Test Group TBM2.8VJGKEK
- Oxygen Sensor Monitoring General Description -
Both oxygen sensors upstream from the catalyst are separately monitored for rich and lean voltage, switching time and response time (time is exposed to rich and lean periods). See Fig 1. These time measurements are performed under the following conditions:
- No secondary air injection.
- Coolant temperature greater than 158°F (70°C).
- Closed loop operation.
- Vehicle speed between 3-75 mph.
- Catalyst temperature between 685-1162°F (353-628°C).
- Limited dynamics
- Throttle angle less than 8.4 degrees.
- Engine speed less than 382 RPM.
- Mean lambda value less than 6 percent.
- Monitoring Procedure
- Switching Time -
For monitoring the switching time, the time from lean to rich mixture and visa versa, are determined and compared with predetermined maximum values which depend on engine RPM and air mass.
Relative thresholds (10-90% of min./max. amplitude) are used to determine the oxygen sensor switching time from lean to rich and visa versa.
The sensor signals are monitored for reversal points while the switching times are being measured. If a reversal point is detected this oxygen sensor signal is not used to determine the switching time. See Fig 2. Due to switching time fluctuations the results are averaged during the monitoring cycle (115 lambda control cycles) before comparing with the thresholds.
- Rich/Lean Periods - For determining the switching time the lean and rich period times are added during 115 lambda controller cycles. A malfunction is registered if one or both of the times exceed(s) the thresholds which depend on engine speed and load. See Fig 3.
- Switching Time -
For monitoring the switching time, the time from lean to rich mixture and visa versa, are determined and compared with predetermined maximum values which depend on engine RPM and air mass.
- Oxygen Sensor Heater Monitoring -
For proper function of the oxygen sensor, the sensor element must be heated. A non functioning heater delays sensor readiness for closed loop control operation and influences emissions. Disconnection, short-to-ground and short-to-battery monitoring and oxygen sensor heater current and voltage monitoring are performed in the standard diagnosis function.
- Heater Circuit Oxygen Sensor Upstream - Due to slow sensor heating, in the case of low oxygen heating output, the rich voltage does not reach its maximum value within a defined time. This reduces the oxygen sensor signal amplitude which permits oxygen sensor heating output to be monitored. The malfunction detection is activated every 240 ms independent of engine operation.
- Oxygen Sensor Monitoring (Downstream Sensor) - Lean sensor voltage is evaluated for diagnosis of the heater circuit. Therefore this check is performed during deceleration/fuel cut-off phases. The sensor downstream must be switched to a load resistance of 100 k/ohms (normal operation 30 k/ohms). During this check, the oxygen sensor signal output voltage is controlled to a fixed value (lean limit). If the number of cycles during which the sensor signal lies within the defined limits is smaller than the limit, a fault will be stored. The sensor signal is checked every 20 ms during the heater circuit diagnosis.