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Thread: Information: Vf

  1. #1
    Living the dream... Kinda
    Supracentral's Avatar
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    Jan 1970
    Woodstock, GA USA

    Default Re: How to: MAFT Pro Vf Monitor


    The Vf1 output has three different types of Vf output.

    • o2 Sensor Feedback Mode
    • Diagnostic Mode
    • Learned Value Mode (<-- this is the one we are talking about for tuning)

    1. Oxygen Sensor Feedback Mode.

    * T1 and E1 Connected (In the diagnostic block.)
    * Engine Running at 2500 rpm

    Result of Ox sensor signal processing:
    0v=Lean or Open loop condition

    2. Diagnostic Mode

    *T and E1 connected
    *Engine off

    Results of Diagnosis:
    5v=Normal, No trouble codes stored.
    0v=TCCS Trouble code stored. View the 'Check Engine' Light, It will blink codes.

    3. Learned Value Mode

    *T and E1 not connected
    *Any operating conditions, RPM's, or speed.

    The Vf line feeds five stepped voltage values of 0,1.25, 2.5, 3.75, 5.0 volts

    Meaning: Result of Air-Fuel ratio voltage feedback

    Learned Value: It is a fuel injection correction coefficient which tailors the standard fuel injection duration to minor differences between engines due to manufacturing tolerances, wear, and minor mixtures disturbances like small vacuum leaks.

    This coefficient is capable of altering the calculated injection {before Ox correction} by as much as 20% to prevent Ox sensor correction from being excessive.

    If you encounter a driveability problem that sets number codes, this Vf voltage feedback can be of some help, especially code 25/26. (engine condition rich or lean)

    • Discard Vf reading when engine is cold- It will be fixed on 'Rich'.
    • The Vf learned value tells you how hard the ECU is working to keep the engine running properly.
    • After loss of power to ECU, it will take a certain amount of time to relearn, depending on the engine model and vehicle driving conditions.
    • The ECU will learn faster if the input signals are stable- Water Temp, Air Temp, Throttle position sensor, Air, O2, etc..
    • At high altitude operation, Vf can be 0v 'Lean' but it is considered to be normal. {lack of oxygen}
    • Vf voltage can be "border line" also, flipping back and forth between 2 ranges.
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  2. #2
    Supramania Regular

    Join Date
    Sep 2009

    Default Re: Information: Vf

    I know this thread is years old but i am getting into the vf tuning/adjusting soon. I am at 6600ft car is not running right now so i dont know my vf voltage, but if it is at 0v can it be possibly to adjust to 2.5 or what would you rec, being an experienced supra owner? my mods will/are:
    57trim ct26, ic kit 2.5 inch, 3in downpipe back, denso 560cc injectors, lex afm, walbro 255, apexi safc, innovate wideband, afpr kit.

  3. #3
    Supramania Regular

    Join Date
    May 2010

    Default Re: Information: Vf

    If you tune your car at that altitude, you should be using the AFC to tune it back to 2.5V.

    This thread must be a zombie... it keep being dug back up every time it gets buried.
    87' 1JZ W58 Holset HY35W

  4. #4
    Supramania Regular
    IBoughtASupra's Avatar
    Join Date
    Mar 2009
    Queens, NY


    Hey Mike, on my 1JZ, the Vf1 pin is showing 2.1v at operating temperature, 180F.

    The car is on a stock fuel system with no piggyback or external tuning and a stock ECU. It has been upgraded to single as well with HKS 9.0MM 264 cams degreed at stock timing on adjustable cam gears for now.

    My wideband is showing a 12.5 for my AFR. When I first start the car, it shows 10.0, then as the car warms up, it goes to 12.5. I am lost here as if my car is idling with a 10.0 fuel ratio, shouldn't I see black smoke and smell heavy fuel which I don't.

    Also, AEM is new but I misplaced the wideband sensor, so I bought one from a supplier.

    Could it be a bad sensor reading rich and what do you think of the Vf1 reading at operating temperature?


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