Rick's Free Auto Repair Advice

Auto AC repair and diagnostics — Step by step approach

Auto AC repair and diagnostics — DIY step by step approach

Diagnose and repair your Auto AC using a step by step approach

You can diagnose and fix your auto AC yourself, but you’ll need the right tools and a step-by-step approach.

Tools you need to fully diagnose and fix your car’s AC system

• Manifold gauge set
• Probe thermometer
• Multi-meter
• Thermocouple thermometers to fit your multi-meter

AC diagnostic tools

AC diagnostic tools

WARNING: Working on AC can be dangerous. Refrigerant can cause frostbite and blindness if it gets into your eyes. Always wear leather gloves with connecting and disconnecting gauges and charging equipment. Wear eye protection at all times when working with refrigerant.

Disclaimer: The advice given here is my advice. Use at your own risk. If any of these steps conflict with your shop manual or the carmaker’s warnings, follow those instructions/warnings, not mine.

Let’s address the most common AC myths and falsehoods first

There are lots of myths, falsehoods and just plain misunderstandings about how AC works. If we don’t get those covered up front, they can lead you down a rabbit hole and prevent you from thinking logically about your AC.

AC Myth #1 If your DIY kit gauge pressure in the green zone that means your AC has the correct refrigerant charge

If you’re starting from the premise that your AC is full of refrigerant because the gauge on your DIY AC kit shows green, we’re already off to a bad start. When it comes to diagnosing an AC system pressure does not equal quantity. The concept of pressure not equating to quantity for AC work is really no different than the fact that your blood pressure doesn’t tell you how much blood is in your body.  The gauge on you kit can be in the green zone and the system can still be low on refrigerant.

AC system charge is ALWAYS determined by weight, not pressure

AC misunderstanding #2 — if the system is low on refrigerant, you can just refill it

It’s a sealed system. If it’s low on refrigerant it’s because you have a leak. That means you’ve lost refrigerant AND oil. It also means you now have air in the system. Air in an AC system reduces cooling and causes higher pressures that can shut off the compressor. Over time, the moisture in that air reacts with the oil and refrigerant to form acids and sludge that causes compressor failure.

If your AC system isn’t working properly and the tests you conduct below lead you to the conclusion that you have air in the system, you’ll have to get the system evacuated so you can fix the leak before you recharge it.

AC myth #3 — A recharge kit with stop leak sealer can fix your AC system

Most AC leaks are due to aging O-ring seals, not pinhole leaks in the condenser or evaporator coil. Stop leak rarely works to fix a leaking AC system.

Worse than that however, stop leak contaminates your AC system and damages a shop’s recycling machines. If you ever want to take it to a shop for an AC repair, their equipment will detect the sealer and you’ll be charged extra to remove the contaminated refrigerant. NEVER use refrigerant with stop leak.

AC myth #4 — more refrigerant provides better cooling

False. R-134a systems have just a 2-oz. charge window. If you add more than 2-oz. you’ll actually reduce cooling. If the charge level is 2 or more ounces below the recommended level, you also won’t get good cooling.

AC myth #5 — You can vent your vehicle’s refrigerant to the atmosphere so you can replace AC components yourself.

First, it’s illegal to vent refrigerant to the atmosphere. But there’s an equally important reason not to do this. The refrigerant in your AC system carries oil that lubricates the compressor. When you vent refrigerant to the atmosphere, you’re also venting the oil. So you’ll never know how much oil you’ve lost during the venting process. That means you’ll never know how much fresh oil to add back in.

When a shop evacuates an AC system, their equipment separates the oil from the refrigerant so they can measure how much came out. That’s how they know how much to add back in.

If you think the oil issue isn’t important, think again. If you add too much oil during the recharge, you’re dramatically reduce the system’s cooling ability. If you add too little oil, you’re burn up the compressor. Addi

Step 1 to diagnosing your car’s AC system

Always start with these preliminary AC checks

1) Check the AC condenser fan operation and condenser fins for blockage

Airflow across the condenser is critical to proper AC operation. That means the condenser fins must be clean and the AC fans must work properly.

