DIY Auto Service: AC System Diagnosis by Symptom

The typical complaint for an AC problem is the AC blows warm air. This condition has a few possible causes: the AC system could be low on refrigerant, an electrical problem may not be allowing the compressor to turn on, there could be an internal problem in the system, or perhaps there is a problem with the heater/AC controls. Since the most common problem is usually the refrigerant, we will start there.

This series is divided into three parts. The AC system operation, AC system service, and the diagnosis of a malfunctioning system. To be able to service and repair an AC system, read through all three articles.

DIY Auto Service: Air Conditioning (AC) System Operation with TXV or Orifice Tube

DIY Auto Service; AC System Service and Component Replacement

DIY Auto Service; AC System Diagnosis by Symptom

There is a relationship between the pressure in the system and temperature. A temperature pressure chart can be used to compare the pressure of the refrigerant at a given temperature. Since there is no way to know exactly how much refrigerant is in the system, without removing it and measuring it, we will use the pressure and compare it to a pressure temperature chart.

• Hook up a gauge set or recovery machine.

• Record the pressures. High and Low Side should be equal.

• Measure the ambient temperature.

• Does the pressure match the temperature pressure chart?

  1. If the pressure matches this means the refrigerant level is close.

  2. If the pressure is lower than the chart, the refrigerant level is low. The system is probably leaking.

  3. If the pressure is zero, that is atmospheric pressure and means the system has a big leak.

  4. If the pressure is higher than the chart, it is either overcharged or may have air (non-condensable) in the system.

Start the engine and operate the AC system. Perform a visual check to see if the compressor is running.

1. Was a click heard when the AC was engaged? Is the center hub turning?

2. Observe the pressure gauges. Does the low side pressure start going down and the high side going up?

3. After the system runs for awhile, raise the RPMs up to about 1,500 RPMs.

4. The system should be about 25 to 30 psi on the low side and 200 to 250 psi on the high side. If the ambient temperature is higher or lower than normal (70 to 80°F) the pressures will go up or down accordingly.

No change in low side and high side pressures, the pressures are still equal.

• Double check to see if the compressor clutch is engaged. If the center hub is turning, the compressor is not pumping.

• Shut the engine off and rotate the center hub by hand. Is there any resistance?

The compressor may be faulty.

Higher than normal pressures on both high and low side with the correct amount of refrigerant, could mean a problem with air flow thru the condenser.

• Is the condenser or cooling fan working?

• Does the condenser have bent or dirty fins?

• Is there blockage inside the condenser?

Lower than normal pressures on both high and low side, with the correct amount of refrigerant, could mean a problem building pressure in the system or too much heat is being removed at the condenser. Is the ambient temperature low?

With the system running place a fender cover in front of the condenser and observe the pressure.

• Does the pressure go up? Can the compressor build the pressure? If not the compressor may be worn out.

• Is the cooling fan running constantly? A pressure switch or sensor could be bad.

This condition usually means there is a restriction in the system or the TXV/Orifice Tube is blocked or closed. Feel the lines, is there a pressure/temperature drop at the TXV/Orifice Tube?

• With a drop, there may be a restriction at the TXV or Orifice Tube.

• With no drop, there may be a restriction upstream from the TXV or Orifice Tube. Follow the liquid line back towards the condenser to see if you can feel a pressure/temperature drop.

Too much refrigerant is flowing into the evaporator.

• The TXV is stuck open or the thermal bulb is not sensing the temperature correctly.

• The orifice tube is too large of an opening or the o-ring sealing the outside of the orifice tube is not sealing.

Some times the refrigerant part of the system is not where the problem lies. The AC and Heater operate out of the same box or plenum typically under the dash. Doors control the direction of the airflow created by the electric blower motor. Depending on the system, there are at least three to four doors controlled by cables, vacuum actuators or electronic motors.

