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Auto Air Conditioning Repair 101
An A/C that is not blowing “cold air” is annoying and worst can be very expensive to repair. This article discusses techniques and strategies written in laymen’s language you can use to fix common A/C problems yourself and save MONEY on professional repairs.
Air conditioning is the process in which air inside the passenger compartment is cooled, dried, and circulated. Heat is removed from inside the vehicle and transferred to the outside air.
All air conditioners, whether it is an auto A/C. household refrigerator, or home HVAC work on the same principles A liquid refrigerant is changed to a gas and then back to a liquid. If a change-of-state of the refrigerant is to take place, heat transfer must take place. The two (2) rules that apply to refrigerant are:
1. Refrigerant in a gaseous state collects, absorbs, and holds heat.
2. Refrigerant in a liquid state releases that heat.
For your auto A/C to blow “cold air”, R134A refrigerant must pass through and change state in three (3) components, and one (1) receiver-dryer, and one (1) expansion valve that makes up the closed air conditioning system. The components of an auto A/C system are:
1. Compressor - A device that pressurizes the heated refrigerant.
2. Condenser - A radiator that transfers the heat that was absorbed in the passenger compartment to the cooler air.
3. Evaporator - A small radiator that is located under dash in the passenger compartment. Liquid refrigerant absorbs heat from the air blowing across the evaporator. It then boils and changes state to a vapor before it enters the suction port of the compressor.
4. Receiver-dryer - A canister that stores the liquid refrigerant when the system has cycled and the compressor is not “ON”. It contains a desiccant that removed moisture from the system.
5. Expansion valve - A metering device that controls the amount of refrigerant entering the evaporator.
A/C Quick Check
Check for obstructions that are lodged in the condenser fins. With the engine running, turn the A/C control to “ON: and the blower on “HIGH”, Warm the engine until the A/C system pressures stabilize. Raise the hood and locate the large tubing connected to and routed from the compressor (low-side” to the expansion valve (inlet) to the evaporator. Locate the small tubing connected to the “low-side” of the compressor. It is routed from the discharge side of the compressor to the “outlet-side” of the evaporator. With the engine running and the A/C system “ON” “feeling” the two (2) lines should have the below results:
- The low-side tubing should feel “cool” to the touch.
- The high-side tubing should feel “warm” to the touch.
If the high side tubing is not warm and the low-side is not “cool”, further tests will have to be made as there is an internal problem in your A/C system.
Testing the A/C System
Before system temperature tests can be made, a checklist should be followed below for testing the A/C system:
- Set the A/C “ON-OFF” switch to the “ON” position.
- Set the temperature control to “maximum cooling”.
- Set the blower on “high” or the highest number on the control switch.
- Temperature inside the passenger compartment should be getting cooler.
- Engine speed should be a minimum of 1500rpm’s.
- All windows should be in the “UP” position.
Compact automobiles and trucks have a small capacity A/C system and a loss of cooling would be more noticeable than a larger capacity A/C system. If a loss of “cooling” capacity is noticed, a temperature check at the vent registers should be made. This check can be made with an instant read or digital temperature thermometer. The discharge air from the vents in the passenger compartment should range from 38 to 42 degrees F.
A/C System Diagnosis
Assuming that the discharge air test measured at the vent registers is out-of-range, a system pressure test will have to be made. Professional A/C technicians connect a pressure tester known as the manifold gauge set to the “low” and “high-side service valves of the system. The manifold gauges set is your “eyes” and for you to look inside you A/C system you will have to learn and interpret a temperature/pressure chart. Secondly, you will have to learn to interpret the readings on the dials of the “low-side” and “high-side” gauges of the manifold gauge set.
A manifold gauge set suitable for your needs can be ordered on Internet A/C parts and supply store fronts. They can be purchased at most auto supply stores for less than $50.00. Look for a set that has a sight glass built-in the manifold of the gauge set.
Locate the “low” and “high-side service valves on your car or truck A/C system. Connect the hoses to the service valves (they are a snap fit) in the following manner:
- Connect the blue hose of the gauge set to the “low-side service valve from the compressor.
- Connect the red hose of the gauge set to the “high-side” service valve from the compressor.
- Connect the yellow hose of the gauge set to a vacuum pump or refrigerant tank to evacuate or add refrigerant to the A/C system. The yellow hose is not connected to the system at this time.
If you find an out-of-range temperature reading, the manifold gauge set should be installed on the “low-side” and “high-side” service valves. With the engine “OFF”, the blue (“low-side”) and red (“high-side”) gauges should show equal readings of approximately 80 to 120psi. These readings on both gauges would indicate that there is refrigerant in the A/C system.