The condenser fan should be on any time the AC is on. Some condenser fans are located in front of the condenser, while others are located on the engine side of the radiator. Some vehicles use a single multi-speed fan that runs on maximum speed when the AC is on. In those, seeing the fan move doesn’t prove that it can run at high speed.

If the condenser fan isn’t working, your car AC will never work properly. The most common symptom of a failed condenser fan is: you get cold air when driving, but hot air when idling. That’s because there’s no airflow across the condenser. That causes pressures to rise above the high limit and the high limit switch shuts off the compressor.

Also check for bugs and debris on the face of the condenser. Clogged condenser fins will degrade cooling.

2) Check the cabin air filter

A dirty cabin air filter restricts airflow and that can cause evaporator freeze up. They symptom of evaporator freeze up is: cold air for a while, then warm air with dramatically reduced airflow. After parking, you find a large water puddle under the vehicle. That’s the water from the melted ice. Your car’s AC system must have the proper airflow for the best performance.

3) Check the blend door and actuator operation (heater valve in some applications)

The blend or “air mix” door controls the air temperature in the cabin. If the door is stuck or the actuator isn’t working properly, the AC will cool the air and then the heater core will heat it. That can cause you to mistake a blend door problem for an AC performance issue.

To check the blend door operation or heater valve operation, move the temperature dial to full hot and then to full cold (engine running and AC off) . You should notice a rapid change in air temperature. If you don’t, address the blend door or heater valve issue first before suspecting an AC problem.

4) Check compressor clutch operation

Turn the AC to MAX and start the engine. The center clutch portion of a clutch-type AC compressor should be spinning. If not, see this post to diagnose a compressor clutch won’t engage problem.

Many late model car AC compressors don’t have a clutch; they spin the entire time the engine is running. These variable displacement compressors modulate refrigerant flow through variable piston displacement and a flow control valve. To see how they work and learn how to identify a clutch-less style compressor, see this post.

Step 2 Test the AC system’s static pressure

Static pressure tells you whether your car’s AC system has any refrigerant and whether the pressure is high enough to activate the compressor clutch.  The test is performed with with a cold engine that hasn’t run for at least one hour. NOTE: The test is done using ambient air temperature. If your engine is hot, that heat will skew your results. If you’re testing after running the engine, factor in how much the engine has heated the refrigerant.

You’ll use your manifold gauge set and your probe thermometer for this test.

1. Record the ambient air temperature near the condenser coil in front of the radiator. (Do not use the weather service temp. It must be the temp in front of the condenser!)
2. Connect the manifold gauge hoses to the high and low pressure ports.
3. Read the pressures for the high and low sides. They should read the same pressure. If not, you’ve got a more serious problem which we’ll cover later.
4. Compare the pressure readings to the static pressure charts below based on the type of refrigerant used in your vehicle (R-134a or R-1234yf).

R-134a pressure temperature chart 2

r1234yf static temp pressure chart

If the static pressure in your vehicle is below the value shown in the chart, the system is seriously low on refrigerant. If it’s higher, the system is overcharged, has air in the system or the refrigerant is contaminated.

If the gauges aren’t the same during a static pressure test

1) You haven’t waited long enough for the pressures to equalize, or
2) The orifice tube, expansion valve, compressor reed valves are stuck or there’s a severe restriction somewhere in the system that’s preventing the pressure from equalizing. In that case, you must fix that problem first.

What the numbers mean

If the static pressure is within a few degrees of the chart

• You have enough pressure to operate the low pressure cutoff switch that switches power to the compressor clutch or operates the variable displacement device in a clutch-less compressor.
• You have enough refrigerant to engage the compressor clutch. But that doesn’t mean you have enough refrigerant to get good cooling.

