How to Repair Automotive A/C




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Step 1: Freon Check

You will need a set of manifold gauges to check you freon pressure.

Here are some general guideline for A/C system pressures and temperatures based on ambient outside temperature. Remember that these are a guideline and your actual temperatures and pressures will vary depending on humidity in the air and the condition of your system. When running the car at idle to check the freon level it is a good idea to have a water hose handy and spray water on the a/c condencer. The condencer is located in front of the radiator.

For vehicles currently running with freon R-134a only
Outside-----low side-------High side------Center vent temp
60 F -----28-38 psi----130-190 psi ----44-46 F
70 F ------30-40 psi ---190-220 psi ----44-48 F
80 F ------30-40 psi ---190-220 psi ----43-48 F
90 F ------35-40 psi ---190-225 psi ----44-50 F
100 F -----40-50 psi ---200-250 psi ----52-60 F
110 F -----50-60 psi ---250-300 psi ----68-74 F
120 F -----55-65 psi ---320-350 psi ----70-75 F
You can also use this good rule of thumb with R134a, run fan on High with all windows open and engine at 1500 rpm. High side pressure in PSI should be about 2.2 to 2.5 times outside air temperature in degrees F. Example if it's 90 degrees F, high side should be no higher than 225PSI

Here is a useful trouble shooting chart for systems currently running on freon R-134a

Trouble shooting

Low Compressor Discharge Pressure
1. Leak in system 2. Defective expansion valve 3. Suction valve closed 4. Freon shortage 5. Plugged receiver drier 6. Compressor suction valve leaking 7. Bad reed valves in compressor

1. Repair leak in system 2. Replace valve 3. Open valve 4. Add freon 5. Replace drier 6. Replace valve 7. Replace reed valves

High Compressor Discharge Pressure
1. Air in system 2. Clogged condenser 3. Discharge valve closed 4. Overcharged system 5. Insufficient condenser air 6. Loose fan belt 7. Condenser not centered on fan or too far from radiator

1. Recharge system 2. Clean condenser 3. Open valve 4. Remove some refrigerant 5. Install large fan 6. Tighten fan belt 7. Center and check distance

Low Suction Pressure
1. Refrigerant shortage 2. Worn compressor piston 3. Compressor head gasket leaking 4. Kinked or flattened hose 5. Compressor suction valve leaking 6. Moisture in system 7. Trash in expansion valve or screen

1. Add refrigerant 2. Replace compressor 3. Replace head gasket 4. Replace hose 5. Change valve plate 6. Replace drier 7. Replace drier

High Suction Pressure
1. Loose expansion valve 2. Overcharged system 3. Expansion valve stuck open 4. Compressor reed valves 5. Leaking head gasket on compressor

1. Tighten valve 2. Remove some refrigerant 3. Replace expansion valve 4. Replace reed valves 5. Replace head gasket

Compressor Not Working
1. Broken belt 2. Broken clutch wire or no 12v power 3. Broken compressor piston 4. Bad thermostat 5. Bad clutch coil 6. Low Refrigerant - low pressure switch has cut off clutch power

1. Replace belt 2. Repair wire or check for power 3. Replace compressor 4. Replace thermostat 5. Replace clutch coil 6. Add refrigerant

Evaporator Not Cooling
1. Frozen coil, switch set too high 2. Drive belt slipping 3. Hot air leaks into car 4. Plugged receiver drier 5. Capillary tube broken 6. Shortage of refrigerant 7. High head pressure 8. Low suction pressure 9. High suction pressure 10. Defective expansion valve 11. Frozen expansion valve

1. Turn thermostat switch back 2. Tighten belt 3. Check for holes or open vents4. Replace drier 5. Replace expansion valve 6. Add refrigerant 7. See problem #2 8. See problem #3 9. See problem #4 10. Replace expansion valve 11. Evacuate and replace drier

Frozen Evaporator Coil
1. Faulty thermostat 2. Thermostat not set properly 3. Insufficient evaporator air

1. Replace thermostat 2. Set to driving condition 3. Check for excessive duct hose length, kink or bend.

If you can't get your a/c to turn on then use this chart to determine if you are low on freon. Remember that this is a general char and will very for different makes and model cars and refrigeration units

