Climate Control

Conversion to R-134

Conversion to R-134

Conversion to R-134 is not as bad as some people say. To convert the system you must purge the system, flush out all oil, and drain the compressor of as much oil as possible. The refrigerant drier must be replace as well as the orifice tube. Replacing all seals is a good move for older cars as well but is not required if the system has been use regularly and therefore has kept the seals moist.

Deatailed inormation is available at which has a great deal of information for free, as well as a retrofit manual for sale at +/- less than $30.

Retrofitting can be done for a reasonable cost, if you know what is required. If you post a question, they will answer you directly in about a week.


The common opinion among many who offer advice on the conversions is that the system should be slightly undercharged for maximum performance. The sight glass lets you look at the stream of gas/liquid from the bottom of the condenser. One of the concerns with R-134a is that the high-side pressures are a bit higher at any given temperature, so condenser efficiency is critical to good system performance.

How does all this play together? Glad you asked... With the sight glass showing bubbles, it means that --almost-- all of the hot gas from the compressor is being cooled enough to condense it into liquid. The gas bubbles are simply gas that hasn't been cooled enough. With stable system pressures and temperatures, and with minimum air in the system, the sight glass indicates the state of charge in the system also.

Now here's where it gets exciting-- You can add more gas to the system to get a full liquid stream from the condenser, keeping in mind that the system is sensitive to ambient temperature. The charge you put in on a cooler day may have bubbles on a warm day. One might be tempted to put just a little more gas in so that the system flows condensed liquid on the warmest days, but that's not really a good answer either. The volume of gas in the condenser is not very large, so any extra liquid in there will reduce the effective area. While this will balance out some on its own, the net effect is less cooling available. This is one of the prime reasons for charging a system based on a standard weight of gas, rather than a somewhat subjective "until there are no bubbles".

What to do: On your car, the symptoms are consistent with a low charge level. The R-134a charge, by weight, should be less than the R-12 charge weight in the same system. Dual air cars need (from memory...) about 1040g of R-134a according to PCNA's conversion TSB. If you know how much you put in, you know how much you need to add to get to 1040g. If you are just doing it by the sight glass, stop adding gas well before the bubbles clear, and allow the system to stabilize for a couple minutes between gas shots. Since you are injecting the new gas in the suction side of the compressor, it will take a little time for the increased charge to make it into the condenser and up to the drier. Be patient, and do not overcharge the system. Again, best (and safest) results are obtained with the system slightly undercharged.

FWIW, I did mine without the TSB info , and put in about 1000g (about 2.2 lbs by weight) of R-134a on a 70 degree day. That was enough to get an almost bubble-free sight glass on that day. Later, as the weather warmed up into the eighties and nineties, there are now a lot of bubbles in the glass. But-- the system is still cold enough. I --may-- add a little gas if I can figure out a way to do it wihout risking any air ingestion into the car. But so long as it's cold, I have no real interest in messing with it at all.

Good Luck!

dr bob


Exactly correct. Had this problem of "overfill" with a 968 I used to own. Saw no bubbles in sight glass, but when in operation the compressor would cut in and out every few seconds. Turned out that the over pressure sensor was triggering due to the "fullness" of the system. There needs to be a little space for the freon to expand and contract properly. Mechanic pulled a little out, problem went away, now saw one or two bubbles in the sight glass every now and then. If you see froth in the sight glass, then it probably is low.

Adam S.
'89 Black 5sp.
'87 Silver Auto

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