Note. Engine with ECU «BOSCH ME 7.4.6 is equipped with two oxygen sensors for each row of cylinders: upper and lower.
Heated sensors
Heated oxygen sensors are switched on by the ECU command if:
- battery voltage below 15 V;
- after a 4-second delay after starting.
The heating of the oxygen sensors is stopped if the regulation is based on temperature. It is maintained at 750°C.
Upper sensor signal voltage
Read parameter value: «upper sensor signal voltage» on the scan tool: this is the amount of voltage that is sent to the computer by the oxygen sensor upstream of the catalytic converter. The voltage value is expressed in millivolts. When the motor is running in closed loop, the voltage should fluctuate rapidly between two values:
- 100 mV±100 lean;
- 800 mV±100 for a rich operating mixture.
The smaller the difference between the minimum and maximum values. the less accurate the information from the sensor (usually this difference should be at least 500 mV).
Note. If this difference is small, check the sensor heating.
Lower sensor signal voltage
It is necessary to read the parameter value: «lower sensor signal voltage» on the diagnostic tool. This is the amount of voltage that is sent to the ECU by the oxygen sensor installed after the catalytic converter. The voltage value is expressed in millivolts.
This sensor is designed to diagnose the catalytic converter and repeat, more accurate, control of the composition of the mixture (slow control loop). This function is activated only after the engine has been running for a while.
When the motor is running in closed loop, the voltage should change in the range of 600 mV±100. When decelerating, the voltage should be below 200 mV.
The voltage reading from the scan tool at idle should not be taken into account.
Correction of the composition of the working mixture
Parameter value «correction of the composition of the working mixture», scan tool readout is the average value supplied by the ECU based on the mixture information. transmitted by the oxygen sensor. installed in front of the catalytic converter (in fact, the oxygen sensor analyzes the oxygen content in the exhaust gases).
The average correction value is 1.
Value below 1: lean request.
Value greater than 1: enrichment request.
Commencement of fuel mixture regulation
The mixture control starts after a time delay after the engine is started and regardless of the coolant temperature.
The time delay after starting the engine can vary between 0 and 70 s.
Phase «control loop opening»
When adjusting the composition of the working mixture, the ECU does not take into account the voltage value of the sensor signal in the following cases:
- with the accelerator pedal pressed all the way down;
- during sharp acceleration;
- when decelerating with the cessation of fuel supply;
- when the oxygen sensor fails.
Standby mode in case of failure of the oxygen sensor
If the voltage signal of the oxygen sensor when adjusting the composition of the working mixture is incorrect (changes very little or not at all), then the ECU will only go into standby mode if the fault has been declared present for 3 minutes. Only in this case, information about the malfunction will be entered into the memory of the computer. In this case, the parameter value «correction of the composition of the working mixture» equals 1.
If a malfunction of the oxygen sensor is detected, if information about the malfunction is already stored in the memory, the ECU immediately switches the engine to work with an open control loop.