Component description for NOx sensor control unit - GF14.40-P-3024MRB

ENGINE 274.9 in MODEL 207, 212 with CODE 920 (Direct petrol injection with stratified charging) 

Exhaust system (shown on model 212) 

Fig 1: NOx Storage Catalytic Converter And NOx Sensor Control Unit

Courtesy of MERCEDES-BENZ USA

Location 

The NOx sensor is located with the firmly connected NOx sensor control unit downstream of the NOx storage catalytic converter. The NOx sensor is screwed into the exhaust pipe. The NOx sensor control unit it located on the underfloor.

Task 

The NOx sensor records downstream of the NOx storage catalytic converter the NOx and O ₂ concentration levels in the exhaust and routes this information in the form of voltage signals to the NOx sensor control unit. This processes the information and sends it via the drive train sensor CAN (CAN I) to the ME-SFI [ME] control unit (N3/10).

Input and output signals 

The following input signals are evaluated by the NOx sensor control unit and appropriate output signals are emitted:

• Direct input signals 
• CAN input signals from the ME-SFI [ME] control unit 
• CAN output signals to the ME-SFI [ME] control unit

Direct input signals 

• Terminal 87

CAN input signals from the ME-SFI [ME] control unit 

• Diagnosis requests

CAN output signals to the ME-SFI [ME] control unit 

• NOx concentration in the exhaust
• O₂ concentration in exhaust
• Diagnostic data

Body 

Fig 2: NOx Sensor Control Unit

Courtesy of MERCEDES-BENZ USA

The NOx sensor (double chamber sensor with evaluation circuit) is inseparably connected via an 8-pin signal line to the NOx sensor control unit.

An integrated sensor heater which is controlled by the NOx sensor control unit ensures rapid operational readiness of the NOx sensor.

Function 

The active ceramic probe bodies consist of many layers with two reaction chambers. The oxygen (O ₂ ) in the exhaust is detected in the first chamber, as in an oxygen sensor.

To do this, a pump voltage is applied to the electrodes in the first chamber. This causes the O ₂ molecules to be broken down into double charged oxygen ions and for them to be pumped out of or into the first contact cavity (depending on whether richer or leaner exhaust gas is present) until a 450 mV voltage difference at the electrodes is reached. The oxygen concentration is calculated from the size of the required pump current "Ip1".

The nitrogen oxide is split at the ground electrode into nitrogen and oxygen in the second chamber if present.

The oxygen is pumped out if the chamber again.

The concentration of the nitrogen oxides in the exhaust is calculated from the resulting pump current "Ip2".

The voltage signals from the NOx sensor are sent to the NOx sensor control unit and processed there. The NOx sensor control unit sends the NOx and O ₂ concentration levels over the drive train sensor CAN to the ME-SFI [ME] control unit.

Using this information, the exhaust treatment can be monitored in stratified operation (lean-burn operation) of the engine and thus the NOx storage catalytic converter purging can be controlled.

The NOx sensor also recognizes damage and "sulfur poisoning" of the NOx storage catalytic converter.

A desulfuring process can then be initiated by the ME-SFI [ME] control unit by increasing the temperature of the NOx storage catalytic converter.

Schematic function diagram of the NOx sensor (double chamber sensors) 

Fig 3: NOx Sensor Function Diagram

Courtesy of MERCEDES-BENZ USA