Engine Size: 1600
Refrigerant Type: R134a
This 20YO car has never had an AC gas top up or service. Last summer my air con would blow vaguely cold for a minute and then blow warm. I figured it was low on gas so this summer I topped it up with 134a. Most of a can went in and it has held that pressure for a few weeks now. Problem is that it will blow cold for a few minutes and then blow warm. Somewhere I read that there can be moisture in the system which freezes in the expansion....valve is it called? I suspect this is what's happening.
So I was going to try to replace the receiver dryer. My question is, do I really need to go to the prohibitive expense of buying or paying someone to vacuum out the system to boil the moisture off? Surely most of the moisture will come out with the gas when I empty the system, and the tiny amount of moisture that is in the air that will then fill the lines would more than easily be absorbed by [a small fraction] of the dessicant in the new receiver dryer, wouldn't it?
1. First thing I'd do would be to see if compressor clutch has disengaged when it starts to blow warm. If not, both high and low pressure readings at about 1500 rpm would be useful.
So it turns out that the compressor is staying on. All the time! I can turn the compressor off by the AC button or by turning the fan to zero, so no relay or mechanical-compressor problems to my understanding. I just went for a drive and kept the AC from stopping (expander freezing?) by switching the ac off and on every 10 seconds or so. That worked fine.
The only gauge I have is for the low end. When I let the system rest - AC off engine off for a good half hour - the gauge reads about 80-90psi. When I turn the AC on the gauge goes basically to zero, of course, because the compressor is constantly on.
Does yours have a fan clutch or electric fans?
You need both high and low side gauges if you try to diagnose and fix yourself. Low pressure when operating should be like 30-40 psi at 1500 rpm, and high side should be like 200 to 250 psi. That 90 psi static pressure doesn't really tell much.
Electric fans. Low pressure side at idle is 0psi due to compressor being on constantly.
Edit: If I yank out the connector for the high pressure sensor, the compressor turns off. I can't find a low pressure sensor (it is an old car, after all. Maybe there isn't one).
Hi side should be fine as a lot of gas went in, and also because I have the same problem now that I had before I recharged: cool for a few minutes, then warm. The only thing that has changed is that it's cooler for those few minutes.
Edited: Mon November 09, 2015 at 2:39 AM by peanutaxis
high and low side pressures
The answer to your original question is yes, any time you disconnect a line to replace a part, the system will fill up with air, and you need to pull a vacuum to remove that air before recharging. Trying to run a mixture of air and refrigerant will have poor performance.
It is also possible for a TXV to get sticky and stick closed, even though there is not a freeze-up. It takes a LOT of water to freeze the expansion valve, like a car that was in a flood or had an A/C line left open for a time and not properly decontaminated.
There are pressures lower than zero psig, a cheap gauge will not show that.
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