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COMPRESSOR OILS - THE NEVER ENDING SAGA

Okay, you've heard that the new refrigerant (HFO-1234yf) will be used with a special formulation of PAG oil that will require some additional space on your storeroom shelves. Not unexpected, and nothing you have to really worry about right now.

But we're still seeing issues with PAG oils in the compressors we have in service now, and interestingly enough, with some of our oldest compressors. Maybe that shouldn't be surprising, because they were made with more generous tolerances and required more viscous oils. And because they went into vehicles that are used for decades, including construction and other off-highway equipment and commercial vehicles, those compressors are more likely to have been CFC-12 units, in some cases retrofitted to HFC-134a. 

Particularly since Y2K, GM has been using a wide variety of compressors from many sources, not just the Delphi units that were exclusive to GM during the period Delphi was a GM division. As a result, the specified oils varied according to compressor supplier, until GM said some seven years ago that a lightweight PAG it wells (55-65 Centistoke) could be used as a top-up oil on all new late-models, including Delphi compressors. However, Delphi, now an independent supplier, had always said although the lightweight PAG is okay for its new ("CVC") compressors, it wants a heavyweight PAG (125-150 Cst) in the V-5, V-7, H-6, and Radial-4 compressors, which are older in design. 

Several of the older design General Motors/Delphi compressors spanned the change in refrigerants from CFC-12 to HFC134a. It may be hard to believe, because we see so many vehicles with them, but those Radial-4 compressors haven't been installed on a new model since 1996. 

But the "golden oldie" is the GM A6, the 37-lb. cast iron behemoth that was used on passenger cars starting in the early 1960s, and through the 1970's. It even went through 1980's on some vehicles, and still today, an updated version is made by Alma Products for OE installation on heavy duty applications (and of course, it is available for service under the APCOAir label).

Alma (and companies that distribute the compressor, like Red Dot Corp.) sell the A6 for both CFC-12 and for systems retrofitted to HFC-134a, either with a full fill of mineral oil (10 oz) or just a small amount of assembly oil. So if it's used in a HFC-134a system, it requires a heavyweight PAG, and Alma provides specifications for its recommended PAG 125 oil. The specs are basically those typical for the industry, i.e. 24.65 Cst at 100 degrees C./212 degrees F., a viscosity index of 221 and a pour point of minus 42 degrees F., although the additive package is a specific formulation, according to Alma. 

It might seem that a PAG 150 would be even heavier, and it is at 40 degreesC./104 degrees F. - with a viscosity of just over  150 at at that temperature. But it's actually  a bit lighter (under 18 Cst) at 100C/212F. However, heavyweight PAGs actually may be rated somewhere between those specs and still do a satisfactory job  of lubricating those older compressors.

A MACS member Pennsylvania machine shop that had become a heavy duty A/C specialist, however, found that a basic verification of the oil it was using would have saved it a lot of grief. During a relatively short but busy period, it replaced eight A6 compressors with Alma APCOAir A6s, and they all failed quickly. The compressors which were factory-filled with mineral oil because the original vehicle application was CFC-12, were drained first and a fill of PAG oil installed. The shop was installing from a container labeled as PAG 150 oil, from a marketer whose products it long has used, and without a prior issue. However, the shop kept checking and rechecking every step in its service procedures, and it couldn't see anything wrong, so when MACS director of training Paul DeGuiseppi suggested that the problem sounded like oil, the shop got a laboratory test, and found the viscosity was only 5.27 Cst.

That explained the failures, but why would the oil be so light? The shop checked its stock and found an older PAG 150 container from the same marketer, and a simple pour-it-out test confirmed that this oil was much, much thicker. In fact, the oil the technician was using seemed almost water-thin. THe shop furnished a fresh container from its new stock to the marketer, which confirmed that ITS supplier apparently had made a packaging mistake. Sl last we heard, discussions were underway to compensate the shop for parts and labor, and presumably the marketer by its supplier.

When we review the car recalls going on these days, it's pretty obvious that you can't expect perfection anywhere, even at the OE level. So there's not necessarily any particular advantage to buying your oil only from the compressor supplier, which after all, also has an oil supplier from which it buys.

However, we do perceive an impression among many technicians that aside from GM/Delphi compressors, everything else takes lightweight PAG. The typical lightweights are al low as PAG 46. And with a new-design compressor, and certainly as just a top-up, a reputable brand as low as PAG 46 should be fine, (including against the 55-65 Cst PAGs that may be used OE). Interestingly, that 5.27 Cst is even lower than the normal 9.5 Cst at 100C./212F. for a PAG 46, so whatever that incorrect oil was, it wasn't even up to PAG 46 requirements.

Also very important to note: many compressors require PAG 100, and here again, a couple of ounces lightweight PAG might not trash a compressor. But it certainly isn't likely to extend its life either, so keep a container of 100 handy.

Be aware that many new systems are down to 3 1/2 to 5 ounces of PAG oil, as the compressors are designed with baffles that keep most of the oil within the compressor. This is definitely an engineering trend for these simple reasons : (1) less oil being circulated with the refrigerant reduces the compressor power required for the system; (2) less oil in circulation means less oil that can coat the walls of the tubing of the eveporator, reducing heat transfer; (3) with clutchless compressors, the compressor never shuts down when the engine is running. A leak will reduce the amount of refrigerant flowing through the system. If the compressor relies on less on refrigerant circulation for lubrication, it is less likely to fail before the loss of refrigerant causes enough of a loss of performance to induce the motorist to bring the car in for service.

So one or two ounces of oil installed when a single component is replaced (condenser, evaporator, compressor, hose), is a much more significant percentage of the total oil fill than wthh systems that hold 8-10 ounces. In fact, two ounces in a four ounce system is 50%, vs. 20% in an older 10-ounce system.

-By Paul Weissler - MACS Senior Technical Corespondent

 MACS Service Reports, April 2010

Austin Rebuilders, Inc.
505 West Oltorf Street * Austin, TX 78704

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info@austinrebuildersinc.com

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