These doors are:

• Hot/Cold temperature door - controls the amount of airflow through the heater core. During AC (Cold) operation, this door may force the air to bypass the heater core. When the temperature lever is set to Hot, all the air is forced through the heater core to use the heat from the engine coolant to heat the cab.
• Fresh Air/Recirculate door - This door selects whether the air is drawn from the outside or circulate the air inside the vehicle. Recirculating the cooler drier air inside the passenger compartment can help maintain a cooler temperature instead of cooling the hot moist outside air. Typically some outside air is always added to keep the passenger compartment pressurized to prevent exhaust from entering the passenger compartment.
• Floor/Vent/Defrost door or doors - This door or a combination of doors controls where the cooled or heated air is sent. Floor mode typically for heater operation since heat rises. Vent out the center of the dash for AC operation. Top of the dash to defrost the windshield during cold weather. There may also be combinations of floor/vent or floor/defrost to accommodate different situations.

There are two basic designs for the airflow:

1. All the airflow goes thru both the evaporator then the heater core. A hot water valve closes the flow of hot coolant thru the heater core during the AC operation. If this valve doesn’t close, the AC cold air will be heated by the heater core. If the valve sticks closed, there will be no heat. The result is hot air out the vents. In the defrost mode, the air will be dehydrated by the evaporator then heated by the heater core for hot dry air on the windshield.

2. The airflow goes thru the evaporator and a door directs the air either thru the heater core or around the heater core directed by the HOT to COLD selector. During AC COLD operation, the “Blend Door” routes the cold air around the heater core. During Heat operation, all the air passes thru the Evaporator and the Heater Core. When the temperature is set in between, part of the air will pass thru the Heater Core. No hot water valve is needed because in cool mode the door closes off airflow thru the heater core.

When the Air passes thru the evaporator and heater core as in example 1 above, a hot water valve is used to shut off the HOT water to the heater core. These Hot Water Valves can be; Cable operated, Air Operated and Electric Solenoid operated.

To control the airflow thru the heater box or plenum a series of doors are used. To control these doors a variety of actuators have been used such as:

• Manual cable operated doors have been around a long time. Cables connect levers in the dash control to the doors. When a lever is moved back and forth, like the hot/cold lever, it moves the door from one position to another. Typically moving the lever quickly from one extreme to the other results in a thumping sound as the door hits the stops. The Hot/Cold door would control airflow around or thru the heater control. Cables popping off or improper adjustments are typically what will go wrong with this style of control.
• Vacuum operated actuators, used on cars and light duty trucks use (gas) engine vacuum to act upon a diaphragm which is connected to the doors. As the controls are moved, a hissing sound is heard. Vacuum leaks are the biggest problem with this type of control. If the vacuum supply is disconnected, the controls usually default to putting air on the windshield. Diesel engine light trucks using vacuum controlled systems use an electric or belt driven vacuum pump to supply the vacuum.
• Pneumatic actuators operate similar to the vacuum type, except the trucks air system pressure is used to move the actuator diaphragms. Hissing under the dash could be from a leaking actuator or connection.
• Electronic actuators use an electric motor to position the doors. These electric motors usually have a sensor built in to allow the system to know the location of the doors. When the controls are moved, a small electric motor buzz can usually be heard.

Service Tip: A common problem with all the actuator types are paper clips, pens, keys and other foreign objects falling down into the heater box from the dash defroster vents and jamming the doors. This could also strip out the door connection to the actuator rod.

Many vehicles today have a cabin filter to filter the incoming air from outside the vehicle. The filter looks like a small air filter from a car. Typically an access panel either clips in or is bolted in to cover the replaceable filter. The filter may be located under the dash or on the heater box in the engine compartment. This filter is often overlooked until it is entirely plugged up and causes very little airflow into the cab.

If the cabin filter is dirty the result will be low airflow. This may cause the AC or heater output to be reduced. Many times this is confused with poor AC or Heater performance due to the fact that the low airflow cannot keep the vehicle cool/warm.

The airflow across the evaporator is used for cooling the air. If dirt, fuzz, leaves or plastic bags are covering the surface, the airflow will be reduced along with the cooling capacity. The AC system may be in good working order but the air cannot have the heat removed as it should. The complaint will be lack of cooling the same condition as a plugged cabin filter.