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Thread: E38 AC management in an engine swap...

  1. #1
    Senior Member
    Join Date
    Apr 2008
    Posts
    197

    Default E38 AC management in an engine swap...

    not exactly a tuning question but also interesting :

    Here's the story

    After a looong weekend with lots of trouble I finally got the AC compressor in my Tahoe.

    I also installed the modified hoses and created a temporary wiring harness to run the 2007 compressor (internal variable displacement type out of 2007 6 cylinder truck) with the 1997 HVAC unit. I used the AC request signal ( I had to change the polarity to active low..) and wired it through the cycling pressure switch in the receiver drier.
    This pressure switch, when closed, switches the low AC request signal to the engine fuse box where the compressor relais is switched on.
    The 2nd pressure sensor is driving the recirculation motor when the pressure in the AC system raises above a certain value.

    Ok now the interesting stuff:

    I am wondering how to set the fans when the AC is switched on. I use the 2007 OEM dual electric fans. They have two speeds, slow when switched in series and fast when switched parallel.
    Compared to the 5.7 mechanical fan I guess the slow setting draws more air through the radiator than the old system. For my personal feeling the high or fast setting is a little overkill.

    Does anybody have more specific idieas about the fans settings I should use?

    Then, when I looked at the 2007 Chevy Tahoe AC schematics I thought about the following: What would the E38 do if there was no AC request signal on the CAN bus, but the hard wired AC pressure sensor ( three wire type, GND, 5Vref and output) is getting high pressure readings? Would the E38 switch on the fans??? Maybe our EFI Gurus could trick the ECM in that way that it watches the AC pressure sensor and switches the fans according to pressure in the system.
    I could also imagine to use the compressor clutch output of the E38 for additional safety in the above wiring. I could use the simple wiring for switching on the AC but I could also check if the E38 tries to switch off the clutch because of high engine rpm or high AC system pressure...

    Harald

    more info and pics on :
    http://www.pacificp.com/forum/viewto...?p=73860#73860
    If nobody has done it before, somebody has to do it....

    1997 Tahoe 2dr 4WD
    2002 Alfa Romeo Sportswagon 2.0 JTD
    1982 Deutz Intrac 2004A
    2003 friesian Horse " Henry"

  2. #2
    Lifetime Member
    Join Date
    Dec 2007
    Posts
    1,143

    Default

    Quote Originally Posted by harascho View Post
    not exactly a tuning question but also interesting :

    Here's the story

    After a looong weekend with lots of trouble I finally got the AC compressor in my Tahoe.

    I also installed the modified hoses and created a temporary wiring harness to run the 2007 compressor (internal variable displacement type out of 2007 6 cylinder truck) with the 1997 HVAC unit. I used the AC request signal ( I had to change the polarity to active low..) and wired it through the cycling pressure switch in the receiver drier.
    This pressure switch, when closed, switches the low AC request signal to the engine fuse box where the compressor relais is switched on.
    The 2nd pressure sensor is driving the recirculation motor when the pressure in the AC system raises above a certain value.

    Ok now the interesting stuff:

    I am wondering how to set the fans when the AC is switched on. I use the 2007 OEM dual electric fans. They have two speeds, slow when switched in series and fast when switched parallel.
    Compared to the 5.7 mechanical fan I guess the slow setting draws more air through the radiator than the old system. For my personal feeling the high or fast setting is a little overkill.

    Does anybody have more specific idieas about the fans settings I should use?

    Then, when I looked at the 2007 Chevy Tahoe AC schematics I thought about the following: What would the E38 do if there was no AC request signal on the CAN bus, but the hard wired AC pressure sensor ( three wire type, GND, 5Vref and output) is getting high pressure readings? Would the E38 switch on the fans??? Maybe our EFI Gurus could trick the ECM in that way that it watches the AC pressure sensor and switches the fans according to pressure in the system.
    I could also imagine to use the compressor clutch output of the E38 for additional safety in the above wiring. I could use the simple wiring for switching on the AC but I could also check if the E38 tries to switch off the clutch because of high engine rpm or high AC system pressure...

    Harald

    more info and pics on :
    http://www.pacificp.com/forum/viewto...?p=73860#73860
    Actually the stock E38 turns the fans on if the pressure is too low or too high, or if the pressure sensor is missing.

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