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Thread: E38 A/C Pressure Sensor Signal / Cooling Fan Control

  1. #1
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    Question E38 A/C Pressure Sensor Signal / Cooling Fan Control

    Thanks in advance for the help. I tried a thorough search and didn't see anything related. I've been a Master ASE tech for years, but this is my first experience in the tuning world, so I'm trying to get a handle on some things.

    The quick background: I swapped a stock '08 5.3 (LC9) into my '84 C10 pickup last fall. Everything has been working pretty well with minor tuning tweaks. I wanted the ECM to control the fans, so I wired the fans just like the 2008 setup. There are dual fans with the 3 control relays. They are working normally at high and low speeds for engine temperature. The problem comes with the a/c. I built some a/c lines and hooked everything up this Spring. I have the a/c high side pressure sensor wired in just like factory. The compressor is not engaged by the ECM because the factory setup uses the BCM to send a request over the CAN to the ECM for compressor engagement. I've left the compressor wired into the 1984 controls.

    Here's the issue. The fans never turn on with the a/c. The a/c pressure signal in the ECM on my scan tool says 0 psi, 0 volts. I check the 5 volts, low reference ground, and signal circuits at the ECM and everything is good. The signal circuit at the sensor reads 1.2 volts and backprobing the ECM at the signal circuit shows the same 1.2 volts. So I'm a diagnostic tech and I'm thinking the computer must have a problem. We happen to have a 07 Denali (6.2L) in the parking lot with a E38 ECM, so I decide a quick "swap diagnostics" is in order. Now, I can't start my truck with the donor ECM, of course, but when I swap it in, I see 1.2 volts on the scan tool from the pressure sensor. I come to the conclusion that the computer has failed internally.

    So, I buy a junkyard ECM, it's out of a Equinox, but the same E38. I pay for new credits and flash the new (used) ECM with the same tune from the original 2008 computer. The pressure sensor still reads 0 psi and 0 volts.

    At this point, it must be in the tune. I've poured over all the options in the editor and don't really see anything. I'm wondering if the donor truck didn't have a/c at all, but I don't see any option to set that. Sorry for the long winded question, just hoping someone might have a tip or experience with this. Let me know if any more info would be helpful. I know I can run the fans off of a pressure switch and more relays, but if I can get it to work with the factory wiring, I'd rather go that route. Thanks again for the help.

  2. #2
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    I had an issue with a '06 E40, in the scanner the AC Volts read normal but AC Pressure(kpa) always read 1275kpa. Notified Support, issue was fixed in a Beta release after a few tweaks, now both read as they should. So you could have two problems, a Scanner issue and a clutch/fan control issue.

    Do you have channels in Scanner you can add to look at the AC request status and current state? If the ECM never sees the clutch engaged it won't activate the fans (at least not based on what's in the Pressure vs Desired table). Posting the tune file and a log with all the relevant channels added would help a ton.

  3. #3
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    Quote Originally Posted by blindsquirrel View Post
    I had an issue with a '06 E40, in the scanner the AC Volts read normal but AC Pressure(kpa) always read 1275kpa. Notified Support, issue was fixed in a Beta release after a few tweaks, now both read as they should. So you could have two problems, a Scanner issue and a clutch/fan control issue.

    Do you have channels in Scanner you can add to look at the AC request status and current state? If the ECM never sees the clutch engaged it won't activate the fans (at least not based on what's in the Pressure vs Desired table). Posting the tune file and a log with all the relevant channels added would help a ton.
    So, I'll add the log file. Not much to see, really, because the AC stuff never changes. You can see where I turn it on then off with the engine RPM and load. I guess the real question is something you mentioned, will the fans be commanded on with high AC pressure even if the request for the compressor isn't going through the ECM? I knew this might be a sticking point, but short of sourcing a BCM so that I can input an AC request through it, I can't get the computer to command on the compressor. I was leaning toward the fact that the ECM would run the fans with high pressure regardless of the request. Either way, the ECM should see some pressure from the sensor. Thanks for the response!AC switched on and off.hpl
    Last edited by Griffco2002; 08-09-2019 at 09:02 AM.

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    Are you seeing the same 0psi/0v sensor readings with a Tech2 and VCM Scanner? With my issue, I proved it was a VCM Scanner problem because sensor read normal when I plugged in the Tech2.

    Not sure about the fan output responding to the sensor independent of the compressor output control state. I have seen one that worked that way but that doesn't mean all do.

    What's the exact 2008 platform this swap came from? Silverado, Tahoe, or ? Trying to find a wiring diagram to look at is kinda hard without that.

