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  • Pin A28 - how do I locate it on P1A harness connector?

    How do count the holes to find pin A28? I found a blue/white wire near the middle of the connector, but every time I count pin holes to double check, I get a different place. Trying to connect a standard Auto Meter tach. Is this where I would connect the signal wire? Thanks.

  • #2
    Originally posted by cabronco View Post
    How do you count the holes to find pin A28? I found a blue/white wire near the middle of the connector, but every time I count pin holes to double check, I get a different place.
    Tachometer +12V square wave wire to operate a typical tachometer:
    It's the blue/white wire, pin A28 on the P1A ECU connector.
    Look very closely, the start & end of each row is numbered.
    It's routed to cavity H on the 10-pin ignition harness connector.

    Holley EFI Wiring Manual:
    EST Output – Activates the EST Output trigger (square wave trigger) that is used to drive an HEI ignition module (Pin A28).
    When a DIS/CNP system is configured, this outputs a +12V square wave that can be used to operate a typical tachometer.
    Trying to connect a standard Auto Meter tach. Is this where I would connect the signal wire? Thanks.
    For a distributor ignition (with or without CD box), read this link:
    http://forums.holley.com/showthread....-TACH-Question (Example - GM HEI Tach Signal)
    http://forums.holley.com/showthread....4913#post44913 (Additional RPM Signal Wiring Information)
    http://documents.holley.com/techlibr...10555rev17.pdf (Holley EFI Wiring Manual, Section 2.1 "ECU Pin-Outs")

    Voltage triggered vs current triggered tachometers: https://documents.holley.com/8920.pdf (MSD 8920 Tach Adapter Instructions)
    MSD Tachometer Operation, Wiring & Info: https://www.holley.com/blog/post/tachometer_operation/ (MSD Tachometer Tech)
    https://forums.holley.com/showthread...361#post226361 (Holley Sniper EFI Tachometer Wiring Information)
    http://forums.holley.com/showthread....7583#post47583 (Holley Terminator, HP & Dominator EFI Tachometer Signal Information)

    For a distributorless ignition system, use pin A28. See below.

    Holley non-LSx Main Harness:
    A28 (P1A connector) blue/white wire - EST/Spout Output operates a tachometer.
    It's already "wired in" to the 10-wire ignition connector - cavity H.
    It outputs a 12V square wave, same as the Tach Out on an MSD box. No adapter required.
    Some factory OEM tachometers may need a "tach signal amplifier" to function properly.

    Holley LSx Main Harness:
    The end of this document has a note about the LSx loose blue/white wire for a tachometer:
    http://documents.holley.com/199r10597rev2.pdf (LSx Main Harness Notice)
    It outputs a 12V square wave, same as the Tach Out on an MSD box. No adapter required.
    Some factory OEM tachometers may need a "tach signal amplifier" to function properly.

    Originally posted by Holley Terminator X Quick Start Guide
    (Optional) Tach Output – Color = Blue with white stripe – This wire provides a 12V square wave output and can be used to trigger a conventional tachometer. Note: This loose wire is not present on all harnesses (LINK - Section 10, "Loose Wires", Page 13).
    If your main harness doesn't have this tachometer wire, you'll have to install it into the P1A ECU connector using the information in this post.
    https://forums.holley.com/showthread...als-Holley-EFI (ECU Connectors & Terminals - Holley EFI)

    There's custom tachometer programming information in the Holley EFI Tier 2 Training Manual (page 17 - "Inputs & Outputs" section). See last paragraph below.

    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, Eagle forged steel crankshaft & H-beam rods, Wiseco forged pistons, Trick Flow Twisted Wedge 11R 205 CNC Comp Ported heads, 12:1 compression ratio, 232°-244° duration/.623" lift/114° LSA H/R 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 SS dual 2.5" exhaust, SS off-road X-pipe, SS 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.

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