Car AC Diagnostics

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image of ac manifold guages
AC Manifold Guages

It may seem crazy to talk about car AC diagnostics in the middle of a snow storm. But by the time this page gets indexed and ranked you should be ready to turn on your vehicles air conditioning for the first time after a long winter of not using it.

On a previous post about air conditioning auto repairs I discussed how to test your system using manifold gauges. We also discussed how important the total charge of the system is to air-conditioning performance.

On this page will be some tips on how to deal with pressure readings that are out of specifications. Also included will be some best practices for Car AC diagnostics as well as running system AC performance testing properly. And don’t forget to wear safety equipment because freon is dangerous.

High side pressure readings

When you’re basing your diagnostics off of system pressure it is very important to refer to the pressure charts in your online auto repair manuals. These pressure readings will vary greatly with humidity and outside temperature.

Improper diagnosis is often caused by expecting an unrealistic reading on a hot humid day. With that said if you have determined that your pressure readings are truly off here are some tips on what they could actually mean.

AC Condensor
AC Condensor

If the high side pressure is too high there could be air in the system, too much refrigerant, or a restriction in the high side of the system. Also poor airflow across the condenser can cause excessive high side readings.

Poor airflow across the condenser can be caused by a radiator fan not working properly or even debris or bugs caught in the condenser fins. In either case the airflow problem must be corrected before further testing takes place.

If the highside pressure is too low the most likely cause would be a low refrigerant level. But also a defective compressor can also cause a high-side pressure not to reach its full potential. Worn parts inside the compressor can cause this but it is recommended to assure the proper amount of Freon is in the system before condemning the compressor or replacing it.

Low side pressure readings

When diagnosing AC, low side readings are often the most critical to the output temperature of the system. To follow are a few examples of how to evaluate improper low side pressure readings on your Cars AC. If the low side pressure is higher than normal the most likely cause would be a refrigerant overcharge or a defective compressor. As mentioned above it is best to remove all of the Freon from the system and then specifically charge the correct amount before determining a bad compressor.

orifice tube image
Orifice Tube

As mentioned on my car AC information pages on the website the manufacturer provides an exact amount of Freon that belongs in the system. This is usually indicated on an under hood label in most cases either on the radiator shroud or on the heater case on the firewall side of the engine compartment.

If the low side pressure is lower than normal and it has been determined that the correct amount of Freon is in the system a metering device could be problem or cause. On most General Motors vehicles they use an orifice tube metering device.

It is not uncommon for this to become restricted and cause low side readings to be below specifications. Other manufacturers will have different types of metering devices although the orifice tube has become the most common type of Freon metering component.

Also note that the orifice tube has a screen on it that can become clogged. If your orifice tube is clogged it is most likely from either moisture in the system or failure of internal components that have been chewed up and spit out by the compressor. If this screen is clogged with metal chips the system should be flushed thoroughly before recharging.

Automotive A/C diagnostic tests

A rundown of automotive AC diagnostics. The procedure will vary with your problem but here is an overview of how I would approach performance testing a system. I start by connecting the manifold gauge set to the respective high and low pressure fittings again you can find the location of service fittings in your service manual.

Close all the doors and windows and set up a fan in front of the radiator to simulate going down the road. Also to simulate road conditions it is recommended to raise the idle to about 1500 to 2000 RPMs. Now you can take accurate pressure readings on the high and low side and compare them to specifications.

You can also take an AC temperature output reading at the center duct with the blower on a medium speed or lower. With the pressure readings and the output temperature recorded this will give you a good baseline to start your diagnostics. Tree charts in your auto repair manual will guide you through the specific repair of problems indicated by your readings.

Can't fix your car problem on your own? Ask a live mechanic!

23 Replies to “Car AC Diagnostics”

  1. Syed

    I have a 2000 Dodge Caravan. I am having a lot of headache trying to diagnose why my AC system won’t hold freon. Once I put the freon in the van it runs fine for 10 minutes. And then all of a sudden the freon starts spewing off from the compressor relief valve. I have vacuumed the ac system and checked the fans to make sure they are coming on as supposed to. I am not sure why the ac system is building up so much pressure that it has to come out through the relief valve on the compressor. I am suspecting the orifice tube is plugged or a faulty expansion valve but wanted your expert opinion before I proceed. Thanks

  2. Mark

    Syed: A few things can cause this type of condition so in the end a diagnostic test should be performed. Also note that either an orifice tube or an expansion valve is used (not both). I’m not sure which one is on a 2000 Caravan. The first thing I would do is confirm the high side problem and get a pressure reading. The high side relief valve should not operate until it hits about 350psi. It could be a bad compressor relief valve BUT your right the orifice or expansion valve is more common. But remember it could be several other things. A damaged compressor is another possibility.

