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  • Custom Tach for GM

    Alright Danny, I need a hand. This is my second time setting up a custom tach output, but the first one was to drive a diesel tach for a low RPM engine, that took something like a 168 triggers per revolution.

    I'm trying to get my Camaro tach to work and I was getting nothing. So I hooked up the A28 pint to and got double reading like it should. So I went back to my wire and made some changes. It moves it now, but it's all wrong as the GM Tach pegs at startup and only came down a little at 850 rpm. I have an LED installed to test out different output settings, and when it was setup like the tach, RPM Derived and set to come on with time it would flash like crazy with the motor not running.

    I would love to know what signal you get from RPM derived gives, I would think 1 pulse per revolution or 4, but I want to know so I can make intelligent adjustments and not a guess. I wish they would have put in my idea to have the actual math the output shown and change with the settings, so when you are doing something for the first time you could figure it out without guessing and testing.

    After messing around a little before deciding I might as well just ask someone who has done this before. I as you see it would read about 1400 when idling at 950.

    Click image for larger version  Name:	Tach Setup.jpg Views:	1 Size:	106.4 KB ID:	349575
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    Last edited by The Dickens; 04-17-2016, 10:42 PM.
    Shawn Dickens
    Builder/Tuner at www.waterthunder.com
    Chevy LS Airboat Motor Specialists

    Technical Adviser for www.holleyinjection.com
    A site owned & managed by Cyber Space Automotive.
    Authorized Holley EFI Dealer

  • #2
    Holley EFI Tier 2 Training Manual:
    If you want vary the frequency with engine speed, you set the Type to "RPM Derived" and enter
    either/or a Multiplier or a Divider. This deserves some explanation. If both the Multiplier & Divider
    parameters are both zero, the output frequency in Hz is equal to the engine speed in RPM. If the engine
    speed is 1000 RPM, then the output frequency would be 1000 Hz. A 1:1 ratio usually results in a
    frequency that is too high for most applications. For a one pulse per fire frequency for a V8, you would
    need a divider of 15. This would result in a 4 pulse/rev. A divider setting of 30 would result in 2 pulses/rev,
    which happens to be what many GM tachometers use.










    So set your Multiplier as 1 and your divider as 30 for the GM tach which is actually looking for a 4 cylinder signal.
    (And why GM chose to be stupid like that, we will never know.)
    -Scott
    Don't forget to check out progress on my Race Car:
    Project Blasphemy - 8.07 @ 171
    Low 8 Second Street Car

    Comment


    • #3
      Awesome, that's the info I needed & couldn't find. I had no clue what signal was coming out for that, and didn't want to hack my way to an answer. I really think a changing equation for the current output settings would be a great tool and save a lot of confusion, questions, & tech calls. Thanks for the help. Tom probably went over that in the classes and I didn't think I'd ever use, so put no effort into retaining.

      On a different not what is the most reliable fuel pressure regulator for you. Wanting to ditch the in tank one for an inline with dual inlet and single return. Aeromotive has not worked well for our shop ever. Right now, I'm leaning toward a Weldon and would have one on the way, if I could find one of our venders that sold them. But doesn't seem to be a line any of ours carry. Thanks again.
      Shawn Dickens
      Builder/Tuner at www.waterthunder.com
      Chevy LS Airboat Motor Specialists

      Technical Adviser for www.holleyinjection.com
      A site owned & managed by Cyber Space Automotive.
      Authorized Holley EFI Dealer

      Comment


      • #4
        Originally posted by The Dickens View Post
        Tom probably went over that in the classes and I didn't think I'd ever use, so put no effort into retaining.
        There's some instruction on it, in the Holley EFI Tier 2 Training Manual (page 17 - "Inputs & Outputs" section). See last paragraph below.
        https://forums.holley.com/showthread...4640#post74640 (Related Forum Thread)


        Holley EFI Tier 2 Training Manual:
        Output Types

        PWM+
        ‐ Like the +12V output type is used when the other side of the load is grounded. When the
        output is active the ECU “pushes” 12V to the load. This is often called an active high output or high side
        output. Unlike the +12V type it is not simply on or off. You can set it up to smoothly control the duty
        cycle as a function of multiple variables.
        The letter code for “+12V” in the Pin Map is “H.” Think of “H” as active High. After enabling an output
        and assigning it this type it will show up in the Pin Map as an unassigned output with an “H” immediately
        to the left. This signifies it can be clicked and dragged to any available pin that has an “H” beside it.

