Rick's Free Auto Repair Advice

Diagnose car AC with gauges

How to diagnose your car AC with gauges

Using a manifold gauge set to diagnose car AC issues

You’ll need manifold gauge set if you want to properly diagnose your car AC with gauges. You can’t really diagnose with just a low pressure gauge. You’ll also need temperature probes and a multimeter that accepts K-style fittings. For more information about conducting tests using temperature probes, see this post. However, in this post I’ll concentrate on understanding pressure readings to diagnose car AC issues.

You can’t really diagnose charge level in your car AC with gauges using static pressure

Many sources tell you to start your charge diagnosis by checking static pressure. It’s a good starting point, but you should never rely on static pressure readings as the sole indication of system charge. Measure static pressure with the engine off and AC not run for at least one hour. That allows the high and low pressures to equalize.

But static pressure readings can only tell whether the system is completely empty or has some refrigerant in it. It can never tell you whether the charge is correct! Let’s look as some examples to see how static pressure can fool you.

static AC pressure

3-ounce partial charge at 70° shows the SAME pressure as a fully charged system

 

 

 

 

 

 

 

 

 

 

 

 

What’s going on? Well, R-134a boils at -15.34°F. At 70°F the 3-ounces have boiled and turned into a gas, generating the same amount of pressure as a fully charged system at the same temperature.

The ONLY thing a static pressure reading can tell you is if there is enough pressure in the system to satisfy the low pressure switch and allow the compressor clutch to engage. Most low pressure switches require at leas 20-psi before they’ll engage the compressor clutch. If your static pressure shows more than that, the compressor clutch should engage. If it doesn’t there’s another problem. If there’s NO refrigerant in the system. If the high and low gauges read 0, the system is empty.

What the AC gauges can tell you

Let’s get this straight from the get-go, gauges can tell you if the system pressures are in a normal range, over-pressurized, or under-pressurized, but they CANNOT tell you if the system has the correct charge. The ONLY way to know if your system is properly charged is to evacuate it and add new refrigerant with a scale. In other words, if your system is low on charge, you can’t use your gauge set to add refrigerant to get to factory specified fill capacity. The same applies if your system is overcharged. You can’t recover refrigerant through the gauges to get down to the factory specified fill capacity.

But gauges can give your a rough idea of whats might be going on with your system when it’s running. Let’s take a look at some scenarios.

What you have to know about your system before using AC gauges

• You must know ambient air temperature.

Measure temperature about 1-ft in front of the grille. This is really important. Don’t go by the temperature from a news or weather service. It must be the temperature of the air coming into your car’s condenser.

• You must know the type of system in your vehicle — orifice tube

AC orifice tube or expansion valve

Click on the image to find which type of system is used on your vehicle.

or expansion valve. To find out what in your vehicle, click on the image to the right and find your vehicle.

• You must know the type of compressor in your vehicle. For example, you might see a low high side pressure and determine the compressor is bad, when in fact, your vehicle is equipped with a variable displacement compressor.

• You must use a pressure temperature chart or be using diagnostic gauges

Step 1: Find out what type of refrigerant metering device your car uses

Click on the image above and find the type of refrigerant metering device your vehicle uses. What is this so important? Because expansion valve systems usually show show a lower suction side pressure than an orifice tube system. If you don’t know which system you have, you can misinterpret the low side readings right off the bat and think your system is undercharged. See the pressure chart below to see what I mean.

R-134a TEMPERATURE PRESSURE CHART FOR ORIFICE TUBE SYSTEM — ENGINE AND AC RUNNING

Ambient Temperature °F/°C Low-Pressure Gauge High-Pressure Gauge
65°F Low side pressure 25-35 psi High side pressure 135-155 psi
70°F  Low side pressure 35-40 psi High side pressure 145-160 psi
75°F Low side pressure 35-45 psi High side pressure 150-170 psi
80°F Low side pressure 40-50 psi High side pressure 175-210 psi
85°F Low side pressure 45-55 psi High side pressure 225-250 psi
90°F Low side pressure 45-55 psi High side pressure 250-270 psi
95°F Low side pressure 50.55 psi High side pressure 275.300 psi
100°F Low side pressure 50-55 psi High side pressure 315-325 psi
105°F Low side pressure 50-55 psi High side pressure 330-335 psi
110°F Low side pressure 50.55 psi High side pressure 340.345 psi
Ambient temp is the outside atmospheric temperature.