Temperature -------------------134a Pressure(Psig)
-55.0 -----------------------------------20.1*
-50.0 ---------------------------------- 18.5*
-45.0 ----------------------------------- 16.7*
-40.0 ----------------------------------- 14.6*
-35.0 ----------------------------------- 12.3*
-30.0 ----------------------------------- 9.7*
-25.0 ------------------------------------ 6.7*
-20.0 ------------------------------------ 3.5*
-15.0 ------------------------------------ 0.1
-10.0 ----------------------------------- 2.0
0----- ------------------------------------- 6.5
5.0 ---------------------------------------- 9.2
10.0 ------------------------------------- 12.0
20.0 --------------------------------------18.5
25.0 ------------------------------------- 22.2
30.0 --------------------------------------26.1
35.0 --------------------------------------30.4
50.0 --------------------------------------45.5
60.0--------------------------------------- 57.4
70.0--------------------------------------- 71.1
75.0 ---------------------------------------78.7
80.0--------------------------------------- 86.7
85.0--------------------------------------- 95.3
90.0 ------------------------------------ 104.3
100.0 -------------------------------------124.2
135.0------------------------------------- 213.6
145.0 ------------------------------------- 245.5
150.0-------------------------------------- 262.9
155.0-------------------------------------- 281.1

  • - (in Hg) Vacuum

Step 2: Expantion Valve and Flushing

If you decide that the A/C system needs to be flushed in order to remove debris and oils you will need to remove the compressor, expansion valve, and the drier. DOT NOT FLUSH THROUGH THESE COMPONENTS. Also if you are retrofitting your system from R-12 you will need to flush out all the old mineral oil as oils tend not to be compatible with each other. Mixing oils or refrigerants will result to what is reffed to as as black death by mechanics.

The pictures below cover the most difficult part which is the get to the expansion valve. Once you have the evaporator out the car you may find it to be very dirty. I cleaned mines in the shower, remember to be really careful not to get water in the pipes which freon runes through. If you already spent all that time getting it out the car its not a bad idea to replace the expantion valve and do the be best you can to wrap it in sum thermo insulation material. I used some packaging material and tape to hold it in place. (Now that I think about it maybe glue would have been a better idea)

The flushing proses I will not cover in detail, but all that you need to do is to spray a A/C flushing compound it the freon pipes. Then wait 20 min and flush it out with shop air. I was fortunate enough to be able to use the shop compressor at my school. If you use a potable one it most likely will not be able to keep up with demand and you will have to wait for the tank to refill.

Step 3: Compressor

If your compressor is leaking freon from the central o-ring seal or making clicking noises then its probably time to replace it. The replacement is straight forward, I didn't find any surprises when I replaced mine. One thing to keep in mind is to add oil to the suction side and rotate the clutch 15 to 20 times to make sure the oil clears. Also when installing the compressor be sure to tighten the four bolts evenly in a circular pattern to prevent the compressor case from being deformed. And when installing new seals for the freon pipe be sure to rub a little bit of oil on them. Below are pictures of an my oil compressor.

If you are returning you compressor to be refurbished be sure to wrap the oily compressor in some newspapers so the box can be reused. This not only keeps the price down, but also creates less paper wastes.

Step 4: Vacuuming

It is important to change the drier each time the a/c system has been opened or loss of pressure has occurred. Since the drier obsorbs moister that may have entered the the system. When you installing a new drier make sure to drain the oil from the old one and place the same amount of new oil in the new drier. This should be done right before vacuuming, since the drier will become useless ofter two hour or so of being in the open.

Now it is time to pull a vacuum. If you are using a general purpose vacuum pump like I did and the fitting don't match the manifold gauge hoses you should go to you local a/c supply store, and the most likely will find you the right fitting for under a few dollars. Electric pumps sell for a couple of hundred dollars, so if you already own an air compressor you might what to look into an air powered vacuum pump for under fifty dollars.

Once you have to vacuum pump connected, you will need to evacuate moister and air out of the system. It is recommended that this be done when the temperature is above 80 degrees. So try not to be like me and do it in the middle of winter, if possible. Once a vacuum pump has reached nearly 30 in/Hg ( inches of mercury) let it run for a 20 min and then turn it off. If the vacuum drops a little bit that means that some water has evaporated so just turn the pump on to pull it out. Then let the vacuum sit for a minimum of 45min. If the pressure can not kept then that means that there is a leak some where in the system. Be sure to repair it before filling up the system.

