Mixture control

The engine operating with the S 3000 ECU is fitted with two oxygen sensors, called the upstream sensor and the downstream sensor according to their position on the catalytic converter.

I - SENSOR HEATING

Oxygen sensor heating is controlled by the ECU when the following conditions are met:

• intake pressure is below a threshold that depends on a table as a function of engine speed,
• vehicle speed is below 135 km/h,
• after a certain engine running time mapped as a function of engine TDC (excluding accelerator released) and coolant temperature.

Oxygen sensor heating is stopped when the following conditions are met:

• vehicle speed is above 140 km/h,
• under high engine load.

II - UPSTREAM SENSOR VOLTAGE

Reading parameter PR098: "upstream oxygen sensor voltage" on the diagnostic tool. The value read is the voltage supplied to the ECU by the oxygen sensor located upstream of the catalytic converter.

It is expressed in millivolts. Engine "in closed loop" (when the ECU corrects the mixture richness based on both sensors, the engine is said to be "in closed loop"), the voltage must oscillate rapidly between two values:

• 100 mV and 450 mV for a lean mixture,
• 450 mV and 900 mV for a rich mixture.

Note: The smaller the min-max difference, the less reliable the sensor signal is.

If the variation is small, check the sensor heating.

III - DOWNSTREAM SENSOR VOLTAGE

Reading parameter PR099: "downstream oxygen sensor voltage" on the diagnostic tool. The value read is the voltage supplied to the ECU by the oxygen sensor located after the catalytic converter. It is expressed in millivolts.

The function of this sensor is to diagnose the catalytic converter and perform a second, more precise mixture richness check. This second check is called the "slow control loop". This function is only activated after an engine running time delay.

When the engine is "in closed loop" (when the ECU corrects the mixture richness based on both sensors), the voltage must vary within a 600 mV range (except at idle, where it may have difficulty stabilising). On deceleration, the voltage should be around 100 mV.

Note: Do not take into account the voltage read on the diagnostic tool at idle.

IV - RICHNESS CORRECTION

The value read on the diagnostic tool, parameter PR 138: "RICHNESS CORRECTION", is the average of the richness corrections applied by the ECU based on the mixture richness seen by the oxygen sensor located upstream of the catalytic converter (the oxygen sensor actually analyses the oxygen content of the exhaust gases).

The correction value has a midpoint of 50% and stops at 0 to 100%.

• value below 50%: lean request,
• value above 50%: rich request.

V - ENTRY INTO MIXTURE CONTROL

1 - Closed-loop phase

K4J

Entry into mixture control takes effect after a delay that depends on the coolant temperature at start-up. At 20 °C, entry into closed loop takes effect after approximately 8 seconds.

F4R or K4M

Entry into mixture control takes effect after a maximum starting delay of 10 seconds and if the coolant temperature is above 10 °C (manual gearbox) and 2.5 °C (automatic gearbox).

2 - Open-loop phase

In mixture control, the operating phases during which the ECU ignores the sensor voltage value are:

• wide-open throttle,
• high loads and high engine speeds,
• deceleration with accelerator released (fuel cut-off signal),
• in case of an oxygen sensor fault.

VI - LIMP-HOME MODE IN CASE OF OXYGEN SENSOR FAULT

When the voltage supplied by the oxygen sensor is incorrect (varying very little or not at all) during mixture control, the ECU only switches to limp-home mode if the fault has been detected as present for 3 minutes. In this case, parameter PR138: "RICHNESS CORRECTION" is at 45 or 60%.

When a present oxygen sensor fault is detected and the fault has already been stored, the system goes directly to open loop.