R-134a TEMPERATURE PRESSURE CHART FOR EXPANSION VALVE SYSTEM— ENGINE AND AC RUNNING

Ambient Temperature °F/°C Low-Pressure Gauge High-Pressure Gauge
60°F Low side pressure 7-15 psi High side pressure 120-170 psi
70°F  Low side pressure 7-15 psi High side pressure 150-250 psi
75°F Low side pressure 35-45 psi High side pressure 150-170 psi
80°F Low side pressure 40-50 psi High side pressure 175-210 psi
85°F Low side pressure 45-55 psi High side pressure 225-250 psi
90°F Low side pressure 45-55 psi High side pressure 250-270 psi
95°F Low side pressure 50.55 psi High side pressure 275.300 psi
100°F Low side pressure 50-55 psi High side pressure 315-325 psi
105°F Low side pressure 50-55 psi High side pressure 330-335 psi
110°F Low side pressure 50.55 psi High side pressure 340.345 ps

Step 2: Connect the gauges to your car’s AC ports

connect AC manifold gauge

Unscrew the protective port caps and store in a safe place. Connect the quick release connectors to the service ports on the high and low side refrigerant hoses. Some low pressure ports are located on the accumulator, while others are located on the metal portion of the lines. High pressure ports can be located on the receiver/drier or near the condenser

Step 3: Measure ambient temperature

Then use the chart below to find “normal” pressures for that temperature.

R-134a TEMPERATURE PRESSURE CHART FOR ORIFICE TUBE SYSTEM WHILE RUNNING

Ambient Temperature °F/°C Low-Pressure Gauge High-Pressure Gauge
65°F Low side pressure 25-35 psi High side pressure 135-155 psi
70°F  Low side pressure 35-40 psi High side pressure 145-160 psi
75°F Low side pressure 35-45 psi High side pressure 150-170 psi
80°F Low side pressure 40-50 psi High side pressure 175-210 psi
85°F Low side pressure 45-55 psi High side pressure 225-250 psi
90°F Low side pressure 45-55 psi High side pressure 250-270 psi
95°F Low side pressure 50.55 psi High side pressure 275.300 psi
100°F Low side pressure 50-55 psi High side pressure 315-325 psi
105°F Low side pressure 50-55 psi High side pressure 330-335 psi
110°F Low side pressure 50.55 psi High side pressure 340.345 psi
Ambient temp is the outside atmospheric temperature.

R-134a TEMPERATURE PRESSURE CHART FOR EXPANSION VALVE SYSTEM

Ambient Temperature °F/°C Low-Pressure Gauge High-Pressure Gauge
60°F Low side pressure 7-15 psi High side pressure 120-170 psi
70°F  Low side pressure 7-15 psi High side pressure 150-250 psi
75°F Low side pressure 35-45 psi High side pressure 150-170 psi
80°F Low side pressure 40-50 psi High side pressure 175-210 psi
85°F Low side pressure 45-55 psi High side pressure 225-250 psi
90°F Low side pressure 45-55 psi High side pressure 250-270 psi
95°F Low side pressure 50.55 psi High side pressure 275.300 psi
100°F Low side pressure 50-55 psi High side pressure 315-325 psi
105°F Low side pressure 50-55 psi High side pressure 330-335 psi
110°F Low side pressure 50.55 psi High side pressure 340.345 ps

Step 3: Read the car AC gauges

With the engine running and AC on and an ambient temperature of 75°F, a typical normal reading is around 35/150

AC gauges normal pressure

©, 2018 Rick Muscoplat

AC system low on charge

AC manifold gauges show system is low on charge

 

 

 

 

 

 

 

Here the gauges show lower than normal pressures on both the high and low sides. This is an indication of a low charge

 

134a TEMPERATURE STATIC PRESSURE CHART

Ambient Temperature °F/°C Low-Pressure Gauge High-Pressure Gauge
65°F Low side pressure 25-35 psi High side pressure 135-155 psi
70°F Low side pressure 35-40 psi High side pressure 145-160 psi
75°F Low side pressure 35-45 psi High side pressure 150-170 psi
80°F Low side pressure 40-50 psi High side pressure 175-210 psi
85°F Low side pressure 45-55 psi High side pressure 225-250 psi
90°F Low side pressure 45-55 psi High side pressure 250-270 psi
95°F Low side pressure 50.55 psi High side pressure 275.300 psi
100°F Low side pressure 50-55 psi High side pressure 315-325 psi
105°F Low side pressure 50-55 psi High side pressure 330-335 psi
110°F Low side pressure 50.55 psi High side pressure 340.345 psi
Ambient temp is the outside atmospheric temperature.

There is a DIRECT relationship between ambient temperature at your location and the pressures you see on your gauges. For example: with the engine off and AC not run for at least an hour, a 70° your readings should be 35-40 psi on the blue gauge and around 145-160

Posted on by Rick Muscoplat



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