool the vehicle. The electric side controls the refrigeration side. We diagnose electrical problems with electrical testing equipment. For problems with the refrigerant-side, we use refrigeration pressure gauges and temperature testing.

The first thing we need is an explanation of how air-conditioning works. Please read the Detailed Topic, How Automotive Air Conditioners Work for a simple explanation. We will need to understand the high and low pressure sides of the system. Reading the pressure relationship between them, is the way we find problems. Briefly, the system has a high-side and a low-side. The relationship of the high-side to the low-side determines cooling. We read these pressures as a guide in finding problems.

air conditioner charge kits can ruin the entire system

Charge kits with only a low side gauge should NEVER be used. Severe damage can be caused by adding refrigerant to a system without knowing the high-side relationship. This is critical on modern, variable-displacement compressors.

Even with both gauges, seeing the correct charge level on these units is impossible. Extracting the system and adding the proper weight of refrigerant is the only way they can be charged.

Paying a professional to test and charge these systems, is far less expensive than to destroy a compressor and evaporator core from an overcharge.

Basic automotive air conditioner pressure testing

air conditioner gauges used to diagnose problems and charge the system

Most automotive refrigerant is today is R134A. Newer vehicles may use R1234YF and this will become more common in the future. Diagnosis of R1234YF systems is different and not covered in this article. Older systems use R12 and while the pressure varies slightly from R134A, the principles are the same. A refrigerant gauge set will have two gauges. The left gauge is marked with blue and used to read the low-side of the system. The gauge on the right is marked with red and reads the high-side of the system. A blue and a red hose are used for attachment to the system. The yellow hose allows a refrigerant to be added and a vacuum to be drawn on the system during service.

Most R134A gauges use quick-connects on the hoses to attach to the appropriate port. These are different from R12 fittings and should not be interchanged. With either system, gauges should only be attached or removed with the engine not running.

Air should be purged from the hoses on the inital use 

When the hoses are attached for the first time, they may be full of air. Purging the lines will remove this air and help keep it out of the system. Connect both hoses to the proper ports and then loosen the other end of the hose at the gauges. The pressure in the system will push any air out of the hose. Quickly tighten the hose and then purge the other line. With quick-connects, the hoses will seal when removed and purging should not be needed again. On systems without an automatic seal, the lines may need to be purged each time they are used.

What is static pressure in the system?

Before the vehicle is started, the static system pressure can be read. Pressure should be very nearly equal in both gauges. Actual pressure will depend on the type refrigerant and the ambient temperature and does not show if the system is properly charged. At 85 degrees Fahrenheit, R134A shows a pressure of around 90 pounds per square inch. Much lower readings suggest the system is low on charge or contaminated with air.  Low static pressure normally shows a leak that must be found and repaired. Finding and repairing leaks are covered in the next Detailed Topic.  At least twenty-five PSI of static pressure is required, or the cycle switch may disable the compressor.

What is normal automotive air conditioner pressure?

average gauge readings with R134A on a warm day

Start the engine and turn on the air conditioner. Actual readings will vary according to ambient temperature. The low-side should be near 30 PSI at 90 degrees Fahrenheit or less. Pressure that is too much lower or higher shows there is a problem.

On a properly working system, high-side pressure will be about twice the ambient temperature, plus 50 PSI. For instance, on a 90-degree Fahrenheit day, twice 90 is 180, plus 50 equals 230 PSI. This is not precise but close enough for the purposes of this article. Both pressures reading within these ranges means the system should produce a vent temperature in the mid, to low forties, with the engine idling.

a cold outlet tube indicates the refrigerant system is working

If the pressures are as above and the temperature of the air from the vents is not low enough, touch the outlet tube from the evaporator. A cold line suggests the problem likely exist under the dash, with the actuators or doors. Often, a temperature door malfunction will allow the cooled air to be reheated.

What does higher than normal air conditioner pressures mean?

insuficient air flow through the condenser raises high side pressure

Low and high-side pressures, higher than normal are very damaging. Air conditioner compressors are designed to operate within a limited range. Higher than normal pressure will distort the aluminum case and very quickly cause compressor failure. High pressure will also cause the coils in the evaporator to leak. This can be very expensive as often the dash must be removed to correct the problem.

Two primary reasons for higher than normal air conditioner pressures include lack of air flow through the condenser and an overcharge of the system. Lack of air flow can result from a bad condenser-fan-motor. A worn fan-clutch or debris blocking air flow through the condenser will cause the same issue.

A condenser fan turning does NOT mean it is moving sufficient air

Many electric fans and clutches operate at variable speeds, ranging from 15% to 100% capacity. A malfunctioning module or a sensor may cause the fan to turn too slowly to cool the condenser. The same applies to a fan-clutch, which operate at many rates. Testing air flow and fan speed is best left to personnel equipped to handle the task.

How does a system get overcharged?

Overcharging an air conditioner may result from adding refrigerant, oil or air remaining in the system. The only way to find an overcharge is to extract the contents and measure them. Overcharging often results from adding additional refrigerant to a system, without knowing the reason it is not cooling.

When any line on an air conditioner system is opened, air enters the system. This can also occur if the refrigerant leaks out, and the system is empty. Charging a system without evacuating, will compress air in the lines. Air occupies space and adding the proper charge results in an overcharge. By applying a vacuum, air is drawn out of the system before charging.

Components such as compressors, condensers and accumulators contain oil. Refrigerant oil circulates in the system and lubricates the compressor. When too little oil exists, the compressor will burn up. This can happen when components are replaced without an accounting for the oil within them. Oil occupies space and randomly adding can result in an overcharged system, after adding the proper quantity of refrigerant.

Old components should be emptied, and the oil content measured so that it may be replaced. Replacement compressors need to be emptied and measured as well. Many new compressors contain more oil than was removed in the old one.

What does it mean when the low-side is high, and the high-side is low?

a weak ac compressor shows as insuficient change from side to side

The compressor lowers the low-side pressure and raises the high-side to remove heat from the vehicle. A damaged compressor will not accomplish as much change in the two sides. When this occurs, cooling will diminish, especially at idle. Raising the engine speed may bring the pressures closer to that needed. Compressors in a clean and lubricated environment do not wear out. If a compressor is bad, the root cause needs to be found or the problem will soon recur. Common causes include an over-pressure system, as described above.

The low-side is way too low and the high-side too high

A restriction in the system will cause a very low reading on the low-side

On a fully charged system, insufficient low-side pressure is normally the result of a restriction. A clogged orifice tube is a common cause of such problems. The high-side pressure may be elevated as the compressor attempts to force the refrigerant past the obstruction. This will quickly starve oil flow to the compressor and cause a catastrophic failure.

With any restriction, the source of the debris must also be considered. Repair and sources of debris in the system are covered in a later Detailed Topic in this series. 

Why would both gauges read low?

Both gauges low may indicate a low system charge

Lower than normal pressure, on both gauges can show a system low on charge. Such conditions can also be caused by a malfunctioning variable displacement compressor. The contents of the system should be evacuated and measured to learn which condition exists. Adding additional charge may cause pressure to rise but may also create overcharge damage.

Any of these conditions may also occur in unison. For instance, a restriction and a weak compressor could occur together. This condition could give varying readings. A good overall inspection, vent temperature measurement and gauge readings will isolate many air conditioner problems. After the problem is fully determined, quality repair of the system is possible.

Please also see:

Part one, electrical problems with air conditioners

Part three, finding and repairing air conditioner leaks

Part four, preventing and repairing compressor failure





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