O2 sensors, component description - GF07.04-P-6100VA
ENGINE 273.961 in MODEL 216, 221 up to Model Year 8
Shown on engine 272.963
Location
Engine 272 CIS-E: The O2 sensors downstream TWC are bolted onto the side of the catalytic convertors. They are located in the space between the first and the second monoliths.
Engine 272 DE: The O2 sensors downstream TWC are bolted into the exhaust pipes downstream of the catalytic converters.
Shown on engine 272.963, right cylinder bank
Task
The O2 sensors downstream of TWC (guide or diagnostic sensors) are designed as planar oxygen sensors and detect the residual oxygen content in the exhaust gas for the following tasks:
- Two-sensor control
- Monitoring of catalytic converter efficiency.
Design
Connector for O 2 sensors downstream of TWC, round and square version (View of connector on O 2 sensor)
Voltage-free insulated O2 sensors are used.
The active sensor ceramic consists of a gas permeable ceramic body made of zirconium dioxide. A protective tube with several slots protects the ceramic body from mechanical stresses and from temperature jumps.
They are connected electrically by means of a 4-pin connector.
Advantages of planar sensor compared to finger sensor:
- Stable control characteristic
- Reduced overall size
- Quick start (control readiness < 10 s)
- Reduced heating capacity at operating temperature(< 7 W)
- Increased temperature resistance.
Functional principle
Function
The sensor ceramic is conductive for oxygen ions from approx. 300°C. If the oxygen concentration on both sides of the sensor ceramic differs, a voltage is produced at the boundary surfaces as a result of the particular properties of the sensor ceramic (Nernst voltage). This Nernst voltage produces the O2 sensor signal, which is a measurement for the residual oxygen content in the exhaust gas.
The evaluation circuit of the O2 sensor signals (in the ME control unit) sends a so-called sensor counter voltage of approx. 450 mV to the O2 sensor.
If the O2 sensor is cold, the sensor internal resistance is so high that the O2 sensor voltage is initially the same as the back voltage irrespective of the mixture composition.
If the O2
sensor is unplugged, the counter voltage at the ME control unit can be measured against the ground of the O2
sensor signal.
O 2 sensor heater
The O2 sensors are heated in order to rapidly heat up the sensor ceramic to operating temperature. The sensor heater is actuated by the ME control unit through a ground signal. The heater current in the cold state is increased approximately by the factor 4.
The sensor heater is switched off at coolant temperatures below approx. 20°C and at high engine speeds in order to avoid overheating (thermo shock).
Route of O 2 sensor signal via multiple signal change (schematic)
Faults at the O2 sensor can be detected by means of the O2 sensor signal.
Defined limit values must be maintained for the O2 sensor voltage, the duration of the period and for the sensor status change.
O 2 sensor signal
The O2 sensor signal has a steep voltage jump (λ=1) at the transition from a rich to a lean mixture. This property is utilized for the measurements.