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Thread: P59 & a/c

  1. #1
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    P59 & a/c

    Hello,

    I've read a lot of posts about getting the A/C working on the Gen 3 ECMs, but only ended up more confused. This isn't my first go at A/C, and I got everything working well on an older Gen 3 ECM (Blue/Red), but can't get my P59 ECM (Blue/Green) to trigger the A/C relay when I turn the A/C on. If I go into HP Tuners scanner and manually turn on the compressor relay, everything works as should, compressor engages, if temp is over 200* fans turn on, idle doesn?t stumble. I know that my P59 used Serial Cycling originally, but setting this to any other setting made no difference, Analog, Analog Cycling. I am trying to trigger at idle with 12v to C2 17, and ground to C2 55 and have tried both the +&-, only + and only -. The only thing that seemed to make a difference was when I switched type to Passive, I could no longer trigger the compressor relay manually. I never tried "Not Fitted". I don't have any pressure sensor(s) connected to the ECM, but have a binary switch in-line with my 12v request signal and I am getting 12v through this switch. Under the tab, System-A/C-Hardware-Recirculation Fitted, I have that set to Fan #2 but tried AC Recirc as well. Setting that to AC Recirc, I was no longer able to turn on my #2 Fan from the scanner.

    I'm at a loss as it seems others have gotten this to work, am I missing something? I'd hate to resort to a trinary switch to control fans and not have any bump in idle or RPM deactivation. This is a stand-alone setup, originally the ECM vehicle was a 2003 Yukon 6.0, only big change was I segment swapped to 4L80. OEM type compressor, 10S17F Four Seasons 78377. The only thing I can think of is, reading the post "A/C request Blue/Green connector ECU" at ls1 tech, I need to add the three-wire sensor (GM 13587668) to the high side refrigerant line. I can't see needing to add the two-wire sensor (GM 89040362) as this just adds/removes ground to the HVAC module, which I don't have. I'm monitoring in the scanner, A/C request, A/C Active, A/C Clutch Engaged, and A/C Disable (pressure). I never get A/C Request to go to yes nor get A/C Disable to go to yes, so is the 3-wire sensor still needed since it doesn't appear to be pressure stopping it from engaging? Manually turning on the compressor will get A/C Active and A/C Clutch Engaged to go to yes , but A/C Request and A/C Disable stays at no . Any help is greatly appreciated! Brendan
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    Last edited by Brendang2000; 19 Hours Ago at 12:07 PM.

  2. #2
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    You need the pressure switches/sensor for whatever 'Type:' you have it set for. It's never going to control the compressor on its own unless it sees valid info from the correct type of switch/sensor.

    Essentially, the pressure switch setup you have corresponds to Type: Not Fitted (i.e., the PCM is not expected to control clutch activation).

  3. #3
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    Quote Originally Posted by blindsquirrel View Post
    You need the pressure switches/sensor for whatever 'Type:' you have it set for. It's never going to control the compressor on its own unless it sees valid info from the correct type of switch/sensor.

    Essentially, the pressure switch setup you have corresponds to Type: Not Fitted (i.e., the PCM is not expected to control clutch activation).
    Thank you for your reply! OK, I think I follow you, but still not 100%. So when mine was originally Serial Cycling, it required the serial data from the HVAC module plus the pressure signal which was originally equipped (GM 13587668). However, if I change type to Analog (Cycling) shouldn't it be triggered via 12v on C2 17 (possibly ground on C2 55 for cycling) and not require any pressure signals since Analog didn't have any pressure signals connected to the ECM, like Express Vans? If so, I am still unable to trigger the relay with type set to Analog or Analog Cycling.

  4. #4
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    What flavor of Express van had no pressure switches?? That's not possible if the compressor is PCM-controlled. It has to have some way of knowing if the compressor needs to be on or off.

    2003 Express compressor controls diagram:

    2003 Express compressor controls.png

    Low-side switch is just sharing the clutch coil ground so don't let that confuse you. Signal from the switch goes back to C2 #55. Switch is normally open: zero PSI equals open circuit. Zero PSI means clutch commanded OFF. Right now without a low-side switch wired in the PCM is reading that as zero PSI and is not going to command the clutch ON when it thinks the A/C system is empty.

    I am basing this off the assumption you're using Express van tune settings, that means you have to use the Express van pressure switch arrangement. If not, you need to pick a system and then copy it.

