Blue Print instruction on Air Con System Diagnostics | Part 3

This is the third in a series of articles written to provide technicians with an improved understanding of manual air conditioning systems and diagnostic procedures for efficient repair.

Early detection of trouble can result in prolonged component life as well as the customer not being bothered by unnecessary problems.

Before you commence with a full diagnostic procedure there are some basic checks which it is good practice to carry out.

Basic checks

Drive Belt

Inspect the drive belt for tension, wear and damage. A slipping belt can have a significant effect on the system operation as the compressor will not rotate at the desired speed. This could result in a lower system operating pressure. Equally, if the drive belt is slipping it is going to wear at an increased rate. This is going to result in early belt failure.

Compressor

Is the compressor noisy? A noisy compressor can indicate an internal fault within the compressor causing the system to malfunction. A full system flush and oil change should be the first plan of attack in this situation. If this has not quietened down the compressor it will be necessary to replace it.

Condenser fins

Visually inspect the condenser, if the condenser fins are clogged with dirt or bent over the ability of the condenser to cool the refrigerant will be greatly reduced. This will result in the refrigerant not being in a fully liquefied state as it exits the condenser resulting in reduced cooling at the evaporator.

Oil stains around joints and gaskets

Stains on joints and gaskets usually indicate a leak from the o rings or gasket failure. If refrigerant has been leaking out if the system the A/C pressures will be down, again resulting in a reduced cooling effect at the evaporator.

Check sight glass

Ideally you should not be able to see any bubbles at the sight glass. Bubbles indicate the refrigerant is not being fully liquefied by the condenser. The relevance of the sight glass has reduced since the introduction of R134a refrigerant. It is common for manufacturers to no longer integrate a sight glass into the system. If it is there however, use it, as it’s another useful tool in the diagnostic technicians’ armoury.

Performance testing

A useful test to ascertain the operating of the A/C system is to test it to see if it doing what it’s supposed to do in its basic form. This is simply to load up the system as much as possible and test to see if the air is actually cooling the air at the vents sufficiently. This is known as performance testing.

• A/C turned on and set to max cool
• Engine speed 1500 rpm
• Blower speed full
• Recirculate selected
• Doors open
• Centre face vents open

Place a thermometer (usually digital for accuracy and speed) at one of the face vents and measure the temperature. This isn’t a precise science but it would be normal to expect the temperature to be a maximum of 10 degrees C. The temperature at the vents will vary depending upon the temperature of the day. On a day with an ambient temperature of 15 – 20 degrees C it would be normal to expect the vent temp to be 6 – 8 degrees. If the temperature is above these example values it would be a fair assumption that the A/C system is not operating perfectly and further investigation is required.

Normal operation pressures

Note. These readings may vary due to the ambient temperature conditions and manufacturers specifications. It is always wise to cross reference with the manufacturers technical data.

Poor cooling – pressures low

Under these conditions, the pressure is low on both the low pressure and the high pressure sides. These readings would indicate that the liquid refrigerant evaporates before the evaporator and will therefore absorb the heat energy in the wrong place thus resulting in insufficient cooling performance. By far the most likely outcome of this instance will be a lack of refrigerant within the system. Before the system is re gassed a leak detection process should occur. Leak detection should be carried out in line with DEFRA guidance. The environmental protection act of 1990 states that the deliberate release of refrigerant is banned. Oxygen free nitrogen pressure testing is now widely recommended.

Poor cooling – pressures high

Under these conditions the pressure is too high on both the low pressure side and the high pressure sides. These readings could indicate a selection of faults. Firstly there could be too much refrigerant in the system. This will lead to the refrigerant evaporating after the evaporator and will therefore absorb the heat energy in the wrong place thus resulting in insufficient cooling performance. An additional possible cause of this can be air in the system. This can result in the pressures being too high which will result in evaporation of the refrigerant occurring in the wrong location in the system. The final common possibility could be that the condenser is not managing to cool the refrigerant sufficiently. This could be caused by the cooling fan not working or the condenser being blocked / fins bent over.

Poor refrigerant flow – Does not cool / cools intermittently

Under these conditions, a vacuum is indicated on the low pressure side. Low pressure is indicated on the high pressure side. In most cases the refrigerant flow is being obstructed as moisture turns to ice at the expansion valve. This causes a temporary blockage resulting in the pressure in the high side fluctuating and the pressure in the low pressure side dropping due to a restricted supply to the compressor. There should not be any moisture in the system and it can be a clear indication that the dryer needs to be replaced along with the necessity to thoroughly vacuum down the system and recharge.

When working with air conditioning systems please be aware of safe working practices. Always wear appropriate personal protection and never vent refrigerant into the atmosphere. Leaks should always be detected using a DEFRA approved procedure.