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    Car Air Conditioning Troubleshooting Repair Tips – ASE Master Tech

    Car AC Troubleshooting Repair

    DennisB © Summary: Practical troubleshooting tips for diagnosing and repairing problems with car air conditioning systems. Questions about fixing car A/C’s, answered by me, Dennis, an ASE master auto technician. When I first starting to work on car air conditioning (in Florida)  it was the 1980’s. R12 was the only refrigerant that was used. Most cars took much more refrigerant during that time. Although vehicle’s systems took more refrigerant (Freon) because the capacities were larger, the systems were less efficient. One reason is because the components like evaporators and condensers were thicker causing slower transference of temperatures with the refrigerant within. The typical amount of Freon a car held back then was 3 to 4 pounds. Now some cars take around a pound with some dual AC systems taking 2-3 pounds. Most cars on the road today use R134A. The new refrigerant that will be taking the place of 134a is R1234yf. It’s being used on some models now, but it will be several years before most of us will need to worry with it. That’s unless you work at a dealership or a collision shop.

    All A/C systems regardless of what type of refrigerant they use work basically the same way. Here’s a very basic description of what occurs. Refrigerant is pumped in a loop by the compressor. The expansion valve controls the flow. The passenger compartment is cooled by the evaporator by air being blown through its coils (and fins). When the air passes through (forced by the blower) the air is not only cooled, but the water (or humidity) is left on the fins of the evaporator. That’s why the system drips water underneath the car on a properly operating AC system. The more humidity in the air the more water drips. The refrigerant then carries the warmness captured from the passenger compartment and it is cooled in the condenser. Then the cycle repeats.

    Since cars now take less Refrigerant than they used to, a leak can affect the cooling ability pretty quickly. Even small leaks can cause an automobile’s A/C system not to perform properly. Below are some tips on diagnosing a car’s AC system when the AC is not cooling or at least not cooling well enough. A good A/C gauge manifold set is recommended to check the system. That’s because both high and low side readings are needed to accurately check an AC system and troubleshoot any problems. I’ll answer questions in the comment section below, but first I have some questions to help troubleshoot any issues that may be occurring.

    Check the following to troubleshoot Car AC problems if NOT cooling properly.

    With a visual inspection of the AC components, are there any oil residues present? Oily residue (sometimes combined with UV dye) is an indication of a leak in the system. If there’s AC oil on one of the air conditioning components it’s most likely leaking. Common leaks are from hoses, condensers, compressors etc. The evaporator is typically in the dash and that makes it hard to visually inspect unless sing an inspection camera with a wand that can access it after gaining access (like removing a blower resistor or blower motor).

    What are the low and high side readings? The static and live readings help tell the story. This is one of the first steps any technician takes when diagnosing a car’s AC system. The system could be completely empty. By hooking up the A/C gauge set and seeing ZERO on the low and high side you’ll know immediately there’s a leak to look for. If the static readings show 125psi on low and high gauges it will be obvious that someone has overcharged the system. See AC gauge readings explained for more information and tips to interpret what the readings mean.

    Do all the blower speeds work? This is not a direct problem with the part of the AC system that holds the refrigerant but it can obviously affect the act moving of air through the evaporator and into the passenger compartment. If one or two speeds don’t work the AC will still work. If none of the speeds work the AC will not work even if there’s no issues with the refrigerant holding AC components. It’s best to use common sense if there’s a blower issue and not suspect problems under the hood.

    Is the compressor coming on? If the car’s AC is not cooling at all, the compressor may not be coming to the party. If the front disc on the compressor is not turning, the pumping part of the compressor is not engaged. Without the compressor pumping the refrigerant pressure readings will stay still, which is the same as the AC being turned off. This could happen if there’s not enough pressure in the system for the low pressure switch to allow the compressor to come on. The air gap may be too large for the electro-magnet (coil) to attract and engage the disc to the hub. The coil may also have an open circuit and not be working at all. If the there’s no power and ground to the coil, there could also be an electrical malfunction like a bad switch or open circuit. Cycling switches are a common failure

    Is the compressor clutch engaging and disengaging very quickly? If the compressor cycles on and off too quickly the most likely cause is low refrigerant. *Caution – Don’t hot-wire the compressor or jump the cycling switch, this could damage the compressor. When the system gets too low, the low pressure (or cycling switch) turns the compressor’s clutch off by turning the power off to the clutch coil. That’s to protect the compressor from being damaged. The compressor gets its lubrication from the oil that circulates throughout the system with the refrigerant. If there’s not enough refrigerant and the system is forced to run anyway, the compressor will overheat due to excess friction (lack of lubrication) and will be damaged. It will possibly lock up (seize) if ran too long. Another cause of the compressor engaging and disengaging rapidly is the system could be over-charged. The high pressure switch will turn the compressor off when the high side reading reaches 450-500 psi.

