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Thread: Tach reading on E38 ECM used in an LS3 swap

  1. #1
    Tuner in Training
    Join Date
    Aug 2020
    Posts
    16

    Tach reading on E38 ECM used in an LS3 swap

    Hi All;

    I have an LS3 with E38 ECU in a C2 Corvette. Using HPTuners scanner, I get no tach reading (engine running of course). The LS3 is out of a '09 Corvette ... which has a factory dash tach ... so I assume the E38 would have had a tach output signal. I have the E38 "tach" programmed to crank & 15/15.

    The dash tach in the C2 corvette is connected to pin 48 on the E38 ECU and also has no tach reading. I have no 1kohm pullup on this wire at this moment, I'll be trying that shortly. But this shouldnt affect the HPtuners lack of a tach reading (which would come across the OBD connection).

    Its almost as if the E38's tach output is somehow "disabled".

    Anyone have any suggestions/ideas.

    Thanks
    eboch

  2. #2
    Advanced Tuner
    Join Date
    Oct 2013
    Posts
    550
    The Tach pull-up circuit (i.e. 12 volt power with an inline resistor) should get you a useable Tach signal.

    The Gen 4 ECMs (E38 or E67) do not output a strong enough "pulse" to excite an older Tachometer. The pull-up circuit serves to amply this signal.

    Setting the ECM Tach High / Low to 15 / 15 will create a 4 cylinder pulse. Setting this to 7 / 8 will produce an 8 cylinder pulse.

    Once you get a useable Tach signal, then adjust the pulses as necessary until the C2 Tach is reasonably accurate.

  3. #3
    Potential Tuner
    Join Date
    Aug 2021
    Posts
    4
    Quote Originally Posted by eboch View Post
    Hi All;

    I have an LS3 with E38 ECU in a C2 Corvette. Using HPTuners scanner, I get no tach reading (engine running of course). The LS3 is out of a '09 Corvette ... which has a factory dash tach ... so I assume the E38 would have had a tach output signal. I have the E38 "tach" programmed to crank & 15/15.

    The dash tach in the C2 corvette is connected to pin 48 on the E38 ECU and also has no tach reading. I have no 1kohm pullup on this wire at this moment, I'll be trying that shortly. But this shouldnt affect the HPtuners lack of a tach reading (which would come across the OBD connection).

    Its almost as if the E38's tach output is somehow "disabled".

    Anyone have any suggestions/ideas.

    Thanks
    eboch

    I have a E67 ECM connected to my 78 factory tach.

    First try this to see if you get any tach movement. My HP tuner on the engine tab has 3 options for tach serial / crank / frequency. Not sure if the E38 ECU has similar options.

    set this to frequency. If you get readings then the next problem is it accurate. Mine reads 1/2 actual RPM (no fix found yet).

    I also needed a pull-up resister of 5k connected to the ECU tach output pin.