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Troubleshooting Car AC Systems (Pressure Readings and More)

Michael Thomas, automotive and diesel technician, is the author of the McGraw-Hill textbook "Truck and Trailer Systems."

Why Would a Car's AC System Blow Warm Air?

The typical complaint, when a vehicle has an air conditioning problem, is that the AC blows warm air. This condition has several possible causes:

  • An AC system that is low on refrigerant
  • An electrical problem that prevents the compressor from turning on
  • An internal problem in the system
  • A problem with the heater/AC controls

Since the most common problem is a lack of refrigerant, we will start there and show you how to use pressure measurements to find out how much refrigerant you have and what is happening to it. Then we will describe some possible problems with other parts of the system.

Measuring Pressure at a Given Temperature to Evaluate the Amount of Refrigerant

There is a relationship between the pressure in the system and temperature. A temperature-pressure chart can be used to compare the pressure of the refrigerant at a given temperature. Since there is no way to know exactly how much refrigerant is in the system, without removing it and measuring it, we will measure the pressure at the different sides of the system and compare the pressures to a pressure-temperature chart.

How to Measure Static Pressures

"Static pressure" is measured when the system is not running. Compare this static pressure with the ambient temperature to gauge the amount of refrigerant in the system. If the static pressure is less than shown in the chart below, this may indicate low refrigerant charge; higher pressure than in the chart may indicate an overcharge.

To measure static pressures:

  • Hook up a gauge set or recovery machine.
  • Record the pressures. The high- and low-side pressures should be equal.
  • Measure the ambient temperature.
  • Does the pressure match the temperature-pressure chart?

    1. If the pressure matches the chart below, this means the refrigerant level is close to what it should be
    2. If the pressure is lower than in the chart, the refrigerant level is low. The system is probably leaking.
    3. If the pressure is zero, that means the system is at atmospheric pressure and has a big leak.
    4. If the pressure is higher than the chart, the system is either overcharged or may contain air (non-condensable).
Static AC gauge pressures

Static AC gauge pressures

Leak testers and refrigerant Identifiers

Leak testers and refrigerant Identifiers

Checking for Leaks

If the static pressure is low, a leak is probably present. Use the leak tester for larger leaks and the dye with a black light for smaller leaks. Identify the refrigerant before hooking up to a recovery machine.

How to Measure Running System Pressures

Start the engine and operate the AC system. Perform a visual check to see if the compressor is running.

  1. Was a click heard when the AC was engaged? Is the center hub turning?
  2. Observe the pressure gauges. Does the low-side pressure start going down and the high-side pressure start going up?
  3. After the system runs for a while, raise the RPMs up to about 1,500.
  4. The system should be about 25 to 30 psi on the low side and 200 to 250 psi on the high side. If the ambient temperature is higher or lower than normal (70 to 80°F) the pressures will go up or down accordingly.

What if the Low- and High-Side Pressures Are the Same?

If there is no change from the low-side to the high-side pressures (the pressures are still equal):

  • Double-check to see if the compressor clutch is engaged. If the center hub is turning, the compressor is not pumping.
  • Shut the engine off and rotate the center hub by hand. Is there any resistance?

The compressor may be faulty.

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Higher pressure on both the high and low sides may indicate an overcharge or lack of cooling at the condenser.

Higher pressure on both the high and low sides may indicate an overcharge or lack of cooling at the condenser.

What Do Higher-Than-Normal Pressures Mean?

Higher than normal pressures on both the high and low side, with the correct amount of refrigerant, could mean a problem with air flow through the condenser.

  • Is the condenser or cooling fan working?
  • Does the condenser have bent or dirty fins?
  • Is there blockage inside the condenser?

Lower High and Low Pressures

Lower pressures may indicate low refrigerant level or a weak compressor.

Lower pressures may indicate low refrigerant level or a weak compressor.

What Do Low Pressures on Both Low and High Sides Mean?

Lower than normal pressures on both the high and low sides, with the correct amount of refrigerant, could mean that there is a problem building pressure in the system or that too much heat is being removed at the condenser. Is the ambient temperature low?

With the system running, place a fender cover in front of the condenser and observe the pressure.

  • Does the pressure go up? Can the compressor build the pressure? If not, the compressor may be worn out.
  • Is the cooling fan running constantly? A pressure switch or sensor could be bad.

What Do Lower Low-Side and Higher High-Side Pressures Mean?

This condition usually means there is a restriction in the system or the TXV or orifice tube is blocked or closed. Feel the lines; is there a pressure/temperature drop at the TXV or orifice tube?