  5. #5
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    Quote Originally Posted by blindsquirrel View Post
    Are you seeing the same 0psi/0v sensor readings with a Tech2 and VCM Scanner? With my issue, I proved it was a VCM Scanner problem because sensor read normal when I plugged in the Tech2.

    Not sure about the fan output responding to the sensor independent of the compressor output control state. I have seen one that worked that way but that doesn't mean all do.

    What's the exact 2008 platform this swap came from? Silverado, Tahoe, or ? Trying to find a wiring diagram to look at is kinda hard without that.
    It's a 2008 GMC Sierra 1500, engine is VIN 3. Both the Snap-On scan tool and the VCM Scanner say the same thing. On the diagram, you'll see a gray wire at the sensor with the 5 volt reference, the tan wire as the low reference ground, and the orange and black wire as the signal. All these have been verified at both the sensor and the ECM.

    Thanks again

  6. #6
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    So you have two issues. 1: Is the ECM getting valid data from the pressure sensor? 2. Is the ECM able to control the fan speed even without the AC request signal via class2 and without activating the clutch control output?

    You can't answer #2 until #1 is resolved. I would really, really want to look at the live data on a real Tech2, to be sure you aren't seeing the same error duplicated in two aftermarket scanner products.

    (AC request on Silverado/Sierra comes direct from the HVAC module as far as I can make out, no BCM involved other than possibly a pass-thru.)

    It's kinda crazy to me that you can buy a class2 chime module for a few bucks, but not an A/C button with a class2 output to tell the ECM to turn on the compressor. Have you asked any of the swap-harness guys about this? PSI, Speartech, or whoever? I'm not finding anything via Google other than "just use a trinary switch" which just seems stupid when it's all already there in the ECM to control everything.
    Attached Images Attached Images

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    You're absolutely right. I was thinking BCM, but it is on the GMLAN network from the HVAC module. I have seen a few facebook posts asking about a module to send the message to the ECM, but never any answers. I'll do a little research, but you're also right that the pressure sensor problem is separate. I'll check out those companies you suggested. Thanks

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    http://67-72chevytrucks.com/vboard/s....php?p=7886196
    That's the guy I sent you a PM about. See post #11: "We now have a muscle car module that gives tap shift and ac serial on command with out a bcm."

    https://youtu.be/vXnBMG-axm4

    But even that box won't make it work if the ECM can't read the pressure sensor.

  9. #9
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    So, I finally stumbled on this thread, but there's no resolution to the problem. Nice to know I'm not the only one with an issue. https://forum.hptuners.com/showthrea...ressure-signal

    I sent him a PM, but he hasn't been on the forum for a few years, so who knows.

    I also found this module, I don't know how I hadn't found it in previous searches, but maybe it will help future people searching the forums. It will provide an AC request to the E38 ECM. http://www.rpmextreme.com/Product/23...st-Module.aspx

  10. #10
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    Quote Originally Posted by Griffco2002 View Post
    So, I finally stumbled on this thread, but there's no resolution to the problem. Nice to know I'm not the only one with an issue. https://forum.hptuners.com/showthrea...ressure-signal

    I sent him a PM, but he hasn't been on the forum for a few years, so who knows.

    I also found this module, I don't know how I hadn't found it in previous searches, but maybe it will help future people searching the forums. It will provide an AC request to the E38 ECM. http://www.rpmextreme.com/Product/23...st-Module.aspx
    Yeah, I'm talking to them now. They're not sure if it will work with the E40, but they are willing to test it.

  11. #11
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    It should work... there are platforms (Trailblazer) that used the E40 and then in later years switched to E67, the class2 messaging between HVAC & ECM didn't change when the ECM changed. Then, there are other platforms (CTS-V) that started out with E67 and changed to E38 and HVAC didn't change there either. So, if it works for E38, it should work all the way back to E40.

  12. #12
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    And I forgot about the C6 Corvette, that one went straight from E40 in '05 to E38 in '06. Derp.

  13. #13
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    Our AC request module should with with the E38 and E67. It may even work on the E40, just haven't tried it yet.

    The module just sends the AC request. You still need the 3 wire high side pressure sensor wired up and functioning and the compressor clutch relay wired into the GM ECU. Its best to run the fans from the GM ECU as well.

    This module can also send the charging request signal so the ECM controlled alternator will function.

  14. #14
    Just my 2cents.

    I'm all for the ECM controlling the fans and I completely agree that using a trinary switch isn't ideal however $250.00 bucks seems really expensive for that module. If it were me I'd either use one of those trinary switches to activate the fan or just program an Ardunio to send the can message.

    I'm not knocking the product but that's a steep price.