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  #1  
Old 08-15-2008, 07:58 PM
tomsmith tomsmith is offline
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AC compressor cycling

Hello all,

I have a 2004 TrailBlazer LS and have developed a recent problem with the A/C. I can see the compressor kick in, the front 'plate' starts to spin for around 5 seconds and then stops. Around 10 seconds pass by and then it spins for another 5 seconds and so on.

I connected a manifold gauge to the lower port (the one near the windscreen) and when the compressor is spinning, the pressure slowly drops to around 20 and then when it clicks off, it shoots back up to 80.

Or was it the other way around? I can't recall but it the drop/increase in pressure definitely co-incided with the compressor spinning and then stopping

With the vehicle at rest and around 25C degrees outside, the lower port registered an even 80

I was unable to check the high port (the one near the radiator) as I didn't have the right sized connector for it.

I tried using electric contact cleaner on the switch that comes off the accumulator (the one on the low side, near the windscreen) but that doesn't help.

Also, my A/C doesn't blow cold when I'm driving on the highway

Any ideas?
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  #2  
Old 08-15-2008, 08:51 PM
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Super 88 Super 88 is offline
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Quote:
Originally Posted by tomsmith View Post
Hello all,

I have a 2004 TrailBlazer LS and have developed a recent problem with the A/C. I can see the compressor kick in, the front 'plate' starts to spin for around 5 seconds and then stops. Around 10 seconds pass by and then it spins for another 5 seconds and so on.

I connected a manifold gauge to the lower port (the one near the windscreen) and when the compressor is spinning, the pressure slowly drops to around 20 and then when it clicks off, it shoots back up to 80.

Or was it the other way around? I can't recall but it the drop/increase in pressure definitely co-incided with the compressor spinning and then stopping

With the vehicle at rest and around 25C degrees outside, the lower port registered an even 80

I was unable to check the high port (the one near the radiator) as I didn't have the right sized connector for it.

I tried using electric contact cleaner on the switch that comes off the accumulator (the one on the low side, near the windscreen) but that doesn't help.

Also, my A/C doesn't blow cold when I'm driving on the highway

Any ideas?
Sounds like you are low on Freon (134A) which would mean you have a leak somewhere. The system has a low pressure switch that kicks the a/c compressor clutch out at a predetermined value - usually 20 - 25 lbs or so. Sounds like everything is working normal otherwise!
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  #3  
Old 08-15-2008, 08:59 PM
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petevw petevw is offline
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Originally Posted by Super 88 View Post
Sounds like you are low on Freon (134A) which would mean you have a leak somewhere. The system has a low pressure switch that kicks the a/c compressor clutch out at a predetermined value - usually 20 - 25 lbs or so. Sounds like everything is working normal otherwise!
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Old 08-15-2008, 09:45 PM
tomsmith tomsmith is offline
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Thank you both

So does a reading of 80 at the low port suggest a leak? I couldn't find any specs on what it should be reading
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Old 08-15-2008, 10:49 PM
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Super 88 Super 88 is offline
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My understanding is the pressure with the engine off (called static pressure) is NOT an accurate way to measure the amount of charge in the system.

Here are a couple of good articles. I am not a professional, and I would defer here to anyone that is. I have worked on a few of my own cars with a/c so I am mostly talking from experience and/or what I have read!

Here is an article from another forum:
http://forum.miata.net/vb/showthread.php?p=3078401
See especially post number 13 - 15.

And some more in depth reading:

http://www.aircondition.com/tech/questions/82/

I would say if your compressor is kicking out and the pressure drops to around 25 within about 5 seconds, then you are low on Freon. It should drop down to 25 and kick the compressor off, but it should takes much longer than 5 seconds, even on a 75F degree day (Somebody correct me - does 25C = 75f?)
The normal way to check the low pressure is with the car idling (some recommend slightly above idle - around 1200 RPMS or so) a/c on max, fan on high and doors closed. Let the car idle for at least 5 minutes or so.
Many gauges will have a chart that shows what your low side pressure should be as compared to the ambient air temperature. As you may know, the higher the ambient air temperature, the higher the low pressure side will be.

I might also suggest many places do a relatively inexpensive check of your a/c system for like around 30 bucks or so. Might be worth it to have them tell you what the problem is and then go from there.

I also might state - I don't know how much experience you have - but working on a/c systems CAN be extremely dangerous! It's a case where a little knowledge can INDEED be very dangerous!
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