Understanding the Vf signal

Note: This article was lifted in total, and without proper permission, from the Supra.com site: ftp://ftp.supras.com/pub/SONIC/MA70/Vfoutput.txt . It was in a sloppy FTP/text file format, so I took the liberty of making it more presentable. I hope this doesn't bend anyone's nose out of joint, because it's very useful stuff...

VERY IMPORTANT: I did not write this, and I am not an expert in interpreting your Vf signal!! I'm still learning this myself, so PLEASE do not send me an email that says, "Hey, my Vf signal is 1.25 volts at idle and drops to 0 volts at WOT, how do I solve this?", because I won't know. You are on your own with this material!!

For the uninitiated, the Vf signal is one of the more important ECU output signals, as it essentially reports on how well the ECU is adjusting the fuel trim to meet engine load levels. To locate the Vf signal lead, refer to the EFI system wiring chart available elsewhere on my site. (The lead is labeled "Vf".) All referenced connections exist on the 22RE/RTE ECU's.

From the Supra Tech Tips web site:

The Vf output has three different types of Vf output:

Oxygen sensor feedback mode
Diagnostic mode
Learned value mode

Oxygen sensor feedback mode

To test in this mode:

  • Connect T and E1
  • Make sure the idle contact point is Off (Note: this likely refers to the contact switch between pins 1 (E1) and 2 (FC) in the AFM -- jeff)
  • Idle the engine at 2500 rpm

Result of oxygen sensor signal processing:

    5V = rich

    0V = lean

0v...........Open loop condition (Note: this was in the original text, but I'm not sure how it applies here. -- jeff)

Diagnostic mode

To test in this mode:

  • Connect T and E1
  • Make sure the idle contact point is On
  • Shut the engine off

Results of Diagnosis:

    5V = Normal

    0V = Trouble code stored

Learned Value Mode

To test in this mode:

  • Disconnect T and E1
  • Run the engine at any speed (e.g., drive it around)

Result of Air-Fuel ratio voltage feedback:

    0V = Rich

    1.25V = Normal

    2.5V = Normal

    3.75V = Normal

    5V = Lean

Learned Value 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 Oxygen correction) by as much as 20% to prevent Oxygen sensor correction from being excessive.

If you encounter a derivability problem that sets no codes, this Vf voltage feedback can be of some help. Especially code 25/26 (engine condition rich or lean). (Note: these appear to be Supra specific codes. -- jeff)


  • Discard Vf reading when engine is cold.
  • The Vf learned value tells you how hard the ECU is working to keep the engine running properly. After loss of power to ECM, it will take a certain amount of time to relearn, depending on the engine model and vehicle driving conditions. The ECM will learn faster if the input signals are stable <Water Temp, Air Temp, Throttle position sensor, Air, Ox etc>
  • At high altitude operation, Vf can be 0v but it is considered to be normal (lack of oxygen).
  • Vf voltage reading may be different, depending on accuracy of voltmeter, and connections. However, the number of steps (5) are still the same (e.g. 1.10v instead of 1.25, 2.2v instead of 2.5v, 3.5v instead of 3.75v). Vf voltage can be "border line" also, flipping back and forth between 2 ranges.

Learned value mode fuel trim conditions:

ECU decreases fuel by
11% - 20%
ECU decreases fuel by
4% - 10%
+/- 3%
ECU increases fuel by
4% - 10%
ECU increases fuel by
11% - 20%


What does this all mean?

Think of the Vf signal as a peak inside your ECU. At a glance, you can see how hard your ECU is trying to compensate for any discrepancies between the signals it receives from the AFM, TPS, water temp, air temp, and engine speed (RPM), and the preset values it has stored within it's little silicon brain. For example, if you've cranked up the boost and haven't made any other adjustments, the engine is going to run lean. The ECU will be somewhat aware that things aren't what they should be -- engine is running hotter, AFM is flowing more air for a given throttle plate angle (TPS value) than it expects, etc. The ECU will try to compensate, and the extent of this compensation will be shown, in gross terms, by the value of the Vf signal. Since the Vf signal only comes in five sizes, the true extent of the ECU's compensation can't be known by this signal alone. It should, however, be enough to point out obvious problems (if they weren't already apparent), such as a need for bigger injectors, a need to change the AFM spring tension, or a need to turn down the boost. It can also tell you if your ECU's fuel table is way off the mark.

And that, in a nutshell, is what the Vf signal is all about. Thanks to the unknown Supra owner who first pulled this information together.

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