Control Valve Parrafin 1

DENSO: Air Conditioning best practice

Oil contamination is a major problem for air conditioning systems, so how should the condition of the oil/refrigerant mixture be assessed?

Air conditioning (AC) is now a common vehicle feature, which means that workshops not only need to be able to tackle AC service and repair with confidence, but also understand some of the problems that can arise if replacement AC component installation procedures are not followed.

One of the processes that is often overlooked is assessing the condition of the oil/refrigerant mixture in the system, which is particularly important should the workshop not know the service history of the vehicle or have previously serviced or repaired its AC system.

As well as helping when diagnosing other potential faults, understanding the condition of the oil/refrigerant mixture also provides the technician a base line from which to make subsequent decisions concerning the overall system.

The assessment process

The process begins by using a sight glass, which really is a key piece of equipment for AC service and repair, as it allows technicians to view the oil/refrigerant mixture in the system and make an accurate assessment of its condition and crucially, can prevent them from inadvertently damaging an expensive AC service machine with contaminated oil and refrigerant.

What to look for

The ideal outcome of the oil/refrigerant assessment is to see a clear, transparent mixture, as this means the refrigerant and the oil are perfectly mixed and in good condition. Alternatively, if it’s greenish yellow with a similar consistency, it is the same, but with UV-dye in the mix, which would have previously been added to the mixture to show if there were any leaks from the system when looked at with an ultraviolet light. However, the discovery of UV-dye has implications.

Be careful of UV-dye

UV-dye should only be added to the mixture once, it must be SAE approved, and limited to three to five cc for systems up to 1,000grams of refrigerant. Higher levels of dye will reduce the lubrication properties of the oil, which can lead to problems with other components within the AC system, including compressor failure.

Another thing to be careful of is the fact that some AC refilling machines are set to add UV-dye automatically, which is why the sight glass test is so crucial to begin with. Also, it’s important to know that some car manufacturers, such as BMW and Toyota, do not allow the use of UV-dye during the vehicle’s warranty period.

Negative outcomes

If the mixture is light grey, which indicates that although the refrigerant and oil combination is right, there is too much moisture in the mix, it means that as well as the system having to be flushed, it will also require the receiver-drier, or filter element of the subcool condenser, to be replaced.

If the mixture is clear, but has two different layers, it means the oil and refrigerant are not mixed. This is because despite what some suppliers might suggest, universal PAO oil is not compatible with R134a refrigerant, as the two do not mix properly. In addition, PAO oil must never be used in a DENSO compressor.

If the mixture has a light and milky appearance, but with layers, two types of oil have been used in the system. So, although the refrigerant has mixed with the correct, original PAG oil, the PAO oil is separated.

If it’s with layers, but cloudy in appearance, it’s the same problem as the previous example, but with the additional problem of too much moisture, which makes things worse, because as well as not being able to mix, it creates paraffin particles that can clog the control valve, for example, and destroy the compressor.

Should the mixture contain black particles it’s likely to be rubber residue from the discharge hose, which can, over time, deteriorate and break down, causing it to enter the system and contaminate the mixture.

If when working on electric vehicles, hybrids and vehicles with electric driven scroll compressors, which should use DENSO ND-oil 11, the appearance is milky and although apparently mixed, it’s likely to be contaminated with a PAG oil. ND-oil 8 and ND-oil 12 (PAG) should not be used together with ND-oil 11 (POE) because although the refrigerant mixes with both, the two different oil types do not. In addition, if ND-oil 8 or ND-oil 12 is used in an electric driven scroll compressor lubricated by ND-oil 11, it can create a short circuit in the electric motor of the scroll compressor.

There are two other examples that are worth highlighting, one relatively common, the other, fortunately, less so.

If the oil is black, it indicates that there was a previous problem that meant the system had to be flushed to remove residue before it was refilled. However, it was clearly not flushed properly, so some dirty, burned oil remained behind to contaminate the new mixture, which means it will need to be flushed again.

Finally, if you see residue around joints or connections in the AC system, be wary because it indicates that leak stop has been used in the system. However, leak stop should never be used in an AC system because when it’s exposed to air or moisture it becomes a solid.

So, if evidence of its use is found, do not connect it to an AC filling machine because, as well as it being impossible to remove from the vehicle’s AC system, which will need to be replaced, it will contaminate and ruin the AC refilling machine.


After establishing the condition of the mixture in the system, technicians can make correct decisions in relation to the service or repair of the rest of the AC system. If however, the mixture does need to be replaced and flushing is required, two things need to be kept in mind: first, the quantity of oil to be put back into the AC system, which must precisely match the vehicle manufacturer’s specification and second, that it’s likely that other components in the system will also need to be replaced and the correct installation procedure has to be followed to prevent the subsequent failure of these parts.