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Thread: Can AC run without GM pressure sensor?

  1. #1
    Tuner in Training
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    Can AC run without GM pressure sensor?

    I have an LC9 in my 2004 Land Rover Discovery 2 and am sorting out the AC. The Rover has a pressure sensor that cuts power to the compressor so I was hoping to avoid adding the GM pressure sensor. Is there a way to do this? Or do I need to add the GM pressure sensor to get the E38 to trigger the relay?

    FYI, I have a CANbus gateway that is sending the AC request signal.

    Thanks!

  2. #2
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    Technically you don't "need" the pressure sensor, but you will have to wire something in to at least tell the ECM that the pressure is high enough to allow it to turn the a/c compressor on when your command comes in. It's nice to have it working though because it changes fan speeds based on the a/c pressure. It's extra nice if you have a PWM fan

    FWIW I have an E67 (LH6) running my A/C and I am not able to start it from the CAN, I had to send it a class 2 message. Possibly not helpful for you, but you never know.

  3. #3
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    Thanks. I did find a way to emulate the pressure sensor and get the clutch relay signal to work. Sharing here in case others need this. This worked on my LC9 with an E38 ECM (making the ECM think the refrigerant pressure was within range, without the sensor being present).

    The short version is that you need to put two resistors inline with the 5V Ref, Signal, and Low Ref. This will reduce the 5V reference to ~2V (I think anything between 1.5V and 2.5V should work as it correlates with about 150-250PSI). When the ECM sees the 2V signal, it assumes the pressure is in range.

    I used a 1,000ohm and a 670ohm resistors to get a 2V signal but you can Google "voltage divider calculator" to calculate other combinations of resistors. Pay attention to which resistor goes on the 5V side and which goes on the Low Ref. Here's what my diagram looks like.

    IMG_1039.jpg

  4. #4
    Senior Tuner blindsquirrel's Avatar
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    Yeah that's a great trick for making it turn on, but the sensor is kinda important as it tells the ECM the actual pressure and from there the engine load can be calculated so it can keep things stable. It's also the high side safety. Without it you're relying solely on the physical overpressure valve to vent if pressure goes too high, if the system even has one.

  5. #5
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    I figured he would use the Rover's high/low pressure switches, but he should have something. I don't think the engine load is a big deal.

  6. #6
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    I am using the Land Rover trinary switch to control for over and under pressure. I?ll keep an eye out for engine load issues. Thanks for the heads up on that.

  7. #7
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    If you want the GM ECU to control the compressor, Idle and fans you will need the GM high side pressure sensor hooked up and working.
    You will also need one of these AC REQUEST MODULES to send the "AC REQUEST" on the canbus to the ECU.
    This module has a single input that when activated sends the ac request message. You will want to run a low side cycling switch on the input to the module so it will "cycle" the AC on and off as the pressure gets low and doesn't freeze the evaporator core.
    If you have the low side cycling switch on the wire going to the AC compressor it will cycle it but the ECU will still think the AC is requested and will not adjust the idle to match.


    AC-REQ GM E38 DIAGRAM SAMPLE w-CONTROLS.jpg
    Last edited by rpmextreme; 09-24-2021 at 10:56 AM.