  • If there is a drop, there may be a restriction at the TXV or orifice tube.
  • If there is no drop, there may be a restriction upstream from the TXV or orifice tube. Follow the liquid line back towards the condenser to see if you can feel a pressure/temperature drop.

What Do Higher Low-Side and Lower High-Side Pressures Mean?

Too much refrigerant is flowing into the evaporator.

  • The TXV is stuck open or the thermal bulb is not sensing the temperature correctly.
  • The orifice tube has too large of an opening or the o-ring sealing the outside of the orifice tube is not sealing.

What If System Pressures Are Normal, but Warm Air Is Blowing Out of Vents?

Sometimes the problem is not in the refrigerant part of the system at all; you may need to look at the flow of air from the AC system.

The AC and heater operate out of the same box or plenum typically under the dash. Doors control the direction of the airflow created by the electric blower motor. Depending on the system, there are at least three to four doors controlled by cables, vacuum actuators or electronic motors.

These doors are:

  • Hot/Cold temperature door. This door controls the amount of airflow through the heater core. During AC (Cold) operation, this door may force the air to bypass the heater core. When the temperature lever is set to Hot, all the air is forced through the heater core to use the heat from the engine coolant to heat the cab.
  • Fresh Air/Recirculate door. This door selects whether the air is drawn from the outside or recirculated inside the vehicle. Recirculating the cooler drier air inside the passenger compartment makes it easier to maintain a cool temperature than cooling large quantities of hot, moist outside air. Typically some outside air is always added to keep the passenger compartment pressurized to prevent exhaust from entering the passenger compartment.
  • Floor/Vent/Defrost door or doors. This door or a combination of doors controls where the cooled or heated air is sent. The Floor mode is typically for heater operation, since heat rises. The Vent mode goes out the center of the dash for AC operation. Flow to the top of the dash is used to defrost the windshield during cold weather. There may also be combinations of floor/vent or floor/defrost to accommodate different situations.
The AC Heater box or plenum is located under the dash. Electronic actuators with position sensors are very popular to move the doors to direct the airflow and control temperature.

The AC Heater box or plenum is located under the dash. Electronic actuators with position sensors are very popular to move the doors to direct the airflow and control temperature.

Two Airflow Designs: With or Without a Hot Water Valve

There are two basic designs for the airflow:

  1. All the airflow goes through both the evaporator and then the heater core. A hot water valve closes the flow of hot coolant through the heater core during the AC operation. If this valve doesn’t close, the AC cold air will be heated by the heater core. If the valve sticks closed, there will be no heat. The result is hot air out the vents. In the defrost mode, the air will be dehydrated by the evaporator then heated by the heater core for hot dry air on the windshield.
  2. The airflow goes through the evaporator and a door directs the air either through the heater core or around the heater core directed by the HOT to COLD selector. During AC COLD operation, the “blend door” routes the cold air around the heater core. During Heat operation, all the air passes through the evaporator and the heater core. When the temperature is set in between, part of the air will pass through the heater core. No hot water valve is needed because in cool mode the door closes off airflow through the heater core.

When the air passes through the evaporator and heater core as in example 1 above, a hot water valve is used to shut off the hot water to the heater core. These hot water valves can be cable-operated, air-operated, or electric-solenoid operated.

Door Controls and Actuators

A series of doors controls the airflow through the heater box or plenum. To control these doors a variety of actuators have been used, such as:

  • Manual cable-operated doors have been around a long time. Cables connect levers in the dash control to the doors. When a lever is moved back and forth, like the Hot/Cold lever, it moves the door from one position to another. Typically moving the lever quickly from one extreme to the other results in a thumping sound as the door hits the stops. The Hot/Cold door would control airflow around or through the heater control. Cables popping off or improper adjustments are typically what will go wrong with this style of control.
  • Vacuum-operated actuators, used on cars and light-duty trucks, use the vacuum in a gasoline engine to act upon a diaphragm which is connected to the doors. As the controls are moved, a hissing sound is heard. Vacuum leaks are the biggest problem with this type of control. If the vacuum supply is disconnected, the controls usually default to putting air on the windshield. Diesel engine light trucks using vacuum-controlled systems use an electric or belt-driven vacuum pump to supply the vacuum.
  • Pneumatic actuators operate similar to the vacuum type, except the truck's air system pressure is used to move the actuator diaphragms. Hissing under the dash could be from a leaking actuator or connection.
  • Electronic actuators use an electric motor to position the doors. These electric motors usually have a sensor built in to allow the system to know the location of the doors. When the controls are moved, a small electric motor buzz can usually be heard.

