Expert: Sean Halliday - 9/8/2007

Question
QUESTION: I have an 87 Nissan pickup (D-21 2WD V-6). I recently changed it from R12 to R134A. I replaced the receiver/drier, rented a vacuum pump and sucked it down to -28Kg I believe it is, for a half hour and let it set for a half hour, was still holding. Yes, as you can tell I have a set of gauges. Put in an oil charge of medium viscosity (The oil that mixes well with both R12 and R134A). Then I started adding the 134A and that is when my problems started. It says on the hood decal that it takes a charge of 2 lbs. R12. So I figured about 80% of that should be about right for the R134A. I have added only 2 12oz cans of 134A and the compressor engaged. The problem is, even with letting the truck set over night, the compressor don't disengage. Let me explain what is happening at this moment. I hook up gages, valves closed on manifold, start the truck, engage A/C, and the gauges read 50lbs. low side, about 140 lbs. high side starting out. High side builds to about, but not quite, 250, then starts falling off to 205 or 210 and stays there. Low side just keeps falling almost down to 10 lbs. All the while the A/C in the truck is cold and the compressor is still engaged and has never disengaged, except for when I turn A/C off. OH, and the low side tube gets cold but never sweats as I'm told it should do. I have added a little more 134A, thinking maybe I haven't added enough, that hasn't changed anything. Pressure came up at first when I opened the can but started falling soon after. Any ideas what is going on?

Thank You
Mark

P.S. Yes, I would disconnect the yellow line when hooking up a new can and purge the air from it before attaching it to the manifold.

ANSWER: You have air in the system.

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QUESTION: How could this be when it held a vacuum for more than a half hour without the pump running?

Answer
You must flush the complete system otherwise you may have air/moisture in the system.The oil/gas mix must flow or you will not get the proper cold setting.The cheapest and easiest way is have a shop do it as they have the machine for removing the moisture/air.With old systems its pretty hard to do it yourself.