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Air Conditioner Troubleshooting Guide & Diagnosis

Is your air conditioner A/C not working or blowing hot air? The Air Conditioner system can get very complicated in cars but that doesn’t mean that you can’t perform some troubleshooting yourself. Maybe even charge the A/C system if needed. This article will provide an overview of the A/C system, common problems and repairs that you can perform on your own. While the pictures on this article are of Mercedes Benz cars, this guide is written to apply to all makes and models.

Common A/C Problems
How the A/C System Works.
Troubleshooting the A/C
Common Air Conditioner Problems and Solutions
Technical Specs

Common A/C Problems


  •  A/C Blowing Warm Air
  •  No pressure or low refrigerant levels in the A/C System
  • Compressor Clutch
  •  Freon R134a leaks
  • A/C compressor not engaging.
  • Liquid lock of the compressor
  • A/C blowing hot air
  •  Compressors switches on then off very quickly.

How the A/C Works
The basics of the A/C System.

Mercedes Benz A/C Compressor

One of the key components of the A/C system is the compressor. Compressor’s purpose is to add pressure to the system by taking the refrigerant and compressing it. The operation of the A/C in your car is based on the First Law of Thermodynamics  if you wanted to learn the phycis behind it.   From the compressor the fluid goes to the condenser. By flowing through the condenser the refrigierant looses heat and from hot gas it converts into warm liquid.

Next in the system is the receiver-drier whose main purpose is to remove moisture from the A/C refrigerant. After that the refrigerant goes through the expansion valve. The pressure drops in the expansion valve and this allows the refrigerant to expand. Expansion of refrigant causes the temperature of the refrigerant to drop. This is the oposite process that the compressor performs. Put pressure into a fluid you will increase it’s temperature. Removeing pressure from fluid causes it to loose temperature. So in the expansion unit the refrigenart temperatures drop to levels below the outside temperature. From here the refrigerant goes to the evaporator.

Mercedes Benz Evaportor
Mercedes Benz Evaporator

The evaporator works in a similar way as the radiator except in this case instead of hot coolant there is cold refrigerant flowing through the evaporator. The low temperature refrigernat passes through the evaportaor which causes the evaportator to get cold. Air then blows thourgh the evaportator which makes it get cold.
This is the basic operation of the A/C system so let’s move on the some troubleshooting tips.


Step by Step

  1. Gather the right tools. You will need a Recharge Kit which you can purchase at the local auto parts store or online at Amazon. It has a gauge that you will at first use to test your A/C system.  We recommend the Arctic Freeze Recharge kit because it contains 2 large R134a refrigerant bottles. One of them contains UV dye which you may need if you have a leak and are trying to find it. ac_recharge_kit
  2. Turn the A/C ON. If you have an Economy button make sure that is off.  Make sure fan is on HIGH, temperature selector is turned all the way to COLD, and that the A/C and recirculate light indicators are on.
    • Set the  temperature to cold, the lowest setting.
    • Set the cooling fans on high on the climate control.ac control unit
  3. Check to see that the cooling fans on radiator are running when the air conditioning is on. Most cars are designed to have the radiator cooling fan kick in when the A/C is on.
  4. Check the compressor has engaged. Identify the location of your compressor. Look at the pullyer and you should be able to see if it spining or not. The engine running turn the A/C on max cool, set fans high, and make sure that the clutch is engaging on the compressor. The pulley spins any time the engine is running. The center part of the A/C pulley only engages when the A/C is on. If the clutch is engaging and disengaging rapidly, the refrigerant may be low.
  5. Locate the Low Pressure A/C recharge port. Attach the hose from the A/C Recharge Kit and check the pressure. Make sure the car is runing and the A/C is on while you get a reading.
  6. If the system is low then start recharging the system using the instructions that came with your recharge kit. Pay attention of the direction that you need to hold the can. Some kits ask that you keep the can upright, others ask to keep it upside down. Follow the A/C recharge kit recomenadtion to avoid putting air into the sytem.
  7. If the reading is moving towards the normal range continue until the system is charged.
    • If you are charging the system but the gauge is not moving then you may have a large leak. You will need to use charge the system with some dye so that you can find where the leak is.
    • If the system works but only for a short time, again you have a leak which you need to locate and repair.
    • If the system works for a few months then you have a very minor leak. You can introduce some dye on the next charge and see if you can locate teh leak.  We recommend the Arctic Freeze Recharge kit because it contains 2 large R134a refrigerant bottles. One of them contains UV dye which you may need if you have a leak and are trying to find it.
    • As you are charging the system you may notice the A/C sarts to work. The A/C system is designed to disenage the A/C if the pressure in the lines is to low. This happenes to protect the system. Compressor could be disengaged becasue the system does not have enough refrigerant pressure to trip the low pressure cutoff switch that cycles the compressor.
    • Be careful to not overcharge the system either. This could be dangeours and cause problems to your A/C system. You should be charging the system while you have the car runing.
    • If you have low pressure then check for leaks. Leaks are the most common problem associated with the air conditioning system. Charge the system via the low pressure port with a Leak Detection Kit like this so you can find the lea
      • Hopefully your system was just low and now it is working again. If not continue with these troubleshooting steps.
  8. Check the fuses. Look under the hood or under the dash to find the fuses. Find the A/C fuse and make sure that is not blown.
    mercedes fuses w203
  9. Next check clutch itself. The clutch may be bad. If there is voltage going to the clutch? a cycling switch may be bad, a fuse may be blown and the system may not have enough refrigerant pressure to trip the low pressure cutoff switch that cycles the compressor.
  10. Check the relays.
    fuses w203
  11. Check the operation of the climate control and the A/C button.
  12. Check the cabin air filter to ensure that it is not clogged.
  13. Check the blower fan. If you are not getting any air hot or cold then you may have a problem with the blower fan.



  • The dye check will not work if the system is too low and the compressor is not cycling.
  • There is also a high pressure service port. A high pressure gauge can be very usefull in further troubleshooting the system and is what the professionals use. The AC recharge kit that you have will not work, it actually doesn’t even fit on the high pressure port.
  • Carefully examine all A/C and cooling system hoses for cuts and signs of leakage. Leakage from A/C system hoses is often indicated by an accumulation of dirt and oil, particularly at connections and fittings.
  • Make sure the condenser (in front of the radiator) is free of any obstructions, such as leaves or insects. This could reduce airflow, resulting in reduced A/C performance. You can rinse the condenser clean with a garden hose.






Diagnosing problems with an air conditioning system can be very complicated. This section will cover some common problems associated with the A/C system.Always wear safety glasses and gloves. Keep all loose fitting clothing and body parts away from moving parts. For any problems not covered below, a professional mechanic or technician should be consulted.

The following is an general A/C system troubleshooting guide. Realize that it is generic and many of the things listed here may not apply to the 944.

 Common Air Conditioner Problems and Solutions

Symptom / Possible Cause
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. Check and replace replace valve
      3.  Check operation of suction vale. 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 vents
      4. 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.

AC System Gauge Readings

The following table is a 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.

A/C System Pressure Readings
Ambient Temperature
Low Side Pressure
High Side Pressure
Center Vent Temperature
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




  1. www.clarks-garage.com/shop-manual/hvac-02.htm
  2. aircondition.com/tech/questions/82/Troubleshooting-with-Gauges-FAQ
  3. www.aa1car.com/library/ac98.htm
  4. www.aa1car.com/library/automatic_climate_control.htm
  5. www.movacs.com/How%20it%20works/index.html


  1. I have 2009 Mercedes c300 I Drive to the store the AC works fine I stopped at the store come back out getting my car started up and the AC won’t come on any help

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