  3. syed

    Mark : changed both the expansion valves..still the same issue. I got the van from an auction with the suction line broken in half.(probably due to excess pressure. Replaced the line and since then freon has been coming out the relief valve on compressor……come to think of it the compressor does not sound as smooth as it should.

    I also noticed the condensor fan comes on when the low side reaches 45psi….as soon as it comes down this pressure starts to come down to about 25psi……fan turns off…..psi start to rise to 45 psi and the fan kicks back in.
    Should this fan be on regardless? Already changed fan relay but still functions the same way.

  4. Jason

    I have an 03 honda crv. Compressor went out. So vacuumed flushed whole 9 yards put new compressor in, pressure building to high like 400 psi at 2k. So went ahead new expansion valve, new condensor, charged system and still doing the same thing. Any suggestions?

  5. Bo

    I have a 1992 Acura Legend that had a leaking evaporator core. It has already been retrofitted to R134A. I replaced the drier, expansion valve and the evaporator. I vacuumed it down for 20 minutes and put 1.5lbs of R134A in the system. My pressures are to low. Low side is 15psi and high side is 150psi at idle. And its 95 degrees out. It cools fine until thirty minutes of highway driving. During that 30 minutes it cools at 54 degrees but afterwards it hits the upper 70’s through the vents. I notice a tremendous amount of water pouring like a spicket out of drain tube so I pulled the blower resistor out to look at the evaporator and the expansion valve is an ice block. Thinking I might have a defective expansion valve, I pulled the dash back out and this time got a new valve from the dealer. Put it all back together and no change. Same results. What else could this be? Im now wondering if my drier is bad since the lines were open for a weeks’ time. Any help would be greatly appreciated. Thanks

  6. Mark

    Bo: While reading through your AC symptoms you described I was thinking this sounds like a classic expansion valve problem. Then you stated you replaced it. 2 things come to mind. 1) Is the new expansion valve working properly. 2) Yes the dryer should be replaced if exposed to humid air (in my opinion). Although I do not know if it will solve all your problems. The drier removes moisture and partially frozen moisture in the lines can act like blockages that can cause the pressure problems you indicated.

  7. Kirk Fletcher

    Hi Mark, Just read this light bit of info that you have posted here on A/C Diagnostics. I just wanted to share some of the things that we are currently teaching in AC Clinics the last couple of years. Myself, I am trying to get more guys looking at temps rather than mainly looking at pressures. Too many of the OEM’s are putting the high and low side fittings in the wrong area of the system to correctly diagnosis by pressures. Many of the high side fittings are located in the liquid line so the high side readings can be much lower than what the compressor is really creating.

    We are now on our fifth generation of AC Condensers for R134a systems and the flow passages are now as small as .015 of an inch. When you have a condenser inlet side of just less than 8 1/2 square inches of combined flow path (28 tubes x 20 flow paths per tube x .015 per flow path) the inlet manifold tube of the condenser becomes the primary filter for the A/C system. These small passages become partially blocked and become tiny OT’s. We get substantial pressure drops at these partial blockages and we get corresponding temperature drops. We are teaching tech’s to look at inlet and outlet temperatures of the condenser. We only want to see delta’s of 25 to 35 degrees.

    Too little temperature drop and we have an air flow problem through the heat exchanger. Too much of a temperature drop and we have massive amounts of partially obstructed refrigerant flow paths. This industry continues to try to back-flush these new style condensers with Sure-Shot cans and Flush-Guns with zero results. This is one of the major reasons replacement Compressors have very short life spans. The Compressor was not Defective; the compressor was Damaged because of incorrect service procedures. I know there is a need to educate the DIY on proper A/C System service and repair but most DIY’s don’t have the proper equipment to be successful even if they have the proper knowledge.

