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Automobile Engineering

A/c System Leak Testing

Introduction:

Different leak detection methods should be applied under the appropriate conditions. The greatest source of refrigerant leakage is the compressor seal. Other leaks include Schrader valves, connector seals and flexible hoses. The universal drive (industry and legislation) to reduce the leak rates will lead to sealed compressors (electric) and possibly an all metal pipe work system.

Leak testing procedure

  • Different leak detection methods should be applied under the appropriate conditions. An oil stain test is only appropriate for R12 systems.
  • If a system has no refrigerant in the system at all then OFN pressure testing with bubble spray should be selected. Often the leak will be quite large and easy to find. If the system has a low residual pressure then UV test to find an appropriate area where the leak may have occurred.
  • Run the A/C system for a short period if possible and place the electronic leak detection (sniffer) tester around the system concentrating on areas where UV dye was found.
  • Vacuum testing is particularly useful during servicing and applying a deep vacuum for moisture removal is important.
  • Vacuum testing should never be used to test the correct fitment of components.
  • OFN should be applied to the system to ensure that the system is leak free and components and seals have been correctly applied during the repair procedure.

UV tracer dye

  • A leak detecting agent that mixes with the refrigerant is placed inside the A/C system. Because the refrigerant evaporates under atmospheric pressure, if a leak occurs the dye is left behind.
  • The dye is difficult to remove and is only visible under a UV (ultra violet lamp). The lamp is used in conjunction with PPE (Personal and Protective Equipment) and is a very useful method for detecting leaks. The dye is often placed in the system from manufacture so does not need to be initially added.
  • The more service operations carried out on the system will dilute the dye eventually requiring a fresh charge. A fresh charge is generally injected under vacuum into the low side of the system allowing it to be induced into the compressor where most of the A/C lubricant is stored.
  • Problems with using this method include old dye traces that have not been removed which give false indications of a leak. System component replacement can also cause dye to spread around the outside of an A/C system.
  • Once a leak has been repaired the system must be cleaned using dye removal fluid, removing all traces of the dye on the external surface of the A/C system.

Electronic leak detector (sniffer)

  • Electronic leak detectors are very useful in a system that still has refrigerant charge (e.g. 150 grams).When operating the detector the probe must be positioned at the highest point of the A/C circuit in an environment which is not drafty.
  • Because refrigerant is heavier than air the probe is then placed below connectors and across components to detect a leak working towards the lowest point.
  • Some detectors have audible and visual signal output. Once the detector has been switched on the sensitivity can be adjusted.
  • While the detector is on a constant frequency audible bleeping can be heard.
  • If a refrigerant leak is detected, and the gas concentration increases, this is signalled by a rise in the pitch and frequency of the audible bleep.
  • There are two types of electric leak detector, one for use only with the R12 system, and one that can be used with both the R134a and R12 systems.
  • Note, though, that the sensitivity level of the leak detector designed only for the R12 system is too low to be used for detecting leaks in the R134a system.

Oxygen-free nitrogen testing

If the A/C system is empty then OFN (Oxygen-Free Nitrogen) is a useful method of pressurizing an A/C system without damaging the environment. OFN is cheap and very easy to use and has a small molecular structure enabling easy leakage within an A/C system. The OFN is delivered via an A/C hose connected to a regulator and gauge. The system is pressurized up to 15 bar. While the system is being pressurized it is often useful to check the output of pressure switches and sensors to ensure they are operating correctly.

Vacuum testing

After the refrigerant has been recovered, to aid moisture removal or as a system pressure-test, an A/C system can be placed under vacuum. In a vacuum moisture boils and the pumping action of the vacuum pump helps to remove the moisture in the form of a vapour

Oil stains

An oil stain on a connection or joint indicates that refrigerant is leaking from that place. This is because the compressor oil mixed with the refrigerant escapes when refrigerant gas leaks out from the refrigeration circuit, causing an oil stain to form at the place where the refrigerant gas is leaking out. If such an oil stain is found, parts should be retightened or replaced as necessary to stop the gas leakage.