Vf1 voltage

Subject: Vf voltage information for oxygen sensor testing on 1983 and later TCCS Toyota EFI engines.
Contributing Source: Toyota service manuals (a 1997).
While in the normal diagnostic mode with T/TE1 connected to E1, the Vf voltage (feedback voltage) switches between 0 volt and 5 volts as the 02 sensor voltage varies between 0 volt and 1 volt.
When testing at the Vf terminal, be sure the engine is fully warm and hold the engine rpm 2500 to keep the IDL contacts open. Then watch the voltage. The Vf voltage should fluctuate at least eight times every 10 seconds.
When Vf is measured at closed throttle while the idle contacts are closed and TE and E are jumpered, the operation of the terminal voltage

changes to two steady voltages, which indicate the following:

While the ECM is controlling the fuel injector duration with input or feedback from the oxygen sensor, the ECM also is learning about the amount of fuel correction that it is making. This learned value is used by the ECM during both open- and closed-loop operations to help fine tune the fuel control for each particular engine and its operating conditions.

The Vf voltage should be measured only during closed-loop feedback operation, such as during warm engine idle or during steady cruising. During modes of operation such as cold engine start, acceleration, or deceleration, the Vf voltage will often indicate 0 volt and does not reflect the learned value stored in the ECM.
Also, if Vf is measured when the vehicle has been operating at or above 5000 feet elevation, you may see 0 volt. This is considered normal. Vf learned value has a 20-percent effect on the air-fuel ratio.

Check Vf learned value with DVOM at Vf (or Vf1) and E1 terminals. The engine should be fully warm, and the Vf voltage (02 sensor) should have been checked for correct values.

When the fuel control process is started in the ECM, the first thing that the ECM does is to calculate the basic injection duration. The ECM will use the sensor information and engine data (such as volumetric efficiency) to arrive at this number:
Vs/PIM + Ne + THW + THA + Engine Data = Basic Fuel Injection Duration
The basic injector value is like the ECM's educated guess about what the injector pulse width should be at that given time, (figure). The basic injection value is very accurate, with ±20 percent of what the actual value should be.

At this point, the 02 sensor provides the fine tuning information so that the air-fuel ratio is maintained at about 14.65:1.
Depending on a number of factors, including altitude, engine wear, fuel, and engine tolerances, the amount of correction caused by the 02 sensor feedback varies. If the amount of correction that the ECM has to make is relatively small, such as 3 percent, the ECM can easily accommodate it, figure.

However, the ECM in feedback operation is programmed to make changes in the fuel injection duration only in small steps or increments. Thus, if operating at high altitude, the ECM would have to make a relatively large percentage change to the basic injection value to compensate for the lack of oxygen in the air. Thus, the ECM response would be very slow and performance would be affected.
To avoid this situation the ECM memorizes the amount of feedback correction it is making (like an average) over a period of time. This amount is then used in the initial basic injection value calculation to improve its accuracy. By making the basic injection value more accurate, the ECM will only have to make small incremental changes (±3 percent) to the basic injection value and still have the air-fuel ratio at the optimum value.

When in the diagnostic mode (TE1 grounded), voltage at the Vf terminal should move from 0 volt to 5 volts to indicate that the feedback system operating and the ECM is continually fine tuning the injector pulse width to keep the air-fuel ratio centered at about 14.65:1 The Vf voltage when TE is open tells you how much correction has been necessary to the basic injection duration to maintain that air-fuel ratio. It will range from volt to 5 volts, with the number of steps depending on the type of airflow metering used (figure).

D-type fuel injection systems (which have a pressure sensor instead of an airflow sensor) use only 3 voltage steps: 0 volt, 2.5 volts, and 5.0 volts. The 2.5-volt normal voltage covers rich or lean fuel correction of up to 10 percent. A 0-volt or 5-volt reading has the same meaning as it does on L-type injection systems.

The Vf terminal provides information on the amount of correction the ECM is making while the engine is operating (table). The ECM compares the amount it is correcting the air-fuel ratio to a set of programmed values. If the amount of correction is within its normal range, the ECM puts about 2.5 volts onto the Vf terminal.

Table. Interpreting learned value
Vf1 Signal	Engine Condition	Fuel Trim (ECM Compensation)
~ 0 V	Rich	Go leaner - 11-20 %
~ 1.25 V	Slight Rich	Go leaner - 4-10 %
~ 2.5 V	Normal	Lean/rich ± (0-3) %
~ 3.75 V	Slight Lean	Go richer + (4-10) %
~ 5 V	Lean	Go richer + (11-20) %
*D-type* 2.5V	Normal	Lean/rich ± 10 %

If the ECM is decreasing the amount of fuel that it thinks should be normally used, it will output a lower voltage (either 1.25 volts or 0 volt) on the Vf terminal. This will happen when the ECM is compensating for a rich condition.

If the ECM is increasing the amount of fuel that it thinks should be normally used, it will output a higher voltage (either 3.75 or 5 volts) on the Vf terminal. This will happen when the ECM is compensating for a lean condition.

In general, a 0-volt or 5-volt reading indicates a problem. The 1.25 volts or 3.75 volts is considered normal with 2.5 volts considered optimal. (When Vf voltage is at 2.5 volts, the basic injection duration needs to be adjusted by only ±3%). But if you encounter a driveability problem that sets no codes, this information can help direct you to possible problems.

If there is a lean condition that needs to be corrected, such as a vacuum leak, the ECM will add fuel to the base injection duration so that the air-fuel ratio will be at 14.65:1. If it has to increase the duration by 3 to 10 percent, the Vf voltage will be 3.75 volts. If the increase in fuel is greater than 10 percent Vf voltage will be 5.0 volts.

If there is a rich condition that needs to be corrected, such as when the vehicle is driven at high altitudes, the ECM will decrease fuel from the base injection duration so that the air-fuel ratio will beat 14.65:1. If it has to correct the duration by subtracting 3 to 10 percent, the Vf voltage will be 1.25 volts If the decrease in fuel is greater than 10 percent, Vf voltage will be 0 volt.

Each time the ECM is disconnected or the EFI fuse is removed, the ECM will also lose its learned value stored in memory. For the ECM to relearn the value, the vehicle must be driven in closed-loop, feedback operation for a certain amount of time. The amount of time will vary depending on the engine and the type of conditions the vehicle is driven in.

If memory is lost for D-type systems, it may take up to 20 minutes for strategy to be re-learned, depending upon altitude. Airflow meter types are able to relearn faster.

Default voltage for all TCCS systems is 2.5 volts (±0 25 volt). In other words, if memory is lost, the ECM operates from an internally fixed (default) voltage.

Vf learned-value voltage measurements may not be exactly 0 volt, 1.25 volts, 2.5 volts 3.75 volts, and 5 volts and can vary by about 0.25 volt, depending on the accuracy of a voltmeter. However, the number of voltage steps (either 3 or 5) will always be the same.

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