    It is hard enough to get the professional repair shop to upgrade their A/C Service equipment to the current generation and to purchase the dedicated flushing machines that are really needed to flush and clean these new systems. The goal isn’t to go through the motions; the goal is to get the system perfectly clean. How can we expect the DIY to get these systems perfectly clean when only the very best professional repair shops currently do? To me, selling an A/C Compressor to a DIY at the parts counter is like handing a loaded handgun to a five year old and expecting positive results.

  8. Tina Ellison

    I have a 98 Chevy s10. I am having a lot of problems with my ac we are working on the third new ac professionally installed. The high side is low and the low side is high . when idling I have no cool air when i am driving i can get the air to reach 70 degrees my husband tried to put leak detector plus freon and it read warning. The auto parts store is tired of warranting I’m just wondering what is wrong with our truck to cause all of this please help.

  9. Jack

    My ac compressor comes off and on rapidly and repeats this cycle endlessly. When the compressor engages the high end pressure gets very high and the air does not cool. How can I check for a high side restriction problem and where it might be.

  10. Melvin Martin

    Have 2000 Concorde. Gauges read 30 on low and 250 on high at 2000 rpm 85 degrees. At an idle, low side reads 46 and high side reads 225 in 85 degree weather. Took to shop before I did test and they said my car wasn’t holding pressure and was low on freon. AC was cool but not cold is reason I took to shop. These gauge readings seem normal at 2000 rpm. Not sure when at idle though. What do you think about the difference in readings. Does this mean there’s a problem?

    • Mark Gittelman

      Melvin: This is a tough question because the amount of freon in the system is critical to pass a judgement. With a full charge these readings should make cold air at 2,000 rpm. However if there is only a couple of ounces, inside these readings will not. As for the difference in readings at idle, it can be an indication of a worn compressor. Again the total installed charge is so import. If this car was in my shop I would recover and weigh the amount of Freon I pulled out and compare it to the factory spec for a total charge. Then we would have an answer.

  11. donnie

    Have a 05 Dodge 3500, the high side is reading around 350 psi and low is around 40 psi. It cools good at idle, but when you drive it down the road it don’t cool very well. I replaced the fan clutch and orifice.

    • Mark Gittelman

      Donnie: Diagnosing and solving a problem like this on an 11 year old truck can be complex operation. I wouldn’t want to hazard a guess without being there. I will say I know it’s hot in Texas, but that high side reading is too high.

  12. Macon

    Hi I recently replaced the compressor, condenser, accumulator and flushed evaporator coil and lines. I put about 1 lb of freon in and the pressures are reading 40 low 400 hi. I just ordered new manifold hi side lines to see if that works dodge 2500 diesel 1994. I did replace fan clutch as well.

    • Mark Gittelman

      I’ll assume you pulled 30 psi vacuum on the system for 20 minutes before charging. I believe these old Dodges have an expansion valve instead of an orifice tube. This would be a likely area of concern after a compressor failure and could cause those readings.

  13. David Logsdon

    I have a 97 Ranger. When idling, the compressor clutch switches off and on every few seconds and the air warms. While driving, air is cold even in 90+ temps. I have no way of checking high side pressure. Only low side with autozone filler gauge. It reads 45psi. Any ideas?

    • Mark Gittelman

      There are a lot of possible causes on a 20 year old system. Without a high side reading its hard to guess. With that said, if it works better while driving, then this would have me check the condenser fan operation. You don’t need the fan as much when your moving.

  14. Adam Green

    Mark, I have an 08 F150 with the 5.4. It has 250k miles and runs just fine but the AC hasn’t been blowing cold. I put gauges on and added freon until low side is 35psi and high side is 270. The more i added the colder it blew but every diagram I’ve seen says in 90F it should be 45 to 55 on low side and 250 to 270 on high side. I’m afraid to add more to get the low side where it should be because the high side is already at the threshold. Any thought? Should I just add more until the low side is at 45?

    • Mark Gittelman

      Adam: When you’re that close to the desired specs the more important piece of information is the outlet temperature at the center duct on med to low blower speed. If it’s around 50 degrees then I would stop. You also have to consider that not all (especially cheap) manifold gauges are that accurate.

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