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Thread: Installing a VSS Sensor with Holley EFI

  1. #121

    Default Installing a VSS Sensor with Holley EFI - How to with original TBI optical VSS?

    Quote Originally Posted by Danny Cabral View Post
    Yes, the HP & Dominator ECUs can be wired for a VSS or driveshaft speed sensor (LINK).
    The HP ECU is only compatible with the better, Hall-Effect (digital) "F" Type speed sensors.
    The Dominator ECU is compatible with Hall-Effect (digital) speed sensors, as well as VR Magnetic (inductive) "S" Type pickup speed sensors. (Holley EFI Wiring Manual - Section 9.1 "Inputs", Pages 27 & 28)

    Hall-Effect: 3-wire, square wave signal (digital), pulse generation
    VR Magnetic: 2-wire, sine wave signal (inductive), voltage generation (Hall-Effect vs VR Magnetic)
    Read this post on installing a VSS sensor with either the HP ECU or the Dominator. What isn't clear to me is I have an original '87 TBI truck. The speedometer has an optical VSS in it which generates the 2K PPM as I recall. Not sure if this is considered to be a Hall-Effect or an inductive speed sensor. There is a buffer that attaches to the speedometer that conditions and amplifies the signal, which it then sends to the TBI ECM. I'm not sure if I can just use the output from this to send the signal to the HP or Dominator ECU, or if I need to get something like the Auto Meter 5291.

  2. #122
    Join Date
    Dec 2009


    Quote Originally Posted by hegill View Post
    What isn't clear to me is I have an original '87 TBI truck. The speedometer has an optical VSS in it which generates the 2K PPM as I recall.
    I don't know if Holley EFI is compatible with that type of VSS.
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  3. #123


    Here's what the 1987 R/V Manual says:
    "The ECM applies and monitors 12 volts on CKT 437. CKT 437 connects to the Vehicle Speed Sensor (VSS)
    which alternately grounds CKT 437 when drive wheels are turning. This pulsing action takes place about 2000
    times per mile and the ECM will calculate vehicle speed based on the time between "pulses”.

    According to Jaguars That Run, this outputs a square wave signal. Problem is, since the stock ECM determines the speed by calculating the time between the pulses based on a 12V signal it sends to the VSS, I'm not sure how I would tap into this to get the signal. Any ideas?

  4. #124


    I'm in the same boat. I assume we're talking about a 700R4 transmission.

  5. #125


    Asked a question a while back about using a GM VSS buffer for my VSS input signal. What I'm confused about is that on an '87 Chevy EFI truck like mine the GM ECM sends a 12V signal to the VSS buffer. As the speedometer cable turns it grounds out, which causes the voltage to vary. So if you back probe the wire from the ECM to the VSS buffer, you get a square wave signal that consists of a varying voltage. I just don't understand how to configure this pin on the Holley HP EFI/Dominator ECU. Would this be considered to be an input or an output? My assumption is that this would be an input with an "F" pin type. What I'm not clear on is whether or not the holley ECU can send out a 12v signal to a sensor as an input. Based on everything I have read, I think this would work if I could figure out how to configure the pin. I'm also confused when I look at the inputs and it says that the "G" pin type is triggered when a ground is applied. On a GM ECM, the ground is applied when the ECM closes a switch internally to ground out the circuit. Is this what the Holley ECUs do on a "G" input pin? Any assistance here would be appreciated.

  6. #126


    Did you ever get the VSS output setup done using your 700R4 & Holley HP/Dominator EFI?

  7. #127


    I’ve used the EFI Dominator system with ease and great success in my TT 4.6 4V ‘01 Bullitt Mustang including sorting out systems like Dome Control on my own. Yet, I can’t seem to figure out something as simple as getting a 5V square wave speed output from a US Shift Quick 4 transmission controller to work as an input for speed in the Dominator. What am I doing wrong? First, the output from the Q4 is a 5 square wave. It is the same frequency as the VSS input from the 4R currently, but can be ratioed within the Q4 software for differential final drive ratios. I know it works because it works with my Auto Meter Speedometer currently. This 5V output is wired into Pin J2-A21 and I’ve selected Digital Speed/Frequency as its function and titled "Speed Q4". In Data Monitor Setup, I’ve activated Speed Q4 and added it to my monitor display. I get no readings on the Data Display with the wheels turning. What am I missing here?

  8. #128


    I believe the issue is the fact that it's a 5V signal. The Dominator is looking for a 12V signal. I'm an electronics dummy, but maybe a pull-up resister can be used in this case. Andrew

  9. #129


    I missed the 12V thing somewhere. That must be it. There's an option for a 12V vs 5V output from the Quick4 so maybe an easy fix? One can hope anyway. Thanks for the reply and I'll report the results when I get back to it.

  10. #130
    Join Date
    Nov 2017
    Alpharetta, Georgia


    Quote Originally Posted by andrewb70 View Post
    I believe the issue is the fact that it's a 5V signal. The Dominator is looking for a 12V signal. I'm an electronics dummy, but maybe a pull-up resister can be used in this case. Andrew
    While the name might make it sound as if it could do something like that, a pull-up resistor won't boost voltage from one level to another in a circuit like this. What a pull-up resistor does is allow a branch of a circuit to slowly (in "electronics time") rise to a known state vs. ground (or some other level), so that the circuit it's tied into won't be left at a vague and irregular voltage. If you tied one into this circuit, it'd make the signal go from a 5V vs. ground signal to a 12V vs. 12V signal, which is not a signal.

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