Service Tip: A common problem with all actuator types is paper clips, pens, keys and other foreign objects falling down from the dash defroster vents into the heater box and jamming the doors. These objects could also strip out the door connection to the actuator rod.

A Dirty Cabin Filter Can Make You Think There Is Something Wrong With the AC

Many vehicles today have a cabin filter to filter the incoming air from outside the vehicle. The filter looks like a small air filter from a car. Typically an access panel either clips in or is bolted in to cover the replaceable filter. The filter may be located under the dash or on the heater box in the engine compartment. This filter is often overlooked until it is entirely plugged up and allows very little airflow into the cab.

If the cabin filter is dirty, the result will be low airflow. This may cause the AC or heater output to be reduced. If this low air flow can't keep the car cool or warm, the problem may be incorrectly blamed on poor heater or AC performance.

A dirty evaporator.  The dirt on this evaporator has restricted the airflow and caused a lack of cooling.

A dirty evaporator. The dirt on this evaporator has restricted the airflow and caused a lack of cooling.

A Dirty Evaporator Can Restrict Cooling

For cooling to take place, air has to flow freely across the evaporator. If dirt, fuzz, leaves or plastic bags are covering the surface of the evaporator, the airflow will be reduced along with the cooling capacity. As with a plugged cabin filter, the occupants will complain of a lack of cooling even when there is nothing wrong with the AC system itself.

Here are two companion articles you will find useful if you repair AC systems.

Air Conditioning (AC) System Operation with TXV or Orifice Tube. Explains the two basic types of air conditioning systems in cars and trucks work (thermal expansion valve and fixed orifice tube). Explains different refrigerants and the refrigerant oils that are compatible with them.

Servicing an Auto AC System and Replacing Components. Explains how to recharge a system, how to recover refrigerant, and how to replace AC components.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: How about if the static pressure is not equal. 50 psi on low pressure and 180 psi on high pressure with an outside of temperature of 75F, what does this mean for diagnostics of an AC system?

Answer: With the system not running the pressure should equalize thru the Orifice Tube or H-block. When you shut off the AC you hear a hissing noise. I would say there is something wrong with the gauges. What happens when the system runs, do the gauges both move?

Question: How do you tell if the condenser is restricted ?

Answer: By the temperature in and out. Hot in, cold out is restricted. Usually, the high side is really high and low side really low.

Question: I have a 2008 Rav4. Twice I have tried to charge the A.C. system to the proper pressure. When the proper pressure is reached, the vents blow warm air and the compressor drops out. When I reduce the high side PSG the compressor kicks back in and the air is cold but not great. Could it be the hi pressure switch?

Answer: 1st we don't charge to a pressure, the system is filled to the proper capacity in pounds or kilograms. The system must be evacuated before charging.

Question: My car reads 30psi low side at idle and high side reads 70-75 cooling effect...static pressure is 50psi... Ambient temperature is 30.c... Is my AC is undercharged?

Answer: Sounds undercharged. Leak?

Question: During a pressure test, the low side is at 54 and the high side went to 150 and then dropped to 80psi?

Answer: Since I don't know what system you have, I have to guess. Sounds like either a bad compressor or a bad H-block.

Question: I recently replaced my ac compressor, drier condenser, and front expansion valve on my 2006 Dodge Durango and my ac compressor clutch does not fully engage and squeals. What should be my next step? 2006 dodge Durango slt 4.7l with front and rear ac.

Answer: 3 things to check

1. Clutch plate air gap between the plate and the pulley in specs.

2. Voltage at the compressor, should be 12V

3.Voltage drops in + or - side of the AC circuit, bad connections or corrosion (in the bottom of fuse panel) causing voltage drops.

Question: 2020 mustang air works when it wants to. High side reading is normal but low side immediately pegged the gauge and didn’t move. Didn’t read anything on here about normal high side but high low side. What’s likely the culprit?

Answer: 2020 Mustang should be R1234yf refrigerant. Low side pressure is controlled by the H-block.

Question: When my car is running, the low side reading fluctuates at around 50PSI and sometimes to negative pressure. High side reading stays stagnant at 100PSI. Not-so-cool air is being blown out. Ambient temperature 30 degC. What could be the problem?

Answer: Not enough information. But the high side is extremely low. If the system is properly charged it seems the compressor is not working up to par.

