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Compressor lubrication tips

By Richard Hawkins, MACS contributor

Let’s talk about some compressor lubrication tips!

As indicated in last week’s blog article, questions about compressor lubrication are frequently encountered on A/C tech lines. Last week the topic was proper draining and filling of compressors which have drain plugs.

This week we will focus on lubrication information that is often encountered and how it can be interpreted incorrectly and result in the incorrect amount of oil being installed in a system.

When looking up OEM A/C system oil information in information systems, it is not unusual to find an example the following paragraph:

It is important to have the correct amount of refrigerant oil in an A/C system.  This insures proper lubrication of the A/C compressor.  Too much oil will affect the performance of the A/C system and result in elevated head pressures and higher vent temperatures. Too little oil will result in damage to the A/C compressor.

I agree 100%.

Then the paragraph might be followed with a chart like the following:

Now, this is where things start to get interesting, and confusion can result. The amount of oil that needs to be installed depends on the nature of the service work that is being done. We will use the following example which is frequently encountered in the aftermarket:

  • The vehicle has suffered a catastrophic compressor failure and compressor replacement is necessary.
  • It is a single expansion valve system and has a 24-ounce refrigerant capacity.
  • The condenser which contains a drier is being replaced.
  •  The remainder of the system is being flushed.
  • As a result, the system is dry, and the total system oil capacity needs to be installed in the compressor.

Unfortunately, the OEM does not provide the total system capacity as is sometimes the case. Let’s see what might happen if someone just proceeded to follow the instructions above without taking anything else into consideration.  (Those instructions are written assuming the system contains the correct amount of oil, and it is properly distributed throughout the system.) It is common for a compressor in a system like this to hold about 2.5 to 3 ounces of oil, so let us assume that 2.5 ounces is drained from the old compressor. Now let us do some math:

Compressor:                  2.5     oz

Condenser:                    1.0      oz

Evaporator:                   2.0      oz

A/C Receiver/drier:      1.0      oz

Total:                            6.5      oz

As indicated above, adding the numbers provided together for the components and the amount of oil drained from the old compressor equals 6.5 ounces, so one might assume that is the total system capacity.  But wait, don’t hose and lines contain some oil and they are not mentioned. Is 6.5 ounces really the total system capacity?

Now let’s look at a different scenario. Suppose only a thimble full of oil (or no oil) drains from the old compressor. That can easily happen, where a system might have the correct amount of oil in it but has been running with a low refrigerant charge and most of the oil has collected in the evaporator as a result.  

Installing a thimble full of oil (or no oil) in a new compressor is a recipe for disaster. The system would be dangerously short on oil and running a new compressor with little oil (or installing no oil) in it is asking for trouble.

Now suppose someone had been working on this system before it failed, and it had a gross overcharge of oil in it. Suppose six ounces of oil had drained from the compressor. Taking the six ounces and adding it to the other component amounts equals 10 ounces. That is way too much oil.

An experienced A/C technician will recognize that instructions like these cannot be applied to service work being done on a system where there has been a catastrophic compressor failure and the system is dry as a result of component replacement and flushing.

However, I can tell you from personal experience on an A/C tech line, that a lot of inexperienced technicians often take such information and run with it. As a result, they end up talking to the tech line because of the problems that result.

If the OEMs would provide the total system oil capacity and more specific lubrication information, many problems could be avoided.

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