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Thread: EFAN conversion on 06 trailblazer I6 ECU hangs, fan stays on

  1. #1
    Tuner in Training
    Join Date
    Feb 2009
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    28

    EFAN conversion on 06 trailblazer I6 ECU hangs, fan stays on

    I installed derale der-16927 dual fans this weekend and attempted to make the necessary modifications to the wiring and ecu to have the ecu control them completely by switching from EV fan to discrete. By the way, this universal Derale part fits the trailblazer radiator core to cover about 97% of the core! Like it was meant to be! I wired them with 3 relays for a low and high. Low speed runs them in series and high runs them in parallel just like the new GM fans. When I manually put these babies on high they move A LOT OF AIR!

    Since this is a P12 ECU, I needed to find what pin the second fan state outputs to. I cross referenced a Cobalt SS and found that it was pin #10 of connector C1 to control high speed and pin #51 to control low speed. Sure enough, pin 51 on the trailblazer goes to the solid state relay to control the EV fan. Just have to pull the relay, jump pins 4 and 5 of the fuse box where the relay goes and then the white wire at the EV fan is the low speed switch that the ECU controls. I had to add a pin to 51 as it was empty on the trailblazer.
    THE GOOD:
    1. Fan always turns on at the right time.
    2. Pin 51 is the correct pin for low speed discrete fan control
    3. With the AC off, the low speed fan control works just like it should, it cools the car and turns off.
    4. No codes were thrown
    THE BAD:
    1. I could never get the ECU to run the fans on high; it only ran them on low. Even when the reported fan state was clearly high enough when logging. I also forced the fan state to 100% in scanner. I also programmed the ECU to make fan state 1 turn on all fan states…none of it worked. This tells me that pin 10 is either the wrong pin or my P12 in the trailblazer and the P12 in the cobalt are different? What do you think? Is it possible HPtuners is not doing it’s job in making the necessary programming changes?
    2. If I turned the AC on, the fan state would climb along with the ECT and AC pressure until it turned the fan on. Once it turned the fan on, the AC pressure sensor voltage and the ECT drops but the fan won’t turn off! The ECT just keeps bumping against the thermostat at 180! It basically gets stuck on because the fan state % hangs too high!
    3. The AC pressure and Trans temp do not report out when logging. I could watch AC pressure sensor voltage, but not pressure. The trans temp just stayed at -40.
    I hope someone can help me. It is really disappointing to go through all this research and work only to have it not work like it should. I would really hate to go buy a fan controller because I can’t get this to work. I’ve attached a log and my tune saved with beta 2.23. PLEASE HELP!
    Last edited by bruhaba; 03-20-2011 at 09:35 PM.

  2. #2
    There is a post somewhere with an Excel sheet for the GM fan tresholds. Pin 10 & 51 are the right pins (for an LSJ P12, no guarantees for a trailblazer), but there is a GM logic-catch with the fans: once turned on they will only switch off when the temp is below the treshold AND when the vehicle is moving again. (Or when ignition is switched off.)
    So you need to have a working VSS sensor... (what kind of VSS sensor does the trailblazer use? Also a 2 wire VR-type sensor? (like ABS)

    Is the trailblazer dash working through CAN with the P12 ecu? I need a coolant PWM signal from the P12 for my ECT temperature readout... The Cobalt/Ion :SJ dash is done by CAN.