Question: I have a 1998 suburban with rear a/c. The pressures are 30psi on the low side and 165psi high side. It cools to about 45 in the front and 60 in the rear. The compressor clutch stays engaged and the pressure readings stay the same. It has a new compressor, orifice tube, and accumulator. It had this problem prior to replacing these parts. I pulled a vacuum on the system prior to charging. Any thoughts?

Answer: Usually, the problem is reversed. The rear ac uses a TXV/H-Block and it sounds like the valve is not working. The systems with rear ac hold more refrigerant, usually about 3.8 pounds. Should be a tag on the radiator support.

Question: I charged the A/C system. The air inside cab is down to 50 degrees. Low side of Gauge reading is 40-50 psi. High side of gauge reading zero, but the cab is cool. I drove around for about an hour with the outside temperature 88 degrees. What is up with the high side?

Answer: The system can't be zero on the high side. There must be a connection problem with the high side coupler. The system should cool down to 40 degrees.

Question: I can't turn off my a/c. When it's switched on it works great. When I switch it off it still blows semi cool air. The switch is functioning correctly. Could the compressor clutch be partially sticking? The car is a 2006 Hyundai Accent.

Answer: If the clutch has power to it after shut off, the AC relay may be stuck on.

Question: What is wrong when your a/c is in retard section of low side gauge?

Answer: This means your low side pressure is above 150psi. System must be overcharged or your gauges are somehow connected wrong.

Question: What causes the hig side pressure to be low? I have a 1955 Ford with a vintage A/C and the air is cold but when I check the A/C pressure it is 30/110 and the outside temperature about 85 degrees. Is that normal?

Answer: I assume it is an R12 system that has been converted to R134a. The lower high side pressure may be from an undercharged system and the TXV is keeping the low side at 30 PSI. The old systems used a sight glass to charge the systems. Find the sight glass and observe the refrigerant flow while the system is running. If the sight glass is clear liquid the system is fully charged. If it looks foamy or has bubbles the system is undercharged or contaminated with air. Since it does cool I would think it is close to being charged.

Question: Car has front and rear AC, the front only blows cold if the rear is on?

Answer: Does the compressor kick on with the front ac on only? If not front ac control problem.

Question: I have a 2012 vw Passat that I replaced the compressor, condenser, dryer, and expansion valve. Charged up to correct refrigerant specs of 575g. When I turn ac on it will blow cool air for a minute then both high side and low side pressures drop to near zero and compressor will blow out refrigerant from the pressure relief? Any suggestions?

Answer: Never seen this before. Sounds like a blockage in the high side line to compressor discharge oort.

Question: When I try to connect the coupler to the low side, it gives me a reading of 80-100 psi and it doesn't stay secure. What makes the coupler not to stay secure?

Answer: Wrong or damaged coupler would be my guess.

Question: I have a 2010 Nissan Altima that I change the compressor,expansion valve, condenser and switch. I vacuumed the system and put new refrigerant. It works but, the next day doesn’t work. I did this 2 times and pressure is the on the low and high?

Answer: Sounds like you have a leak. Without knowing static pressure this is a guess.

Question: I have a 1996 Dodge Ram Diesel. Low pressure was normal, but the high pressure was extremely high. Why would my Dodge's AC pressures be so different?

Answer: Sounds like poor airflow across the condenser. Cooling fan problem, plugged or blocked condenser are the probable causes.

Question: The A/C compressor has seized on my 2000 Sienna. I have to bypass with the belt. My mechanic says any bypass belt still will use the pulley or pass through the area of the compressor pulley. Does that make sense? Why would they call it a bypass belt if that is the case?

Answer: If the compressor seized, the pulley should still turn. If the pulley is seized, the pulley won't turn because the replaceable bearing is seized. Usually, to use a belt without ac, you need to remove the compressor as if it didn't have ac.

Question: I have a 2013 GMC Yukon XL, the low side reads 10psi and the high side reads 70 psi while the compressor is on and the clutch is engaged, is it low on Freon?

Answer: Sounds like it. Charge with DYE and retest with a black light.

Question: Why does the ac cut off every time I make a turn?

Answer: The computer senses the extra load added by the power steering and turns off the ac until the load is reduced. Typically used on small engines.

Question: I have a 1998 Monaco motorhome. I evacuated the system to get ready to add 134a. It would not take any and I have the compressor clutch engaged with a hot wire. Both gauges are reading zero. I'm thinking a bad compressor could this be right or something else?

Answer: Sounds like the gauges are not connecting. This is the reason you can't add refrigerant.

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