How to Repair Automotive A/C

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

Repair
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

Repair
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

Repair
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

Repair
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

Repair
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

Repair
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

Repair
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)
-60.0----------------------------------21.6*
-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
-5.0-----------------------------------------4.2
0----- ------------------------------------- 6.5
5.0 ---------------------------------------- 9.2
10.0 ------------------------------------- 12.0
15.0--------------------------------------15.1
20.0 --------------------------------------18.5
25.0 ------------------------------------- 22.2
30.0 --------------------------------------26.1
35.0 --------------------------------------30.4
40.0---------------------------------------35.1
45.0---------------------------------------40.1
50.0 --------------------------------------45.5
55.0---------------------------------------51.2
60.0--------------------------------------- 57.4
65.0----------------------------------------64.1
70.0--------------------------------------- 71.1
75.0 ---------------------------------------78.7
80.0--------------------------------------- 86.7
85.0--------------------------------------- 95.3
90.0 ------------------------------------ 104.3
95.0--------------------------------------114.0
100.0 -------------------------------------124.2
105.0--------------------------------------135.0
110.0--------------------------------------146.4
115.0--------------------------------------158.4
120.0--------------------------------------171.2
125.0--------------------------------------184.6
130.0--------------------------------------198.7
135.0------------------------------------- 213.6
140.0--------------------------------------229.2
145.0 ------------------------------------- 245.5
150.0-------------------------------------- 262.9
155.0-------------------------------------- 281.1
160.0---------------------------------------300.0

• - (in Hg) Vacuum

Comments

[nlaurence]

sir, what would be the possible cause of unwanted odor entering the car through the A/C?

[bchenry]

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.

[ahooper]

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.

[Derin]

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

[Mr. Smart Kid]

relaceing gas type coolent into the ansophere is a goverment offence

[ivanbg]

Letting freon out into the atmosphere intentionally is a governmental offense. But if its leaking out of the system or when you clear the gauge hose its not an offense . If the government was so concerned about freon in the atmosphere they would sell it to the do it your selfer. Freon r-12 and 22 destroy ozone Freon r-134a is a green house gas. I would not have had to fix my a/c myself, if the shop had done there job correctly and fix all the leaks instead of just one and fill the system with freon. The law states that they have to fix a leak, but when you have an old car like mines due to vibration you could have many little leaks.

[GorillazMiko]

Wow... Actually a pretty detailed Instructable, nice job.