Oxygen Sensor Signal

The oxygen sensor input to the ECM is needed to correct the injection duration, to ensure that the ideal air/fuel ratio is maintained (Lambda control). The oxygen sensor consists of a porous ceramic material that is coated with a layer of porous platinum. The sensor becomes conductive when heated to its operating temperature (300 °C minimum). The outside surface of the sensor probe is immersed in the exhaust gas flow and the inside of the sensor probe is open to ambient air through the various venting ports in the body of the sensor.

The differential of oxygen content between the exhaust gas and the ambient air on the internal side of the sensor body creates a potential difference of oxygen ions. The ceramic material of the sensor generates a millivoltage due to the potential difference of the oxygen ions. The generated millivoltage is proportional to the differing levels of oxygen and is the base signal the engine control module uses to monitor oxygen content in the exhaust gas. The sensor is designed to produce a voltage of 0.15V to 0.85V within the limits of the stoichiometric range.

Rich mixture: Oxygen content differential is at it's greatest point. Potential difference is at it's greatest which causes a high millivoltage up to 0.85V to be produced.

Lean mixture: Oxygen content differential is at it's lowest point. Potential difference is lower which causes a millivoltage as low as 0.15V to be produced.

The ECM uses this varying voltage input to adjust the injector on time to achieve the ideal air/fuel ratio for all engine operating conditions.



OXYGEN SENSOR SIGNALS (SIEMENS)

The MS 41.x oxygen sensors function differently from Bosch type sensors. However the end result of the oxygen sensor feedback is the same. The ECM uses the signals from the sensors to maintain LAMBDA = 1.







The probe of the sensor which is exposed to the exhaust gases is made from titanium dioxide (a semi-conductive material). When heated to and maintained to an operating temperature of 600 to 700 °C, the titanium dioxide becomes conductive and will allow current to flow, based on the amount of exhaust in the exhaust.

The resistance value of the sensor changes rapidly when the mixture deviates from LAMBDA = 1. If the oxygen content of the exhaust is high (LEAN), the oxygen molecules will block the flow of electrons through the titanium dioxide. This creates high resistance and a small voltage drop across the sensor tip.

If the oxygen content in the exhaust drops (RICH), the resistance to electron flow drops and the semi-conductive tip allows electron flow, creating a large voltage drop.

The MS 41.X ECM supplies the oxygen sensor with a standing voltage of 5 volts.

The ECM then monitors the voltage drop across the sensor tip as it measure of lambda swing.

The voltage signal at the ECM will switch between approximately 4.6 to 0.1 volts as the mixture changes from lean to rich either side of Lambda = 1.