Understanding the Vf signal

**Kirk** · 11-09-2007, 10:15 AM

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.

The Vf output has three different types of Vf output:

Oxygen sensor feedback mode
Diagnostic mode
Learned value mode

Oxygen sensor feedback mode

* Connect T and E1
* Make sure the idle contact point is Off (i.e. TPS just slightly above idle)
* Idle the engine at 2500 rpm

Result of oxygen sensor signal processing:

5V = rich
0V = lean

Diagnostic mode

* Connect T and E1
* Make sure the idle contact point is On (i.e. TPS @ idle)
* Shut the engine off

Results of Diagnosis:

5V = Normal
0V = Trouble code stored

Learned Value 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).

Notes:
* 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 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
* 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:

0V=rich, ECU decreasing fuel

1.25V=normal, ECU decreasing fuel
2.5V=normal
3.75V=normal, ECU increasing fuel
5V=lean, ECU increasing fuel

What does this all mean?

Think of the Vf signal as a peek 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 add AFM correction, or a need to turn down the boost. It can also tell you if your ECU's fuel table is way off the mark.