    Does the system have the proper charge of refrigerant? An under full or over full system will not cool at it’s best. That’s why it’s best to use an AC gauge set to help determine the proper charge. And really the only way to 100% guarantee that the proper charge is to start with an empty system and use a scale to add the factory specified amount of refrigerant. With that being said, using an AC manifold gauge set and understanding the low and high side readings is the next best method. *Note that it is illegal to release refrigerant into the atmosphere. Using an AC Refrigerant Recovery Recycle and Recharge machine to remove Freon is the legal way to empty a system.

    Does the A/C cool well when driving only at highway speeds and does the cooling fan coming on? One of the easiest AC problems to diagnose is when a Car’s A/C only cools when driving at highways speeds but stops cooling at stops. This is almost always the cooling fan for the condenser (out by the radiator) not working. The air that blows through the condenser when driving down the road takes the place of the air blown from the cooling fan allowing the condenser to cool the refrigerant and be circulated throughout the system carrying out the process. *Note some automobiles have two fans, one for the radiator and one for the condenser.

    Does the car engine overheat? Even if the cooling fan is coming on, if the engine is overheating the heat can transfer over from the radiator to the condenser causing the A/C to not work properly. The computer may also disallow the AC to run when the engine temperature is too high. Fix the overheating condition with the engine and the AC may work properly.

    Is the compressor excessively noisy? Some sound from the compressor is completely normal. It is a pump so when it is working it will make more noise, compared to when it cycles off. Sometimes compressors will make excessive noise if the system is not properly charged. Lack of oil, air in the system or just a low refrigerant charge can cause a compressor to be noisy. If the system is charged properly and the compressor still makes too much noise, it’s best to replace  the compressor before it completely fails. When it fails, the compressor can put contaminants throughout the system causing restrictions in components. This can be caused by the bits of metal, plastic and rubber from internal parts. AC systems can be partially flushed if a compressor fails. The dryer is always replaced whenever the compressor is replaced. The expansion valve cannot be flushed so it is recommended to be replaced it unless the compressor failure was just due to a leak and not a catastrophic failure. Add on filters can be installed if a system is severely contaminated. I used to do “live” flushes on Fords back in the 90’s when a compressor failed and expelled black garbage throughout the system. This was basically installing the add on filter and running the AC for awhile and then replacing the add-on filter again after it collected the contaminants. Sometimes I had to repeat the process several times before the system was cleaned up and wouldn’t cause issues with the orifice tube being clogged up during operation. In really contaminated cases the condenser may need to be replaced because sometimes debris cannot be flushed through the tiny tubes in the condenser.

    Is there noise from the compressor when it is cycled off? If there’s noise when the compressor is off (or disengaged) the bearings in the clutch hub could be faulty. This does not mean that the compressor is bad. It means the clutch hub that is installed onto the front of the AC compressor needs to be replaced. It’s best to inspect the nose of the compressor for damage in case the hub has spun on the compressor. It can cause damage and the compressor would need to be replaced if there are ridges worn from a spun hub. I used to see that occasionally on Nipendenso compressors used on some Chrysler products. If just the clutch hub assembly was installed on a compressor that had a worn nose the hub failure would repeat.

    With the AC turned on, is there any iced up components seen under the hood? If the A/C is cooling well, don’t be concerned if something under the hood looks “iced up”. Remember low pressure equals cold and high pressure equals hot. It’s normal for the AC line just past an orifice tube to be very cold, it’s not normal for it to be totally iced (and the AC not cooling). However, if the AC is not cooling well and something is iced up, this indicates a restriction. The restriction could be from a clogged up orifice tube. It could be from a restricted condenser etc. There could even be a malfunctioning expansion valve that is stuck, allowing Freon to flow too slowly resulting in low pressure and freezing.