        PWM− ‐ Like the Ground output type is used when the other side of the load is powered. When the
        output is active the ECU “pulls down” the low side of the load. This is often called an active low output
        or low side output. Unlike the Ground type it is not simply on or off. You can set it up to smoothly
        control the duty cycle as a function of multiple variables.
        The letter code for “+12V” in the Pin Map is “H.” Think of “H” as active High. After enabling an output
        and assigning it this type it will show up in the Pin Map as an unassigned output with an “H” immediately
        to the left. This signifies it can be clicked and dragged to any available pin that has an “H” beside it.

        PWM Setup Parameters
        - Because actuators and solenoids vary greatly, you need some
        adjustments instead of having a one size fits all solution. One thing you need to specify is the frequency
        at which you drive the actuator. You have two options in the software‐ setting a fixed frequency of your
        choosing or using a frequency that is a function of engine speed. For most actuators a fixed frequency is
        appropriate, so you select "Fixed" frequency for the Type and enter in the desired frequency in the field
        below. This frequency is in Hz, or the number of cycles per second.

        If you want vary the frequency with engine speed, you set the Type to "RPM Derived" and enter
        either/or a Multiplier or a Divider. this deserves some explanation. If both the Multiplier and Divider
        parameters are both zero, the output frequency in Hz is equal to the engine speed in RPM. If the engine
        speed is 1000 RPM, then the output frequency would be 1000 Hz. A 1:1 ratio usually results in a
        frequency that is too high for most applications. For a one pulse per fire frequency for a V8, you would
        need a divider of 15. This would result in a 4 pulse/rev. A divider setting of 30 would result in 2 pulses/rev,
        which happens to be what many GM tachometers use.
        May God's grace bless you in the Lord Jesus Christ.
        '92 Ford Mustang GT: 385"/6.3L SBF, Dart SHP 8.2 block, forged steel crankshaft & H-beam rods, forged pistons, Trick Flow Twisted Wedge 11R 205 CNC Comp Ported heads, 12:1 compression ratio, 232°-244° duration/.623" lift/114° LSA camshaft, TFS R-Series FTI Comp Ported intake, BBK 80mm throttle body, Holley Dominator MPFI & DIS, Holley 36-1 crank trigger, MSD 1x cam sync, PA PMGR starter, PA 200A 3G alternator, Optima 34/78 Red battery, 100HP progressive dry direct-port NOS, R134a A/C, Spal Dual 12" HP 3168 CFM fans, Frostbite 3-core aluminum radiator, Pypes dual 2.5" exhaust, off-road X-pipe, shorty headers, Earl's -6AN fuel system plumbing, Walbro 255 LPH in-tank pump & Pro-M -6AN hanger, S&W subframe connectors, BMR upper & lower torque box reinforcements, LenTech Strip Terminator wide-ratio Ford AOD, 10" 3000 RPM C6 billet converter, B&M Hammer shifter, Stifflers transmission crossmember & driveshaft safety loop, FPP aluminum driveshaft, FPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 M/T Street Comp tires.

        Comment


        • #5
          I figured I'd just not retained it in one of the classes. I now have 4 books from them now, and just didn't catch it while flipping through them. Thanks guys!
          Shawn Dickens
          Builder/Tuner at www.waterthunder.com
          Chevy LS Airboat Motor Specialists

          Technical Adviser for www.holleyinjection.com
          A site owned & managed by Cyber Space Automotive.
          Authorized Holley EFI Dealer

          Comment

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