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

Step 1 Preliminary AC checks

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

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. Still others are built into the radiator fans. In those, the fans immediately go into high speed mode when the AC is on.

If the condenser fan isn’t working, your car AC will never work properly. The most common symptom of a failed condenser fan is: cold air when driving and hot air when stopped at a light.

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 can restrict 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, it can heat the cooled air, causing 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 AC system static pressure

Static pressure tells you whether your car’s AC system has refrigerant and whether the charge level is near where it’s supposed to be. The test is performed with the engine off and cool and when the AC 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 result in a false static pressure reading so let it cool before conducting the test.

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 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.
• The charge level is close to normal.

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 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 are 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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