    Does the belt squeal when the compressor clutch first engages? This could indicate a worn or loose belt, a weak belt tensioner, a tight compressor or a slipping AC clutch. The belt related issues are self explanatory. The clutch on the compressor could have a weak coil, the air gap could be too large or the compressor’s front seal could be leaking oil onto the clutch disc causing it to slip.

    On a system that is cooling but not very well. Is there a difference in the sound of the airflow when maximum air (or recirculate) is selected? When the max air setting is selected a door in the HVAC system recirculates the air in the passenger compartment. This stops air from flowing from the outside making the air colder in the cabin. This is because the air being ran through the system is already cooled substantially because it has already been circulating through the evaporator; as opposed to cooling the air from the outside that has yet to be cooled down. So if the sound of the door isn’t heard or at least the sound of the air going through the system doesn’t change and ramp up, the door or the motor (called actuator) could be faulty. This will prevent the air conditioning system from cooling at it’s best.

    On cars equipped with an ambient (outside temperature sensor) does the temperature seem accurate? Sometimes if the outside temperature sensor is bad it will actually read negative. This may need to be read an the ECU’s (Electronic Control Unit’s) data screen using a scan tool. Some air conditioning systems will not allow the air conditioning to run if the ambient sensor is bad. The inside temp sensor on automatic systems may also cause a problem if they are faulty.

    Do you have an AC manifold gauge set? To help diagnose car air conditioning problems, an AC manifold gauge set is essential. That’s because knowing the low and high side readings are needed to troubleshoot what’s going on with the system. AC Manifold gauge sets for 134A are color coded and have anti blowback features that make using them very safe. It’s always a good idea to use safety glasses, but 134A gauges are a lot safer than the old R12 gauge sets that didn’t have the anti blow back safety feature built into the service port connectors.

    Do you have an electronic refrigerant leak detector? Low refrigerant is the most common cause of an A/C system not cooling. If there are no visual signs of leaks looking for oil or UV dye, an electronic refrigerant leak detector is the most common and most effective way to find a Freon leak. Some will say to use dish wash soap to find a refrigerant leak. While I have used that method, it’s not always practical unless the leak is fast and you already know about where the leak is likely to be. With an electronic leak detector the entire AC system can be checked in a matter of minutes. Including the evaporator which is not easily accessible to use the soap bubble method.


    Comment from Dennis Bandy
    Time September 16, 2013 at 6:16 pm

    Low Freon is the main cause of a car A/C system NOT cooling, but adding too much is not good either.

    Comment from Zachary
    Time September 29, 2013 at 5:26 pm

    Thanks for the information on trouble shooting my AC. I am going to be getting my A/C tools that I ordered from Wise Auto Tools this week and I’m anxious to get my car cooling well again.

    Comment from Wan Mohd Amin
    Time October 20, 2013 at 11:07 am

    I have problem with my air cond (Porsche Panamera). Passenger side is slightly cold, look like fan is blowing & on driver side is cold but not as cold as before. When I further reduce the temp setting, seem to be no different.
    What should I do ?


    Comment from Dennis Bandy
    Time October 22, 2013 at 1:43 am

    The most likely cause is low refrigerant. Other things can cause the problem too (like a dual zone blend door issue) but the first thing to check is to make sure the refrigerant level is correct. Good Luck.

    Comment from Ben
    Time June 27, 2016 at 7:32 pm

    This problem has been going on for years. I have a 1985 Toyota PU that has been converted from R12 to 134a. Since the conversion when the low side is somewhere normal the high side is out of sight. (over 350) Everyone that has worked on it says there is a restriction in the system. Every component has been replaced (at the same time) except the evaporator and it has been removed and flow checked. Replaced the compressor, condenser, all lines, dryer, and the expansion valve (3 times). Nothing changed. I have had people work on it that was supposed to be experts but no one has been able to fix it. It seems to be a simple system. I can’t understand why it is so complicated. In order to keep the high side readings near normal the amount of refrigerant just barely blows cool. Never enough to be comfortable. There is no room to install a larger condenser. I’m at my wits end. Do you know if that when the expansion is changed if the new one is still manufactured for R12 and I am using it in R134a creating a restriction? This has been such a problem all this time any suggestions you may have will be very much appreciated.

