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Thread: Fan A/C Pressure

  1. #1
    Senior Member mtnman's Avatar
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    Default Fan A/C Pressure

    Any ideas on what these should be set at (G0911,G0912)?

    What exactly does it affect (I know, "If the A/C pressure is above this value, then turn on fan #1)?

    What are the effects if the enable is set very low vs very high?

    My fans are (still after many a month) coming on at idle when the AC/Fan is on. I'm wondering if raising the "A/C pressure enable" will keep the fans from coming on until the enable pressure is reached.
    2003 2500HD,E-Fans, LT headers, Corsa catback, Outlaw intake.

  2. #2
    Lifetime Member Chevy366's Avatar
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    I think the fans have two properties that are reached , one ECT and the other A/C pressure , so if one or the other is reached the fans come on .
    Why would you not want the fans coming on at idle , setting still ?
    Under the fuse box cover and outer shroud are the relays that control the fans , you could hook a toggle switch there and toggle the fans if you wanted to , but then again why ?
    Last edited by Chevy366; February 7th, 2007 at 07:04 AM.
    2005 1500 HD , Custom OS3 SD tune .
    2006 Trailblazer
    Dinosaurs and Plants gave their lives so that we may drive , long live fossil fuel .

  3. #3
    Senior Member mtnman's Avatar
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    Quote Originally Posted by tunedbyGM
    I think the fans have two properties that are reached , one ECT and the other A/C pressure , so if one or the other is reached the fans come on .
    Why would you not want the fans coming on at idle , setting still ?
    When the engine temp is below the setting I have for the fans to come on, the fans will cycle on and off at idle. This cycling happens over and over, on - off, on - off. Fans go off for about 5 seconds then come on 5 seconds. They're coming on when they dont need too as far as the engine temps are concerned. They're coming on cause the A/C compressor is coming on.

    I've discussed this 'cycling' problem before with the fans and A/C, but there wasnt really a solution as far as PCM settings go, so I was just wondering what the pressure enable/disable settings were "tied to", wondering if their modification would keep the fans from cycling so much.
    2003 2500HD,E-Fans, LT headers, Corsa catback, Outlaw intake.

  4. #4
    Lifetime Member Chevy366's Avatar
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    Quote Originally Posted by mtnman
    When the engine temp is below the setting I have for the fans to come on, the fans will cycle on and off at idle. This cycling happens over and over, on - off, on - off. Fans go off for about 5 seconds then come on 5 seconds. They're coming on when they dont need too as far as the engine temps are concerned. They're coming on cause the A/C compressor is coming on.

    I've discussed this 'cycling' problem before with the fans and A/C, but there wasnt really a solution as far as PCM settings go, so I was just wondering what the pressure enable/disable settings were "tied to", wondering if their modification would keep the fans from cycling so much.
    Your not running a after market fan harness ? , (Nelson) if you are that is your problem right there .
    The stock fan harness from GM does not cycle with the A/C as you described .
    Here is the stock fan harness part # 15216042 and yes it will fit your 03 .
    2005 1500 HD , Custom OS3 SD tune .
    2006 Trailblazer
    Dinosaurs and Plants gave their lives so that we may drive , long live fossil fuel .

  5. #5
    Senior Member mtnman's Avatar
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    Quote Originally Posted by tunedbyGM
    Your not running a after market fan harness ? , (Nelson) if you are that is your problem right there .
    The stock fan harness from GM does not cycle with the A/C as you described .
    Here is the stock fan harness part # 15216042 and yes it will fit your 03 .
    Hmm, wish I had known that about 1 year ago. DOH!
    2003 2500HD,E-Fans, LT headers, Corsa catback, Outlaw intake.

  6. #6
    Lifetime Member Chevy366's Avatar
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    Quote Originally Posted by mtnman
    Hmm, wish I had known that about 1 year ago. DOH!
    Sorry , but at least you know now , the part number is for the Forward Lamp Assembly Harness which in GM's wisdom contains the fan harness as well .
    I just striped out the fan harness stuff and used it , with new convoluted tubing covering the wire that I exposed .
    2005 1500 HD , Custom OS3 SD tune .
    2006 Trailblazer
    Dinosaurs and Plants gave their lives so that we may drive , long live fossil fuel .

  7. #7
    Joe (Moderator) joecar's Avatar
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    If the enable is set low, then fans will be on always.

    If the enable is set high, the A/C can't cause the fans to come on (but ECT can still turn on the fans).

  8. #8
    Senior Member mtnman's Avatar
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    Quote Originally Posted by joecar
    If the enable is set low, then fans will be on always.

    If the enable is set high, the A/C can't cause the fans to come on (but ECT can still turn on the fans).
    So, if enable is set high, only the engine temp will be causing the fans to come on? Will this not have adverse affects on the A/C system? Isnt there some correlation between the A/C pressure and the fans?
    2003 2500HD,E-Fans, LT headers, Corsa catback, Outlaw intake.

  9. #9
    Lifetime Member Chevy366's Avatar
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    Some PCMs do and some don't depending on year .
    You probably only have one trigger wire that one being the one for PIN 42 , on newer PCM's PIN 33 and PIN 42 control the fans for high and low operation as well as the A/C pressure , some people use the PIN 16 for older PCM's .
    Without A/C pressure control of the fans the A/C can cause major problems .
    I think on ls1tech there is a Pin Out diagram for a 03 PCM might give it a look .
    2005 1500 HD , Custom OS3 SD tune .
    2006 Trailblazer
    Dinosaurs and Plants gave their lives so that we may drive , long live fossil fuel .

  10. #10
    Joe (Moderator) joecar's Avatar
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    When the A/C compressor comes on, the pressure increases, which would trigger the fans to remove the heat from the condensor coils...

    If the fans don't run then the heat isn't removed so you won't get much A/C cooling at low vehicle speeds.

    If fans are coming on too easily/often, check:
    - is engine overheating...?
    - is ECT sensor bad...?
    - is A/C overpressurized (I don't know how)...?
    - is A/C pressure sensor bad...?
    - is there an intermittent short in the fan wiring...?

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