AC Fault Diagnosis

For the vast majority of cars our first check is to ascertain the pressure of refrigerant in the system. If this appears too little for what we know about the car then we start investigating with electronic leak detector and UV lamp to check that there are no leaks in the system. If pressures are reasonable when static we then start to check pressures under operating conditions to see if the compressor is doing its job and any condenser fans are working correctly.

If the fault has obvious ties to low refrigerant pressure and thus a leak we have a number of tools in our armoury. We usually start with an electronic leak detector if that route seems appropriate to the technician. Each van carries a minimum of three different types each with their own advantages. We also may use a dye used in conjunction with UV light. Many leaks can be found using compressed Nitrogen gas. For exceptionally small refrigerant leaks we also have a detections system that involves the detection of Hydrogen, the smallest molecule in the universe in order to find what is able to wriggle through the tiniest gap. Used under the maximum pressure allowed within the whole AC system this is our best bet to find tiny leaks immediately, before any dye has become visible. Sometimes however the vehicle just needs to run for a few days under normal running conditions when the UV dye will show just where the refrigerant is able to escape.

The complexity of modern car air conditioning or climate control can sometimes result in a failing AC system that will not respond to just a routine vacuum and recharge service. In these circumstances the requirement is for good observational skills to detect operational abnormalities, extensive vehicle data files to act as a reference point, a wealth of AC experience and an analytical brain to utilise these factors and come up with the solution.
Our first response to an electrical fault is a Power Probe and a Digital Multimeter. Increasingly however electronics rather than just electrics are responsible for many problems and additional tools have to be be employed. Electronic Diagnostic Tools, able to communicate with the onboard computers controlling much of the cars operation are necessary for virtually all post 2000 vehicles.
These can identify malfunctioning parts and can remove Fault Codes generated after the faulty part has been replaced. These Diagnostic Tools interrogate the small network of computers that control most operations in modern vehicles and which generate a DTC, a Fault Code, to signal that some component is not doing what it is supposed to. An example of this might be that a small electric motor, designed to turn a flap which controls the mixture of hot and cold air entering the cabin which regulates the drivers temperature might have seized up. A Fault Code is now generated to signal to the technician which component is faulty and needs replacing. Fault Codes are not always generated however and in these cases we have to dig deeper to find the problem.
We also use a Multi-channel Oscilloscope to observe the actual waveform generated by whatever control module in the car we are investigating. To complement that we have variable width squarewave generators to simulate the action of the AC Control Module as it controls the latest generation of AC compressors or stepper motors. In the hands of an experienced technician these pieces of specialised test equipment are invaluble and can help diagnose most problems but if the brain behind the tool is inexperienced the outcome may be disappointing.

In the process of investigating AC faults our High Tech equipment can sometimes reveal other related Fault Codes and sometimes Fault Codes completely unrelated to the AC system can be detected which we are often able to correct. For example a problem with a rear door window motor in the past may have generated a Fault Code which may prevent the central locking system working correctly. This may be identified in the process of investigating an AC fault and can be corrected simultaneously.

If you have an aircon problem arise unexpectedly then think back to see if some other work on the car could have caused it. We have come across several faulty AC systems which were the direct result of a radio being installed by someone with insufficient experience and causing serious problems to the aircon system. I would go as far as to say that if you have a modern car, say less than 10 years old, it might be worth delaying a radio installation until some really warm weather arrives so that if a problem with the AC is created it is immediately obvious and can be cured easily. There are many adverts around at-the-moment trying to convince all UK listeners that DAB radio is superior to FM broadcasts on VHF. As many in the population are not convinced of this, the take-up of DAB radio is currently only a small fraction of the number of FM and AM radios in use and the changeover to DAB is likely to delayed by some years. Just in case you may ask, No we do not install radios - we just specialise in vehicle AC.

To be frank, some of our most testing electrical problems have been very low tech - they have sometimes been simply the result of a bad connection, possibly badly corroded or even a wire actually broken. Mostly we can find these but occasionally we are forced to refer these to another specialist, an auto electrician.

As an example of the complexity of fault finding then consider this: in July 2016 a nice Fiat Scudo was seen. This is the Fiat version of the Citroen Dispatch and the Peugeot Expert. The owner had owned the car only for a limited time and the AC had never worked in that time. We diagnosed that the reason the AC was being automatically switched off was that the Exterior Temperature Sensor was reading a temperature of lower than zero degrees C. The temperature sensor on these cars is mounted in the driver's door mirror. What the owner did not know was that at some time before his ownership the previous owner had replaced the broken mirror with an identical looking mirror from a Peugeot or a Citroen but which has a different temperature sensor to that fitted by Fiat.

The New Refrigerant HFO1234yf

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