    Comment from Dennis Bandy
    Time June 30, 2016 at 5:45 pm

    Ben conversions are never as good as when a system has been designed for 134A refrigerant. 134A typically runs at high pressures anyway. If the expansion valve is for 134a that could be part of the problem. Also you mentioned the condenser which will never be for 134A. I’ve had similar problems in the past with conversions and have even added an additional condenser fan. Sorry to say that didn’t really help that much. You may try though. Good Luck.

    Comment from Greg Mullins
    Time June 30, 2016 at 10:28 pm

    My 05 LeSabre cools fine when you first start up the car. If I stop and turn the car off rarely will start back up and compressor clutch doesn’t engage. Swapped relays no luck. Looking for any ideas??? Thanks

    Comment from Dennis Bandy
    Time July 1, 2016 at 7:13 am

    I’d test with a meter to see where the problem is when it’s acting up. Good Luck.

    Comment from rick
    Time July 1, 2016 at 6:30 pm

    hi i have a 1996 lexus es300 my ac blows cold air for only ten minutes then it blows warm air

    Comment from Dennis Bandy
    Time July 1, 2016 at 6:52 pm

    I’d check gauge readings to get an idea what’s going on. Could be many things like condenser fan not working, bad expansion valve, a restriction etc

    Comment from Carmen DeMatteo
    Time August 4, 2016 at 4:46 am

    I just installed a vintage air unit in my streetrod. The air blows cold but after about a half hour drive, there is water leaking from the evaporator unto the floor ( clear clean water). The drain tube is discharging properly, I checked to make sure the evaporator is level, checked low and high side lines, tested air temp at the firewall (below 80*) and have a manual shut off valve as well as an electronic one for the radiator coolant. When I turn the thermostat down, it leaks less, then starts again HELP!!

    Comment from Dennis Bandy
    Time August 4, 2016 at 7:53 am

    Carmen, sounds like a leak in the case plain and simple. The case may need to be removed and sealed, then re-installed. The colder the setting and higher the humidity, the more condensation accumulates. Good Luck.

    Comment from Doug
    Time August 11, 2016 at 12:12 pm

    I have a 2011 Ram 1500 that when I start it up and air blows normal but after about 15-30 seconds it shuts off (fan and everything) and slowly starts back up then runs normal from then on, till I shut if off then start it back up it starts all over. I have only noticed it doing this when its 85 or warmer outside. I’ve checked with a refrigerant gauge and it appeared to be good on refrigerant. What else could cause this? thanks.

    Comment from Dennis Bandy
    Time September 12, 2016 at 7:00 am

    Doug, I would let someone with a scan tool see if there’s any codes for climate control. Sounds like the system may be trying to re-calibrate actuators. But if there’s a problem there will be a code. Good Luck.

    Comment from Jon
    Time September 11, 2016 at 11:48 am

    2001 ford f-350 superduty, air conditioner blows cold when first starting out then after about an hour on the road starts to gain a hotter temperature than normal.

    Comment from Dennis Bandy
    Time September 12, 2016 at 7:02 am

    Jon, I would check gauge readings and engine temp when the problem occurs and go from there. Could be the engine temperature getting too high effecting the AC. You can get heat transfer from radiator to condenser if the engine is running too hot. Good Luck.

    Comment from Mark
    Time May 5, 2017 at 5:36 pm

    My 2005 Mercury Montego has had a recent compressor change( 2years ago) suddenly AC is erratic. Does not like to work in “Auto” setting, only on Manual setting, blows super cold on Manual

    Comment from Dennis Bandy
    Time May 5, 2017 at 6:21 pm

    Works OK on manual, wouldn’t make sense that there’s an issue with compressor. Would have to be checked. Good Luck.

    Comment from Ralph
    Time June 16, 2017 at 11:29 am

    stuck… since fixing the wiring to the clutch, it is engaging. blew colder for a minute when it engaged, now no cool air. have dual gauge manifold set. dynamic pressures are 30 and 175 OAT is 75 humidity low. trying to see if mixing door is closing but what else?

    Comment from Dennis Bandy
    Time June 16, 2017 at 11:44 am

    175 psi on high side seems low especially if that’s with engine raised to 1800-2000 RPMs.

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