Rick's Free Auto Repair Advice

AC pressure gauge readings

How to diagnose your car’s AC using pressure gauge readings

You can’t accurately diagnose your car’s AC by using just the low-pressure gauge on a recharge kit because you only see pressure on the system’s suction or low side. That’s not enough information to diagnose a problem. It’s like trying to diagnose your overall health with only half of your blood pressure reading. I’ve listed normal AC pressure gauge readings and abnormal readings and what to do about them.

AC manifold gauge

Manifold gauge set with hoses

Step 1: Measure the static pressure of your AC system

Static pressure is the refrigerant pressure with the system off and equalized (compressor off for at least 30 minutes). This reading tells you whether the system has enough pressure to override the low-pressure cut-off switch so it can enable the compressor clutch.

Find static pressure charts here.

Static pressure only tells you two things

• Whether the system is completely empty
• Whether the system pressure is high enough to enable compressor clutch engagement.

If the static pressure is lower than 27-psi. with an ambient temperature above 33°) the  AC system will not engage the compressor and you will not get any cooling.

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

If the gauges don’t read the same during a static pressure test, it means:

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

Troubleshooting incorrect static pressures

If your static pressure readings don’t match the temp/pressure chart and are too high (most common), these are the likely causes:
• There’s air in the system
• The system is overcharged
• The refrigerant is contaminated

What are normal AC gauge pressures when the system is running?

Generally, you want around 27 psi on the low side and 200 psi on the high side with the compressor running. An R-134a refrigerant pressure of 27 psi on your gauge means the refrigerant will produce about 32°F at the evaporator, which is just around the freezing point of water.

On the high side, R-134a 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.

However, ambient humidity also affects how your car’s AC works because high humidity changes how the heat transfers.

ac temperture pressure chart
For more information on normal AC pressures, see the charts below.

Normal high and low AC ambient temperature pressure listing

Ambient Temperature in °F Low side Pressure Gauge reading and High side Pressure Gauge reading
65°F Ambient temperature: Low side pressure 25-35 psi High side pressure 135-155 psi
70°F Ambient temperature: Low side pressure 35-40 psi High side pressure 145-160 psi
75°F Ambient temperature: Low side pressure 35-45 psi High side pressure 150-170 psi
80°F Ambient temperature: Low side pressure 40-50 psi High side pressure 175-210 psi
85°F Ambient temperature: Low side pressure 45-55 psi High side pressure 225-250 psi
90°F Ambient temperature: Low side pressure 45-55 psi High side pressure 250-270 psi
95°F Ambient temperature: 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 Ambient temperature: Low side pressure 50-55 psi High side pressure 330-335 psi
110°F Ambient temperature: Low side pressure 50.55 psi High side pressure 340.345 psi

NOTE: High humidity and airflow velocity across the evaporator and condenser have a direct effect on pressure readings.

How to test AC pressures with a manifold gauge

1, Set the AC controls to MAX and RECIRCULATE
2. Blower set to HIGH speed
3. Close the doors. Windows can be open.
4. Once you attach your gauges and start your engine, keep RPMs steady at around 1,200 to 1,500 RPM.

Remember that AC pressures and center duct temperatures directly relate to ambient temps and relative humidity.

AC pressure temperature chart

If your readings are outside the typical pressure/temperature range, see below.

AC pressure chart

See this post to learn what type of AC system is in your vehicle.

Low and High AC pressures are nearly the same

If the compressor is running (the center portion of the pulley is turning), it is spinning but not pumping, which indicates a bad compressor. If the center pulley isn’t turning, see this post for diagnostics.

Symptoms of a bad AC compressor

• Low and high side pressures are about the same (like 80-psi low side and 80-psi high side) AND you have to increase RPM quite a bit to get pressures into the normal range, AND you notice the air only gets cold when you’re driving, AND you’ve ruled out a low refrigerant charge or bad orifice tube/expansion valve.

Low and High AC gauge pressures are high with no fluctuation

The system is overcharged, there’s a lack of airflow across the condenser (clogged fins, radiator/condenser fans not working or not working at the proper speed), or you have air and moisture in the system.

What to do if AC pressures are high:

1. Check the front of the AC condenser (it’s in front of the radiator) to make sure the fins aren’t clogged.
2. Check to make sure the radiator fans are working at the proper speeds.
3. Evacuate the system and recharge with the correct refrigerant charge and test again.

Low and High AC pressures are lower than recommended on an orifice tube system

Symptom: Low cooling
Pressure readings: Low side is low. High side normal to slightly low
Causes: Low on refrigerant charge

In this case, the evaporator is starved for refrigerant. In a fixed orifice tube system, you want the evaporator almost completely full of refrigerant. When properly charged, a full evaporator will spill some boiling refrigerant into the accumulator where it will continue to vaporize before entering the compressor.

However, if the system is low on charge, the evaporator is only partially filled with refrigerant, so half of it will be cold and the other half hot. This often causes evaporator icing, where ice builds up on the evaporator. The ice blocks airflow and the low pressure triggers the low-pressure switch.

