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Thread: Tach not working on you LS3 engine? Read this

  1. #1
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    Tach not working on you LS3 engine? Read this

    I spoke to Autometer about my American Platinum tach not working. This is the base gauge kit that FFR supplies.
    When GM reflashed the ECM to delete the security, the rear O2 sensors, etc it sometimes interferes with the tach signal. He said you have a 50/50 chance it will work.
    Mine doesn't work, I'll be looking into this today or tomorrow and posting more info.
    In the meantime here is more info.
    https://www.autometer.com/resources/...faq_view/id/77

  2. #2
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    Great info! Looking forward to more. I have LS3 just about ready for first start.

    Thanks
    Rick

  3. #3
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    I hooked the tach up to my spare battery, a signal generator, a scope, and a resistor decade box set to 10K ohms. The tach did work.
    Next I'll hook the scope up to the tach ECM signal wire to see if I have a square signal.

    One more issue regarding the gauges, the oil pressure gauge and the LS oil pressure sensor, they are not compatible.
    The gauge wants to see a resistance change to operate it, like the sensor that came with the gauge supplies.
    The LS sensor provides a 0-5 volt signal for oil pressure.

  4. #4
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    Got my tach working...easy fix.
    Even though the LS3 engine info says we don't need a pull up resistor I needed one to get the tach working.
    I hooked a 10K resistor between the ECM tach signal wire and +12 volts.
    The tach needs to be set to 4cyl, not 8 cyl.

    Will checkout the MIL remote wire next.

    If you have the GM tranny and engine ECM there is a bulkhead 12 pin connector that connects both together. The "B" pin has a brown/white wire that is the one to use for the dash mounted MIL light.
    If your using an LED you'll need a resistor in series with the LED, I used a 1K ohm.
    Last edited by JimLev; 09-09-2018 at 10:55 AM. Reason: added MIL info

  5. #5
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    Quote Originally Posted by JimLev View Post
    Got my tach working...easy fix.
    Even though the LS3 engine info says we don't need a pull up resistor I needed one to get the tach working.
    I hooked a 10K resistor between the ECM tach signal wire and +12 volts.
    The tach needs to be set to 4cyl, not 8 cyl.

    Will checkout the MIL remote wire next.

    If you have the GM tranny and engine ECM there is a bulkhead 12 pin connector that connects both together. The "B" pin has a brown/white wire that is the one to use for the dash mounted MIL light.
    If your using an LED you'll need a resistor in series with the LED, I used a 1K ohm.
    What signal wire are you calling the ECM tach signal wire. I have a tach/coil wire in the dash harness but can not get tach to work. Have tried putting resistor in line with 12v. but no luck. I have switches set to 4 cyl. any help appreciated.

  6. #6
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    I think the wire your using is one that is in the FFR harness.
    You need to use the GM wire, it's the orange/black.
    The label on the wire says "RPM".
    Yes, 4cyl is the correct switch position as the tach sig is only coming from one bank of cylinders.

    Update..speedo you asked about
    Pretty sure you need to use the GM connector marked Vehicle SPD and not the one marked TISS for the sensor at the back of the tranny.
    Last edited by JimLev; 09-22-2018 at 12:51 PM.

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