The car ac can really make things miserable when you need cold and can’t get it. Actually, driving a car with no ac stinks doesn’t it? That ac problem can be identified depending on the ac system pressure readings. But who knows anything about pressure and readings? (Most people don’t.) So, here’s an article that tells you all that you need to know about pressure in your car’s ac system to check the pressure and get it pumping cold air again.
Car AC Air Flows
The parts of the automobile ac system are very sensitive and easy to damage. All of the ac airflows go either through the evaporator and heater core or through the evaporator and a door. Doors operate the airflow direction which is generated by an electric blower motor. Most systems have three to four doors that are controlled by the cables, electronic motors, and vacuum actuators.
The hot/cold temperature door controls the air flow that goes through the heater core. The fresh air/recirculate door decides whether to use air drawn from outside and use the air inside the cabin. The floor/vent/defrost doors control where to send the cooled or heated air. The doors are controlled by actuators which may be electronic, manual cable-operated, vacuum operated, or pneumatic.
Refrigerant is what makes the air cold in your car’s ac. The refrigerant comes in different ranges with different properties, some flammable and some non-flammable. Most vehicles use R13 or R134a refrigerant. The ac compressor in your car’s ac is designed to prevent the ac refrigerant from leaking while being pressurized. When the ac compressor (view on Amazon) starts to wear out, the bearings become damaged. The refrigerant is then able to leak through the damaged bearings. It is common to see the leak near the ac compressor. This is sign that your ac compressor bearings have worn out.
The pressure measurements for your ac are great indicators as to how well your ac system is functioning. By measuring the refrigerant pressure in the air condition, problems can be diagnosed such as over or under charging and leaks. Today, ac pressure can be measured without much strain and expense. By measuring
The pressure readings are taken at the ac compressor/condenser. These readings reveal pressure that is too high or too low at the compressor output or at the compressor input or the suction line. These problems mean the compressor is not maintaining the required pressure ranges. The first step to fixing the ac problem is to perform a diagnostic. Find out what is wrong and then form a plan to het it repaired.
Check Your Car’s AC System Pressure
When your car engine is off, the pressure readings should be more than 0 and no more than 150 psi. When the car engine is on, the low pressure readings and high pressure should be around 250 psi (this differs depending on the car make and model). Checking the pressure in your ac system is pretty simple. Here are the steps to follow:
1. Turn your car on and turn the ac to HI.
2. Open the low and high side buttons on the pressure gauge.
3. Check the reading. The PSI for the low side should be 25 to 40. The PSI for the high side should be between 200 and 225. If your reads are not within these parameters, or you do not see any charge most likely the problem is the cutout relay in the compressor. The relay is blocking any voltage from activating the clutch.
4. Charge your ac up with refrigerant.
AC Static Pressures
The air conditioner in your car is divided into two equal parts when it come to the pressure. Very high pressure is held between the condenser up expansion valve and the compressor. Low pressure beyond the evaporator and the expansion valve travels back to the compressor inlet. The refrigerant used in the ac system is pressurized so that new refrigerant can be pushed through the ac system. The pressure and the temperature in the ac system are in direct relationship. Temperature pressure charts are often used to compare the air pressure at any temperature.
To measure the refrigerant using a chart, follow these steps:
1. Hook up a recovery machine or gauge set to the compressor
2. Record the pressure readings that you get.
3. Measure ambient temperature.
4. Find a corresponding pressure and temperature match on the chart and look to see if both the temperature and the pressure match the chart reading.
5. If the pressure is the same as the chart, the refrigerant level is close. If the pressure is lower than on your chart, there is probably a refrigerant leak in the system. If there is no pressure at all, it reads ‘0’, the atmospheric pressure means there is a major leak and possibly all of the refrigerant is gone. If the pressure is higher than the pressure on your chart, the system is overcharged or there may be non-condensable air in the system.
The auto air conditioning pressure switch cycles the compressor on and off. It cycles it off when there is little refrigerant. Since refrigerant does not evaporate, the only way the refrigerant level could become low is if there is a leak.
To test the pressures while running, follow these steps:
1. Start your car and turn on the ac.
2. Do a visual check of the compressor status, see if it is working. Did you hear a clicking sound when the ac engaged? Look to see if the center hub is turning.
3. Check the pressure gauges for the high and low side pressure behaviors.
4. After your car has been running for a while, increase the engine RPMs to 1500. Your ac system should be about 25 to 30 psi for the low side and about 200 to 250 psi on the high side. The ambient temperature should be about 70 to 80 degrees Fahrenheit. If the temperature is higher or lower than it should, the pressure will also be higher or lower.