Low refrigerant charge causes superheat

Keep in mind that AC works when the refrigerant removes just enough heat from the air to cause the refrigerant to change from a liquid to a vapor. However, in a partially filled evaporator, the vapor then absorbs heat. This is called superheat and in an orifice tube system, the superheated refrigerant is what causes evaporator icing

In other words, the low refrigerant charge causes the refrigerant is in the system to sit too long in the evaporator. So the refrigerant picks up too much heat. First, it picks up enough heat to cause the refrigerant to change state from liquid to gas. THEN, the gas absorbs even more heat from the evaporator airflow, causing evaporator temperature to fall below the freezing point of water. Any ambient moisture immediately freezes on the evaporator fins.

Test for evaporator freeze-up

To test for evaporator freezing, turn off the engine and leave the vehicle for a while. When you return, examine the puddle size under the vehicle. A large puddle is an indication of evaporator ice that has melted and drained onto the ground.

OR, continue driving the vehicle with the blower on HIGH while monitoring airflow from the vents. Once the evaporator ices over, you should notice a dramatic reduction in airflow from the vents. The compressor will shut off once the low pressure triggers the low-pressure switch. The blower will melt the ice. Low side pressure will increase to the point where the low-pressure switch activates the compressor. Then you’ll get cold air again—until the evaporator ices up again. The pattern will repeat over and over.

NOTE: Low airflow across the evaporator, caused by a plugged cabin air filter, can mimic the symptoms of a low refrigerant charge. Also, a faulty evaporator temp sensor can prevent the system from shutting down once the evaporator has iced over.

Restricted or plugged orifice tube can also cause low pressure on an orifice tube system

Restriction in the low side of the system. For example, a restricted orifice tube will cause the compressor to suck, but the restriction/clog in the orifice tube prevents the full flow of refrigerant, causing evaporator starvation. Low-side pressure drops below the low-pressure switch threshold, which shuts off the compressor.

What causes orifice tube restriction?

Debris is clogging the orifice tube screen

• Metallic wear particles from the compressor clog the orifice tube screen.
• Rubber hoses deteriorate over time, and rubber particles clog the orifice tube screen.
• Moisture in the system reacts with the refrigerant and oil to form acids and sludge that clog the orifice tube screen
• Moisture in the system that freezes at the orifice tube and restricts refrigerant flow through the orifice. An AC system leak allows moisture into the system. The moisture travels with the refrigerant. Since the temperature is near the freezing point of water right at the orifice tube, the moisture freezes, cutting off the refrigerant flow into the evaporator.

To test for moisture: Turn AC off for 10-15 mins, then back on. If the gauge reading returns to a very low suction-side reading, the orifice tube or expansion valve screen is likely frozen. If the gauge reading is normal when first turned on and stays normal for a few minutes, you get cold air and THEN low side goes low, that’s a sign of moisture in the system. Or, if you can access the orifice tube, apply heat to the orifice tube as you watch system pressures. if the system pressure returns to normal once you’ve applied heat to the orifice tube, the cause may be moisture in the system.

The orifice tube is the wrong size

• the orifice tube is too large
• the orifice tube has been replaced and the O-ring isn’t seated properly. In other words, the system can’t build pressure.

Low-side AC pressure is VERY low, and high-side pressure is low: restriction

Low and High AC pressures are lower than recommended in an expansion valve system where the valve is located on the evaporator outlet

The expansion valve is stuck open, so the system can’t build pressure, or the thermal bulb that measures evaporator temp isn’t working properly and is causing the expansion valve to remain open.

Low and high side AC pressures are normal, but the air isn’t cold

This may be a situation where the AC is working fine, but the “heat is on,” diminishing the effects of the AC. This can be caused by a stuck air temperature door, faulty air temp/blend door actuator or, in older systems, a stuck open heater control valve that’s running hot engine coolant into the heater core even though the heat is set to cold.

What to check:

Check the heater control valve

Determine if the vehicle has a heater control valve. The valve is usually under the hood and opened and closed by a vacuum motor or cable. Look for heater hoses and either a cable or vacuum hoses running to a valve. Use a hand pump vacuum to operate the valve or move the cable by hand to close it off. Then, check cooling.

Check air temperature or blend door and actuator

If the cabin air temperature is controlled by an air temperature/blend door, check the actuator operation to see if it moves as you change the temperature setting. In some cases, the fix can be as simple as replacing an actuator. If the actuator moves but the air door doesn’t, try moving the door by hand. Many times, the door can get stuck or break off from its hinges.

Check the cabin air filter

Yeah, it’s just like a furnace filter. If it’s clogged, it can restrict airflow. That, in turn, causes the evaporator to freeze up, shutting off the AC. Don’t skip this part. Many DIYers replace AC parts only to discover a clogged cabin air filter after spending hundreds on parts.

Check for a dirty evaporator coil

Leaves and dust can clog the fins of an evaporator coil, reducing its ability to remove heat from your car. To access the evaporator coil, remove the blower motor resistor and take a peek inside. To clean the evaporator coil, use a foaming chemical cleaner.

For more troubleshooting tips using gauges, see this post

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Posted on by Rick Muscoplat

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