A/C compressor?

Old 03-01-06, 12:52 PM
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Red face A/C compressor?

my Auto A/C makes a hissing noise when I turn it off, and it takes a while to actually start to cool. Is this a problem w/ the compressor or is it something else? Please tell me something before I have to take it to the shop (like to be prepared)
Old 03-19-06, 07:16 PM
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Let's start from the beginning. The AC won't start cooling immediately because the compressor must build up pressure, the hot, high pressure gas must get to the condenser to be cooled and condensed into liquid, the liquid must get to the evaporator and be allowed to evaporate and cool, and air must pass over the evaporator to be cooled. The car is also possibly hot - so the air coming into the evaporator is hot. It can easily take several minutes to feel cool air under adverse conditions. Assuming that everything is working properly, the AC will continue to deliver cold air and keep you comfortable.

When you turn off the engine, the compressor stops - but there is still pressure built up on the high side of the system. The whistling/hissing sound that you hear is that pressure being normalized - and it can continue for a minute or so until all pressures in the system are equalized. This isn't a leaking sound. If you had a leak that you could hear, you would long ago have lost all of your coolant and would have no AC at all.

If your AC system is otherwise working properly (i.e., keeping you cool), these startup and shutdown experiences don't necessarily indicate a problem.

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