5. If the low and high side pressures are the same, check to see if the ac compressor clutch (view on Amazon) is engaging. Check to see if the center hub is spinning. If the hub is spinning, the compressor is not pumping.
6. Turn the car off.
7. Rotate the center hub using your hand. Do you feel any resistance? If you do, the compressor could be failing.
AC Pressures Higher than Normal
When the pressures are higher than normal on the low and high side, you may have a problem with the air flowing through the condenser. If this is the case, ask yourself these questions:
1. Is the cooling fan or the condenser working?
2. Are there any bent or dirty fins in the condenser?
3. Can you see any blockage ?
Lower Low Side and High Side AC Pressures
The low side pressure is one part of a semi-complex system. When the pressure is lower than it should be on both the low and high sides along with the right level of coolant may mean the system is having a problem building pressure. It could also mean that too much heat is leaving the condenser. Follow these steps:
1. Check to see if the ambient temperature is low
2. Turn the car on and place the fender cover in front of the ac condenser. Watch the pressure.
3. Does the pressure increase? Can the ac compressor build up pressure? If the answers are no, the ac compressor may have gone bad or worn out.
4. Check the cooling fa to see if it is constantly running. If not, a pressure sensor or pressure switch may have gone bad.
Lower Low Side and Higher High Side AC Pressures
Low pressure on the low side in your car’s ac system almost always includes a leak somewhere. When your car’s ac system has this condition, it can also mean that there is a clog or restriction somewhere in the system or the TXV or Orifice Tube is closed or blocked in some way. If there is a drop, it is more likely a restriction in the TXV or the Orifice Tube. If there is no drop, the restriction may be upstream from the TXV or Orifice Tube. Follow the line of liquid towards the condenser and see if you can detect a drop in temperature.
Higher Low Side and Lower High Side Pressures
When this is the condition of your car’s ac, then there is too much refrigerant flowing to the evaporator. One of two things is happening: 1. The TXV Tube us stuck open or it’s thermal bulb cannot sense the temperature appropriately; or 2. The Orifice Tube is too large at its
opening or the o-ring which seals the outside of the Orifice Tube is no longer sealing it.
System Pressures Normal, Still Warm AC Air
Sometimes the pressure is fine in car’s ac, yet the air is still warm. In some cases, the refrigerant in the car ac system is not the problem area. There are also possible problems with the evaporator, pressure switch, or the cabin filter. The cabin filter filters all incoming air from outside. Although the filter is small, when it is not clean, it could slow and even restrict the flow air The same happens with a dirty evaporator.
A dirty evaporator results in restricted airflow and the cooling capacity for the air. The heat is not being removed from the air as it should. The bad ac pressure switch can also cause the ac to stop working all together. The ac pressure switch monitors the refrigerant pressure on both the high and low sides. Your car may have two pressure switches, one on the low side and one on the high side.
Check the OBD2 Codes
Use the OBD2 Scanner (view on Amazon) to find the pressure problem. The scanner will show you the trouble codes that have been thrown for your vehicle. Some of the codes are specifically for the ac system components. For example, you may get the P0645 “A/C Clutch Relay Control Circuit” trouble code which means your ac is probably not working due to circuit.
You Need to Look After Your AC!
The main reason why you’re going to notice a lot of issues with your AC is due to poor maintenance. There are a lot of maintenance techniques that need to be done on your AC in order to keep it in good condition. Failure to do this means that the AC will suffer a lot of problems.
If you want to AC problems or other issues and save $100s of dollars that you’ll spend at the auto repair shop, you’ll need to service your vehicle often – you can use our mechanic-rated Auto Maintenance and Repair Manual to do this. It’s basically what mechanics use to go through your vehicle to check if there are any problems that need fixing. As soon as they notice the most minor problem, they’ll ask you to fork out some money even though it’s a problem you can fix yourself in minutes – the manual will teach you how to maintain your vehicle every few thousand miles and it’ll teach you how to fix minor problems that mechanics will ask you to pay for; saving you money in the long run.
A lot of our readers have the Auto Maintenance and Repair Manual printed on their garage wall and 92% of them haven’t visited the auto repair shop in the last year because they know what to do to avoid problems. All it takes is giving your vehicle a little attention every few thousand miles and you’ll never spend money at the workshop again.
Car AC Servicing
Of course, sometimes when working on a problem with a car, nothing seems to work. The ac is an important part of your overall driving experience. If you were not able to test the pressure in your ac or measure it properly, you can take your car in for servicing and get help with the problem. The professionals will know how to repair, refill, or vacuum your car’s ac system to get it right. Your ac will be back blowing ice cold air while you drive in no time. Get going and be safe.