Step 5: Filling the System

Once you are sure that the system can hold a vacuum for an hour you may proceed to filling the system with Freon. If your car ran on Freon R-12 from the factory and you have the means to purchases it you can use R-12 or Freeze 12 to get maximum performance. If you are not certified then you have the choose Enviro Safe, R-134a and I have heard that propane can also be used. I chose to use R-134a for my retrofit. You probably will not notice difference between using R-134a instead of R-12 if you do not live in Death Valley California.

Once you have the can of Freon hooked up to the service line of the manifold gauge (middle or yellow), you should purge the air from the hose. If you do not have a purge valve then turn the can upside down and unscrew the hose a little bit until air starts to come out and as soon as liquid starts to come out - tight it. When refilling the system, I would recommend that you wear goggles. Once the air is removed you can open the low side and start filling. NEVER fill through the high pressure side as tough this may cause the can to explode. If you are adding a can of Freon with UV dye or oil you need to have the can upside down. This will cause liquid instead of gas to come out; you should open the low side valve just a little bit in order to have a little bit flowing. Be careful not to get liquid inside the compressor; as liquid is not compressible and can cause damage to the compressor. If you are just adding a can of Freon and nothing else than keep the can right side up and the valve fully open. If you want to speed up the refill process then dump the can in some warm water or heat it with a hair drier. Do not use heat guns, gas burners or boiling hot water. Once there is some pressure some pressure in the system, turn on the engine and the a/c fan to max.

Now it is time to determine when to stop filling the system. I find that the best way is to fill the system until it comes close to the chart above chart. Because every car is a little bit different you will need to find the best pressure for your vehicle and to do that you will need helper and digital cooking thermometer. What did was I had my brother in the car looking at the thermometer which is in the center vent. When the pressure in the manifold gauges is around 30 psi away from the chart value, I start to let in less Freon and I wait until my brother says to stop. What my bother is doing is watching until the temperature stops dropping or starts to rise and says stop.

System top off

If you are just adding a little bit of freon to the system and you don't want to purchase an expensive manifold gauge, then you can use the simple hose that comes with no gauge. The way to determine when the system is full is to look at the sight glass until you see liquid with no bubbles or no foam appears. Another way is to add freon until the low side line under the hood starts to have water condensation. The last way is to use the center vent temperature method mentioned above.

Step 6: Leak Checking

Leak checking can be done with a leak checking device. You will need to hold the device up to any connection, compressor, condenser and the drier.
Anywhere there is oil visible on the A/C line could be a leak. When checking the condenser, take your time and check all the rock chips. When checking the service valves do not push the sniffer directly down wards because it can push the valve down and give you a false reading.
If you cannot find a leak you can always turn up the sensitivity on the leak detector.
Unfortunatly there is no way to check the evaporator for leaks.

Step 7: Useful Tools

Here are two tools that can be useful when servicing air conditioners.

The firsts one is a valve removal and installation tool. The difference between this tool and a tire tool is that this one is longer so it can reach deep down in the A/C port.

The second on is used to straighten heat exchangers fins.

Step 8:

Here are some links with extra explanation:

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    22 Discussions


    3 years ago

    I believe I have moisture in my car's air-conditioning system. We put freon in and now it will blow freezing air for about 5 minutes & then blow hot air. How do I fix this?

    1 reply

    Reply 5 months ago

    You are most likely correct with that diagnoses. I will try to explain this as simply as possible. If it has moisture it was introduced in one of two ways. Either all the Freon leaked out and someone just added Freon or someone added Freon and did not purge the air out of the hoses. 1) If it loss all the Freon it has a leak and you CAN NOT just vacuum the system and recharge. The reason is when you try to vacuum it will be constantly pulling in air as you are vacuuming. The air contains moisture. Solution : Repair the leak, Evacuate, and recharge the system.
    2) If someone added freon and did not properly purge air from the hoses that would introduce moisture. But here is the problem : Why would someone add Freon if there wasn't a leak. Answer: they wouldn't unless they didn't know what they were doing.
    Let be sum up something that driveway mechanics don't know and you tube videos don't tell you. FREON (Refrigerant) never wears out or loses it's chemical compound. If some try's to tell you they are going to put new Freon in your car because the refrigerant is old you should run as fast as you can. Freon, because it is sealed in the system will last NEVER goes bad and will last thousands of years which is why they banned certain refrigerants because they will get released into the atmosphere and stay there and supposedly ruin the ozone!!!