If the static pressure is too low

• The system has a leak and is low on refrigerant
• The low pressure may prevent the low pressure switch from energizing the compressor clutch or the variable displacement mechanism (see low pressure switch cutoff pressures below).

If the static pressure is too high

• The system is overcharged
• The system has air inside due to a leak
• The system is contaminated with different refrigerants

If the system is overcharged, has air in the system or is contaminated, take it to a shop for a complete evacuation. DO NOT VENT THE EXCESS REFRIGERANT TO THE ATMOSPHERE.

Minimum static pressures needed to activate the low pressure switch

Each carmaker has their own specs for the low pressure switch. Here are some typical lower pressure switch values.

Allows compressor clutch operation at at 34-psi, prevents compressor clutch operation at less than 10–psi
Allows compressor clutch operation at 47-psi, prevents compressor clutch operation at less than 25–psi
Allows compressor clutch operation at 40-psi, prevents compressor clutch operation at less than 32–psi
Allows compressor clutch operation at 34-psi, prevents compressor clutch operation at less than 8–psi
Allows compressor clutch operation at 47-psi, prevents compressor clutch operation at less than 25–psi

If the static pressure is correct, but the compressor clutch doesn’t engage, check the low pressure switch,  compressor clutch fuse and relay, clutch air gap and clutch coil.

If they’re good.

Adjust compressor clutch air gap

What to do before conducting a running pressure test

If static pressure is too low to operate the low pressure switch, you must add refrigerant in order to operate the compressor. Add just enough refrigerant to get static pressure to where it should be based on the charts.

If static pressure is too high, STOP and take it to a shop. It is illegal to vent excess refrigerant into the atmosphere.

Step 3 Start the AC diagnosis by testing running pressures.

Set the AC set to MAX and blower on high. Set the mode to recirculate and vents to dash. Close all the windows. Place the probe thermometer in the center air duct. Start the engine and let the AC run for 5 minutes.

Record the high and low pressure readings
Record the temperature from the center duct
Record the ambient air temperature

Normal AC gauge pressures when the system is running

Proper running AC gauge pressure and center duct readings are dependent on outside temperature and humidity. As a very general rule, expect around 27-psi on the low side and 200 on the high side. The center duct dash thermometer temp should be below 40°F

In a properly operating R-134a AC system, a 27-psi. low side pressure should result in about 32°F at the evaporator (as long as the orifice tube/expansion valve is operating properly and there’s no air in the system that’s artificially inflating the pressure to 27-psi).

High side pressures usually run 2.2 to 2.5 times the ambient temperature entering the condenser. So, if the ambient temperature is 80°F, you’ll see high side pressures running between 176-psi and 200-psi. on an R-134a system. At 200-psi. the refrigerant entering the condenser will be around 130°F.

car AC high side pressure readings
car AC low side pressure chart
However, humidity also affects pressure and temperature readings

Humidity decreases heat transfer so “normal” pressures will be higher when the humidity is high. See the chart below.

AC pressure temperature chart
It your AC pressure readings are off
AC pressure chart

Step 4 — If the pressures are off, conduct a heat load test on your car’s AC

For an expansion valve system, conduct a superheat heat load test
For orifice tube systems, conduct a maximum heat load test

Step 5 — Evacuate the system and make the repair based on the test results

Rent or buy a vacuum pump and pull a vacuum on the system. Replace the expansion valve, orifice tube or condenser as indicated by the results of the tests. Add the proper refrigerant oil based on the carmakers recommendations.

Step 6 — Recharge the AC system using a refrigerant scale

The refrigerant charge is based on weight, not pressure. Rent a refrigerant scale and refill the system. Over or under-charging an R-134a by as little/much as 2-oz can dramatically decrease performance.

For all AC related articles, go to this page

For refrigerant capacities and oil specs, go to this page.

©, 2022 Rick Muscoplat

Posted on by Rick Muscoplat



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