For example, if the readings were 50psi on each gauge, this would indicate that there is refrigerant in the system. However, there is a LOW CHARGE.
In another example, if the readings on each gauge were 10 to 20psi would indicate there is basically no refrigerant in your car or truck A/C system.
Engine Running Test
With the engine running, turn the A/C switch control to the “ON” position. Turn the blower switch to “high” and run for fifteen (15) minutes. Observe the readings on the manifold gauge set.
A manifold gauge set with a built-in sight glass will save you diagnostic time. Bubbles from refrigerant oil and refrigerant would tell you that the compressor is functional. Refrigerant is being moved from the “low-side” to the “high-side” of the system in the refrigeration cycle.
System Full Charge
With the engine running, turn the A/C switch control to the “ON” position. Turn the blower switch to the “high” position setting. With the manifold gauges installed, a fully charged A/C system would show approximately 35psi on the “low-side” gauge and a pressure reading of 225 to 240psi on the “high-side” gauge. This reading would be on a 90-degree F day.
These readings would indicate that the compressor is functional, clutch is engaged, and refrigerant is being moved from the “low-side” to the “high-side” of the system in the refrigeration cycle.
System Low Charge
With the engine running, turn the A/C switch control to the “ON” position. Turn the blower switch to the “high” position setting. With the manifold gauge set installed, a “low-side” pressure reading of 20psi on the low-side gauge and 150psi on the “high-side” gauge would indicate there is a LOW CHARGE in the A/C system. The low reading on the “high-side” gauge (150psi) would tell you that the A/C system is doing very little work in the refrigeration cycle.
A LOW CHARGE usually is an indicator that there is a leak from a component in the “low-side” and “high-side” of the A/C system.
A refrigerant leak would have to be repaired for your car or truck A/C system to blow “cold air”.
REFRIGERANT DOES NOT WEAR OUT IT LEAKS OUT
Some symptoms to look for in you car or truck A/C system that has been diagnosed with a LOW CHARGE is listed below:
- Compressor clutch short cycling.
- Clutch will not engage.
- Little or no cooling.
- Oily residue on hoses and components.
An A/C system that is not “cooling”, in most cases, has a leak in the system. Refrigerant leaks will have to be found and repaired. Some leaks can be easily spotted and other leaks can be caused from a loose clamp on a hose.
However, most leaks in an A/C system have to found with a UV leak detector. Some common targets to look for leaks are listed below:
- Schrader valves are used on the “low-side” and “high-side” of the A/C system. The red and blue and yellow hoses of the manifold gauge set are attached to these valves to enter the system. The valves use a valve core that is screwed into the valve. When a leak has been found, a valve core removal tool to use to remove and replace a new valve core. This is the first area that a technician should look for a leak in the A/C system.
- Oily deposits on refrigerant hoses attached to the compressor and other components should be checked for leaks with a UV leak detector.
- Loose bolts securing the hoses to the compressor suction and discharge ports should be checked for leaks with a UV leak detector.
When tightening the bolts to the suction and discharge ports of the compressor with hand tools, do not over tighten. O-rings are used for a leak free connection. Only tighten snug.
When your can or truck A/C system has a LOW CHARGE, a refrigerant charge will have to be added to the system to check for a leak. R134A refrigerant that contains a dye is added to the system for this purpose. The manifold gauge set, and a can tap valve will be needed to add refrigerant to the system to check for a leak in the A/C system.
Connect the hoses to the manifold gauge set as listed in the below procedure:
- Connect the yellow hose to the can tap valve and front seat (turn clockwise) the valve.
- Front seat (turn clockwise) the hand wheel on the blue gauge of the manifold gauge set.
- Back seat (turn counterclockwise) the valve on the can tap valve.
- Slowly back seat (turn counterclockwise) the hand wheel on the blue gauge of the manifold gauge set. The refrigerant will flow into the A/C system. Continue adding refrigerant until the blue gauge (low-side) shows a reading of 60psi.
A battery-powered UV leak detector and UV glasses can be ordered from an Internet A/C Tool supply store front. It can also be purchased at most auto supply stores for less than $50.00 that is suitable for you to leak check your car or truck A/C system.
The UV glasses should be worn when checking for a leak. The tip of the leak detector is placed on a component or hose. When a leak is found a greenish color is seen through the UV glasses.