    1 year ago

    Most times when an A/C fails it's a blown capacitor. How do I replace that in a car?

    1 reply

    Reply 5 months ago

    That doesn't apply to automotive AC. The motor (compressor) in a auto does not have a capacitor. Most times when auto AC fails it is because it has a leak. The leak must be repaired. Everyone thinks they can JUST ADD FREON!!! That's where the problem begins.


    Question 1 year ago on Introduction

    I got an o-ring/gasket set for my ac lines in my car. I want instructions on how to change them correctly. My car is a 2006 Chrysler 300 touring awd with the 3.5 L engine. It’s showing pressure loss from the machine the mechanic used. I’ve tried to refill it with Freon, it blows cold air for 10 minutes then hot air again. It goes empty with Freon within 10 minutes. It’s not the compressor. I had a new compressor installed last year. My uncle who is an auto mechanic said it could be the compressor relief valve being stuck open. I got a set of o-rings and gaskets. That was a small price, I’ll try that first. My uncle said it would be rare for the o-rings or gaskets to go bad for no reason at all.


    4 years ago

    Great instruct able but a few things are off.

    A vacuum should be pulled to under 500 microns measured with a micron gauge. Without it your not sure if there is still moisture present in the system or the pump is capable of going that low to get all your non-condensables out.

    Leak testing with a vacuum doesn't mean there is not leak. At most a deep vacuum is only putting 30psi pressure on the outside of the pipe pushing in, and dirt can get sucked into a crevice temporarily stopping any noticeable leaks. R134a can achieve what like a little under 200 psi? If possible use dry nitrogen and fill to 150 psi then wait to see if it leaks. Don't use air and definitely don't use oxygen unless you like explosions.

    1 reply

    Reply 3 years ago

    Yes, you are correct that the best way to check leaks is with 150 to 200 lbs dry nitrogen and wait at least an hour with no pressure drop.

    Checking with a vacuum is even less effective than you state; at sea level the most pressure you will get pushing in from outside is atmospheric pressure, about 14.7psi :)


    3 years ago

    We got the 12v socket car use air cooling fan from , it helps so much that now we are OK to keep the weak air conditioner now for the summer.


    4 years ago

    Thank you for the very extensive instructable sir. That was probably hard work. Your detail is much appreciated.


    6 years ago on Step 8

    I have a 2005 chrysler town and country the ac compressor has a whinning noise, when you start the car.Even when the the ac is off.

    1 reply

    Reply 5 years ago on Introduction

    its your belt maybe its time to replace it maybe a little bit loose but this almost never happened I fix mine with w40 just spray it to the belt inside and out


    5 years ago on Introduction

    Hi I have a ute and the AC works for a short time like 10 / 20 mins then stops, any clues? It isn't freezing up.




    6 years ago on Step 8

    Has a new compressor on it.(the car)


    Reply 7 years ago on Introduction

    The smell is coming from the evaporator. The smell is a problem on older cars, that is why newer vehicles come with a cabin air filter to prevent dust and other things from getting into the evaporator and decomposing.

    They do make products to try and clean it without disassembling the dash. Sometimes they work and most of the time they don't.

    The bast way to cure the smell is to take dash apart and clean the evaporator with bleach or some type of recommended coil cleaner.


    7 years ago on Introduction

    Using propane and other flammable refrigerants is illegal in many states and really upsets those who do it professionally, as it contaminates very expensive equipment. Also evaporator leaks can be found with an electronic leak detector at the evaporator drain or by taking out the blower motor resistor and checking through there (dye often will show up there as well).
    Recovering hoses is also required by law if the hose is longer than 1foot.
    Also the EPA license to buy R12 is really easy, its an open book test and can be taken through ASE.


    7 years ago on Introduction

    A good friend of mine worked for a refrigeration company and after talking with the engineeers replaced the gas in his AC unit with Propane. worked pretty well apparently and real cheap.


    11 years ago on Introduction

    here is how mechanics find leaks: 1 fill system with nitrogen 2 spray foam all over engine(they did that) if it bubbles you got a leak 3close the leak 4 retrieve freon,seperate nitrogen 5 fill old freon back,add more if necessary