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Old 11-07-2012, 05:48 PM   #1
pnasty101
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Default Turn on heater = engaged AC compressor?

Replaced my AC compressor this summer and its been a tad weird since-

With heat on, outside-to-inside air flow, the ac compressor will constantly go on for 1 second then go off for 20 seconds or so. I've read how compressors are supposed to engage every so ofter to cycle the system, but this seems quite often.
With heat on, if i press the recirculate button the ac compressor immediately turns on and stays on.

No matter if i turn the ac button on/off it does not engage/disengage the compressor.

How does this happen?
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Old 11-07-2012, 09:12 PM   #2
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Default Re: Turn on heater = engaged AC compressor?

Floor, or defog?

Defog will engage the compressor to dehumidify the air blowing up onto the windshield.

As to pure heat, unless it's because the ECU has decided that it needs to dehumidify, I dunno.

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Old 11-08-2012, 04:43 AM   #3
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Default Re: Turn on heater = engaged AC compressor?

Recirc automatically turns on the AC but does not show the light. It's something someone did a generation ago and they continue doing. I believe the Urban Legend goes something like this:

An owner wondered why the AC turned on during the winter when it was so cold outside. It made them feel colder seeing the AC light when they were trying to heat the interior. So an engineer added some fancy-shmancy circuitry that kept the AC light off when they selected Defog or Recirc. And now our cars don't tell us what's happening - they just tell us what makes us feel better.

Truth takes it on the chin again!

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Old 11-08-2012, 08:26 AM   #4
pnasty101
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Default Re: Turn on heater = engaged AC compressor?

Quote:
Originally Posted by RalphP View Post
Floor, or defog?

Defog will engage the compressor to dehumidify the air blowing up onto the windshield.

As to pure heat, unless it's because the ECU has decided that it needs to dehumidify, I dunno.

RwP
Is goes on/off on all settings except when on recirculate, in which it stays fully engaged.
Quote:
Originally Posted by DrivenDaily View Post
Recirc automatically turns on the AC but does not show the light. It's something someone did a generation ago and they continue doing. I believe the Urban Legend goes something like this:

An owner wondered why the AC turned on during the winter when it was so cold outside. It made them feel colder seeing the AC light when they were trying to heat the interior. So an engineer added some fancy-shmancy circuitry that kept the AC light off when they selected Defog or Recirc. And now our cars don't tell us what's happening - they just tell us what makes us feel better.

Truth takes it on the chin again!

Quoting a member:
"The problem with the internet is you can't verify who said what. Abraham Lincoln"

(OK, so I paraphrased, but that's what he said!)

So my question is, why would the engineers have the compressor engage when in recirulate mode? I would think that would be unnecessary wear/tear and lower mpg. So in theory, never use recirculate unless your actually using your ac, or else you'll get lower mpg's?
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Old 11-09-2012, 09:13 AM   #5
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Default Re: Turn on heater = engaged AC compressor?

They probably have it on to dehumidify the air, so that it won't fog up the cooler windows.

RwP
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Addendum: Rest in peace. Surge tank blew, motor overheated, I think she's lived a long and useful life.
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Old 11-11-2012, 10:49 PM   #6
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Lightbulb Re: Turn on heater = engaged AC compressor?

Quote:
Originally Posted by pnasty101 View Post
Replaced my AC compressor this summer and its been a tad weird since-

With heat on, outside-to-inside air flow, the ac compressor will constantly go on for 1 second then go off for 20 seconds or so. I've read how compressors are supposed to engage every so ofter to cycle the system, but this seems quite often.
With heat on, if i press the recirculate button the ac compressor immediately turns on and stays on.

No matter if i turn the ac button on/off it does not engage/disengage the compressor.

How does this happen?
We have a 2001 Malibu, and here is what I observed when I adjusted the settings on my climate control attempting to recreate your problem. Here's what I gathered.

The AC compressor should not engage unless the following modes are activated:

AC button depressed
Recirculate engaged (in any function and thermostat adjustment)
Defrost or defrost/floor mode

Defrost/recirculate engages the AC for safety reasons; to keep humidity down and help clear humidity inside the windows on very humid days, and happens by default as stated by a previous poster. I have read, but am not entirely sure, that this is required by the NHTSA on vehicles made after the mid to late 90's.

The AC did not engage in any of the other modes including heat on fresh air intake, at least not on any other chevy I have access to. I have heard of high failure rates of the internal electronics inside of the thermostat control module on these cars, causing the AC to engage or not engage correctly or at weird times, and I have even seen instances where the AC light actually flashes when depressed.

As for the quick engagement and disengagement of the AC clutch, how well does your AC perform during the summer? It could be possible that your AC performs so well that the AC is disengaged quickly because it is reaching it's set temp to prevent freezing over. Though, doubtful since the compressor wouldn't be on long enough to de-humidify the air when defrost or recirculate was activated.

Some reason's (that I can think of) for unnecessary AC clutch engagement.

Faulty control board:
As said above, they fail more frequently than they should. It would probably pay to find a known working one from a friend's car of similar make/model or cheap pull from a junk yard to see if your problems aleviate. This is my bet, since the thermostat control is electronic, and isn't supposed to allow engagement of the AC in fresh air intake. It could also be stuck on recirculate even though the fresh light is illuminated.

Electrical short or loose connection:
It is possible that when replacing the compressor or over time of the car, that a wire was accidentally pinched, grazed, scraped, etc and allowing jacket to expose and the wire to touch ground and engage the AC. I have heard of shorts and lose connections causing quite the assortment of weird problems, and have encountered one or two puzzling ones myself.

I hope my information is useful and pertains well to the subject matter. Thanks all!
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