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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 overview the A/C system, common problems, and repairs you can perform independently. While the pictures in 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
  • The A/C compressor is not engaging.
  • Liquid lock of the compressor
  • A/C blowing hot air
  •  Compressors switch on and 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. The compressor’s purpose is to add pressure to the system by compressing the refrigerant. The operation of the A/C in your car is based on the First Law of Thermodynamics if you want to learn the physics behind it.   From the compressor, the fluid goes to the condenser. By flowing through the condenser, the refrigerant loses heat, which converts into a warm liquid from hot gas.

Next in the system is the receiver-drier, whose primary 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, allowing the refrigerant to expand. The expansion of the refrigerant causes the temperature of the refrigerant to drop. This is the opposite process that the compressor performs. Put pressure into a fluid you will increase its temperature. Removing pressure from fluid causes it to lose temperature. So in the expansion unit, the refrigerant temperatures drop below the outside temperature. From here, the refrigerant goes to the evaporator.

Mercedes Benz Evaportor
Mercedes Benz Evaporator

The evaporator works similarly to the radiator, except in this case, instead of hot coolant, there is cold refrigerant flowing through the evaporator. The low-temperature refrigerant passes through the evaporator, which causes the evaporator to get cold. The air then blows through the evaporator, which makes it get cold.
This is the essential operation of the A/C system, so let’s move on to some troubleshooting tips.


Step by Step

  1. Gather the right tools. You will need a Recharge Kit to purchase at the local auto parts store or online at Amazon. It has a gauge you will first use to test your A/C system.  We recommend the Arctic Freeze Recharge kit because it contains two large R134a refrigerant bottles. One 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 it is off.  Ensure the fan is high, the temperature selector is turned to COLD, and the A/C and recirculate light indicators are on.
    • Set the temperature to cold, the lowest setting.
    • Set the cooling fans on high climate control.ac control unit
  3. Check to see that the cooling fans on the 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 puller, and you should be able to see if it is spinning or not. The engine is running, turn the A/C on to max cool, set the fans high, and ensure 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 engages and disengages 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. Ensure the car is running 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 to the direction that you need to hold the can. Some kits ask that you keep the can upright, while others ask to keep it upside down. Follow the A/C recharge kit recommendation to avoid putting air into the system.
  7. If the reading moves toward the normal range, continue until the system is charged.
    • If you are charging the system, but the gauge is not moving, you may have a large leak. You will need to use charge the system with some dye to find where the leak is.
    • If the system works but only for a short time, again, you have a leak that you need to locate and repair.
    • You have a minor leak if the system works for a few months. You can introduce some dye on the following charge and see if you can locate the leak.  We recommend the Arctic Freeze Recharge kit because it contains two large R134a refrigerant bottles. One contains UV dye which you may need if you have a leak and are trying to find it.
    • As you charge the system, you may notice the A/C starts to work. The A/C system is designed to disengage the A/C if the pressure in the lines is too low. This happens to protect the system. The compressor could be disengaged because the system does not have enough refrigerant pressure to trip the low-pressure cutoff switch that cycles the compressor.
    • Be careful not to overcharge the system, either. This could be dangerous and cause problems to your A/C system. You should be charging the system while you have the car running.
    • 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 leak.
      • Hopefully, your system was low, and 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 it is not blown.
    mercedes fuses w203
  9. Next, check the clutch itself. The clutch may be bad. If Is there 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 hot or cold air, 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 useful in further troubleshooting the system and is what the professionals use. Your AC recharge kit will not work; it doesn’t 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.
  • Ensure the condenser (in front of the radiator) is free of 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. A professional mechanic or technician should be consulted for any problems not covered below.

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

 Common Air Conditioner Problems and Solutions

Symptom / Possible Cause
Low Compressor Discharge Pressure

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

    1. 1. Repair the leak in the system
    1. 2. Check and replace valve
    1. 3.  Check the operation of the suction vale. Open valve
    1. 4. Add freon
    1. 5. Replace the drier
    1. 6. Replace the valve
    1. 7. Replace reed valves
High Compressor Discharge Pressure

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

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

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

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

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

    1. 1. Tighten the valve
    1. 2. Remove some refrigerant
    1. 3. Replace the expansion valve
    1. 4. Replace reed valves
    1. 5. Replace the head gasket
Compressor Not Working

    1. 1. Broken belt
    1. 2. Broken clutch wire or no 12v power
    1. 3. Broken compressor piston
    1. 4. Bad thermostat
    1. 5. Bad clutch coil
    1. 6. Low Refrigerant – low-pressure switch has cut off clutch power

    1. 1. Replace the belt
    1. 2. Repair wire or check for power
    1. 3. Replace the compressor
    1. 4. Replace the thermostat
    1. 5. Replace the clutch coil
    1. 6. Add refrigerant
Evaporator Not Cooling

    1. 1. Frozen coil, switch set too high
    1. 2. Drive belt slipping
    1. 3. Hot air leaks into the car
    1. 4. Plugged receiver drier
    1. 5. Capillary tube broken
    1. 6. Shortage of refrigerant
    1. 7. High head pressure
    1. 8. Low suction pressure
    1. 9. High suction pressure
    1. 10. Defective expansion valve
    1. 11. Frozen expansion valve

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

    1. 1. Faulty thermostat
    1. 2. Thermostat is not set properly
    1. 3. Insufficient evaporator air

    1. 1. Replace the thermostat
    1. 2. Set to driving condition
    1. 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 the ambient outside temperature. Remember that these are a guideline, and your actual temperatures and pressures will vary depending on the 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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