System Air Flow
An often overlooked cause of your car or truck A/C system not blowing “cold air” is debris clogging the condenser. Bugs, plastic bags, and leaves from city and highway driving will prevent outside air from passing through the fins of the condenser. The condenser should be cleaned with a garden hose and cleaning solvent. Bent fins on the condenser coil should be straightened with a fin comb.
A restriction in your car or truck A/C system is a blockage of air flowing over a component, primarily the condenser. Some form of debris is causing a restriction of refrigerant flow through the condenser tubes. The passages in the condenser tubes of your compact car and truck A/C condenser are very small (6mm or less in diameter). These small passages are a prime candidate of a refrigerant restriction on the “high-side” of the system.
An effective way to diagnose a “high-side” restriction in your A/C system is the manifold gauge set. The “high-side” red gauge would move into the DANGER zone, over 300psi.
When your car or truck A/C system has been opened, moisture has entered the system. To restore the A/C system to working order, that moisture will have to be removed. The tool to remove the moisture is the vacuum pump.
For you to finish the repair, you will have to borrow or buy a vacuum pump. A small two (2) stage vacuum pump that is suitable for removing the moisture from your A/C system can be purchased for less than $100.00 at most auto supply stores.
After replacing a component or opening the system, the moisture that entered the system has to be removed. Install the manifold gauge set to the “low-side” and “high-side” service valves and follow the below steps:
- Connect the blue hose to the “low-side” service valve.
- Connect the red hose to the “high-side” service valve.
- Connect the yellow hose to the suction port of the vacuum pump.
- Front seat (turn clockwise) the hand valve of the vacuum pump.
- Back seat (turn counterclockwise) the hand valves on the blue and red gauges of the manifold gauge set.
- Connect the electrical power to the vacuum pump.
- Back seat (turn counterclockwise) the hand valve of the vacuum pump.
- Run the vacuum pump for three (3) minutes.
- The needle on the blue “low-side” gauge should show to be in a vacuum of 28.3 or higher.
- A system with no leaks will pull a vacuum of 28.3hg in three (3) minutes.
- If he needle on the blue “low-side” gauge did not fall to 28.3hg, the evacuation will have to be aborted as there is a leak in your car or truck A/C system.
- The battery-powered UV leak detector and UV glasses is used to find the leak in the A/C system.
- Once the leak has been found and repaired, repeat the evacuation Steps 1 through 7 as shown above for twenty-five (25) minutes.
- Front seat (turn clockwise) the hand valves of the blue and red gauges. Observe the needle of the blue gauge.
- The needle on the blue gauge (“low-side”) should hold steady at 29hg indicating a tight system.
- Any movement toward “0” on the blue (“low-side”) gauge would indicate a leak in the system. The evacuation process would have to be aborted. The UV leak detector and UV glasses is used to find the leak.
- Look in the owner’s or a service manual for the factory recommended refrigerant charge. Systems range from 16 to 28 ounces of R134-A refrigerant.
- Attach a can tap valve to can of refrigerant. Make sure that the valve on the can tap valve is back seated (turned counterclockwise).
- Front seat (turn clockwise) the valve of the can tap valve.
- Turn the A/C switch control to the “ON” position and the blower switch on “high”.
- Start the engine and run at 1500rpm’s.
- Place the refrigerant can in a pan of hot water. Do not invert (turn up-side down) the refrigerant can as liquid would enter the system and ruin the reed valves in the compressor.
- Back seat (turn counterclockwise) the hand valves on the blue and red gauges of the manifold gauge set.
- Continue this process until all of the recommended factory charge has entered the system (approximately 2 ½ cans).
- After ten (10) minutes, the system should be refrigerating. The system pressures on the blue and red gauges should range from 35psi on the “low-side gauge (blue) and 225 to 240psi on the “high-side” (red) gauge on a 90-degree F day.
- Shut the engine “OFF” and let the A/C system stabilize.
- When the readings on the blue and red gauges are equal, remove the blue (“low-side”) hose from the service valve.
- Restart the engine and remove the red (“high-side”) hose from the service valve.
- Close the hood of you car or truck and take the vehicle for a three (3) test drive.
- Place an instant read or digital temperature thermometer in the vent register in the passenger compartment.
- The temperature should range from 38 to 42 degrees on a 90-degree F day.
Tools and Materials
- Manifold gauge set
- Vacuum pump
- Hand tools
- Battery-powered UV leak detector
- UV glasses
- Can tap valve
- R134A refrigerant with dye
- R134A refrigerant
- Instant read temperature thermometer
- Digital temperature thermometer