  5. #5
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    Oh, forgot. The high-side switch is drawn incorrectly in the diagram, it should be shown as normally closed. So it's not required for the clutch to engage, but it needs to be there as a safety device.

    A/C Pressure Switches

    The A/C system is protected by two A/C pressure switches.
    ? A/C low pressure switch
    ? A/C high pressure switch

    The A/C high pressure switch interrupts the A/C request signal when the A/C line pressure is more than a predetermined value. The A/C low pressure switch interrupts the A/C low pressure switch signal when the A/C line pressure is less than or more than a predetermined value. When the powertrain control module (PCM) stops receiving the required signals, the A/C compressor clutch relay control circuit is no longer grounded, disengaging the A/C compressor clutch. The A/C compressor clutch is disengaged under the following conditions:
    ? A/C low pressure switch is less than 124 kPa (18 psi).
    ? A/C low pressure switch is more than 338 kPa (49 psi).
    ? A/C high pressure switch is more than 2896 kPa (420 psi).

  6. #6
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    Quote Originally Posted by blindsquirrel View Post
    Oh, forgot. The high-side switch is drawn incorrectly in the diagram, it should be shown as normally closed. So it's not required for the clutch to engage, but it needs to be there as a safety device.
    I'm using the truck tune (GMT800), I just looked at an express van ECM that I have. Sorry, I saw the pressure switches but they doesn't send a signal like the three wire sensor, so that could be tricked by applying/removing ground or +12v, correct?

  7. #7
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    According to the diagram, yes, for testing it should read a jumper wire from C2 55 to GND the same as a valid low pressure switch.

    Now, the catch is, I don't know if anybody can guarantee that all P59s with all operating systems have all the parts inside to enable reading all possible variations of pin configurations across all the various platforms. Do you know if the '03 Express and '03 Yukon use the same Service Number? The Yukon diagram shows C2 55 as unused, of course. If they are different SNs the Yukon part may just not have the board-level components inside connected to that pin. Also the Yukon OS may not respond properly when the 'Type:' settings are changed over to match the Express van. I've seen that before on some fan controls on other ECMs. The menu of options is there in the tune but it doesn't change the operation of that assigned pin like it should.

    Looks like you can pick up a pre-programmed P59 on ebay for $65... if you were to supply them with VIN 1GCHG35U131147731, you'd get one for a 2003 Express 3500, 6.0 & 4L80E.

  8. #8
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    Thanks blindsquirrel, you've been very helpful!! It seems that to get this to work, I still need to send the ECU pressure signals via the 3 wire sensor, or trick the ECU into seeing the correct voltage by jumping a 50k ohm resistor between C1 45 & C2 14 (the resistor might not be a permanent fix though), set Type to Analog (NOT cycling), and set fans to correct pressures 190 low, 240 high (got most of this from autocomman's post @ ls1tech). If I am still unable to get it to work, then I think maybe this P59 does not have the hardware to activate the compressor via C2 17.

    I can no longer see the service number on the Yukon ECU as it's installed (trapped) in the car. It may take me a while to make the hardware changes (get resistor or install sensor), but I'll reply when I do to hopefully say it worked. Again, thanks for your input blindsquirrel!!

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    No, you know the internal hardware for C2 17 is present as you can manually command it on with the special functions. It's the reading-a-signal-on-C2-55 that would be the potential issue. I only checked Rock Auto for the PCM lookup, there's no listing for the Yukon PCM but several options for the Express, that leads me to believe they are different, otherwise they would be the same parts listed under both models.

    Is there a channel in the scanner for that low pressure switch status? There probably won't be since the supported channels list is based on the operating system, and the Yukon OS didn't use that.

  10. #10
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    And I don't think faking the Yukon's sensor signals would tell you anything useful as a test. You already know the Yukon PCM with the Yukon OS & settings will work with the Yukon pressure sensor inputs.

  11. #11
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    Finally realized a batter way to say what I was getting at in the last post. I'm slow that way sometimes.

    The Yukon is set up for Serial Cycling. The low side switch status is sent to the PCM over serial data.

    2003 Yukon compressor controls (serial cycling).png

    So say you set up a test replicating a valid pressure sensor signal at C2 14, and change the tune to Analog Cycling. What pin do you put the low side switch (or test ground) to?? The same pin we're suspecting of not being connected to anything inside the PCM?

    There's really no good way to nail down whether it's an internal hardware issue with that pin in that specific Service Number, or an issue with that specific OS not able to look at the pin to get the switch status.