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Old 07-21-2003, 04:00 PM
erubin erubin is offline
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Join Date: Mar 2003
Location: Los Angeles, CA
Posts: 445
Thanks for the link to Mastercool's phone #'s.

I finally got to the bottom of this:

We know that R12 and R134 refrigerant will not damage the manifold hoses or the gauge set. The concern is cross contamination within a car's A/C systems and recovery tanks. Mastercool said that when R134a was being introduced as a replacement for R12 the rules established by SAE and several A/C groups was that any oil that remained in the manifold hoses could contaminate a system when moving between R12 and R134a. It was only later after extensive field experience and a sufficiently long passage of time that it was observed that a converted R12 (to R134a) system was not chemically contaminated by trace amounts of incompatible oils. I'm only talking about trace amounts of incompatible oils which is what reamains of the oil after a flush.

This being the case, a very small amount of incompatible oil (lets say a drop or so) that is carried from your hose set is not going to ruin your compressor or even come anywhere close to damaging the A/C system. As an example, when converting a car from R12 to R134a your system will have traces of mineral oil imbeded in the hi and lo pressure hoses and this will be the case even if countless flushes are done before converting. These R134a systems don't suddenly go south because of traces of mineral oil clinging to the hoses! If it makes you feel better just blow out the hoses to your manifold gauge set to eliminate the unlikely case that they are loaded with oil, this is what i plan on doing.

In conclusion, it may be handy for backyard mechanics who do systems regularly to invest in two sets of devoted manifolds or a 6 hose set up. But it is not a problem for the manifold set or A/C system to use one set of hoses (with removable couples) for both R12 and R134a. These facts will be rejected by some technicians because they were trained at a time (or with information from a time) that correctly observed incompatibility between oils and refrigerant but incorrectly stated that contamination between oil was a problem. Mastercool's old instructions state this assumption but no longer.

This has nothing to do with contaminating R12 or R134a. I'm not talking about that. You will not have refrigerant contamination (which is a real problem/concern) by using the same gauge set with different systems.
1972 450SL
1982 300D Turbo

Last edited by erubin; 07-21-2003 at 04:09 PM.
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