(Component Description For Nox Sensor Control Unit - GF14.40-P-3024OIA)

Engine 656.9 in model 213, 238, 257 

up to model year 2021 

Engine 656.9 in model 222 

Partial view of the exhaust system 

Courtesy of MERCEDES-BENZ USA

Partial view of the exhaust system 

Courtesy of MERCEDES-BENZ USA

Location 

The NO_x sensor upstream of the diesel oxidation catalytic converter is located in the exhaust pipe downstream of the diesel oxidation catalytic converter and the NO_x sensor downstream of the SCR catalytic converter is located in the exhaust pipe downstream of the SCR catalytic converter. The NO_x sensor control units are each mounted on the underbody.

Task 

The NO_x sensors register the NO_x and O₂ concentrations in the exhaust gas downstream of the DPF and the SCR catalytic converter.

This information is forwarded to the NO_x sensor control units in the form of voltage signals. The NO_x sensor control units process this information and transmit it via the drive train sensor CAN (CAN I) to the CDI control unit (N3/9).

Input and output signals 

The NO_x sensor control units evaluate the following input signals and output corresponding output signals:

• Direct input signals 
• CAN input signals from the CDI control unit 
• CAN output signals to the CDI control unit 

Direct input signals 

• Terminal 87

CAN input signals from the CDI control unit 

• Diagnosis requests

CAN output signals to the CDI control unit 

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

Every NO_x sensor (double chamber sensor with evaluation circuit) is inseparably connected via an 8-pin signal line with the respective NO_x sensor control unit. An integral sensor heater which is controlled by the respective NO_x sensor control unit, ensures rapid readiness to operate of the NO_x sensors.

Structure 

Courtesy of MERCEDES-BENZ USA

Function 

The active ceramic probe bodies consist of many layers with two reaction chambers. The oxygen (O₂ ) in the exhaust is detected in the first chamber, as in an lambda senor.

To do this, a pump voltage is applied to the electrodes in the first chamber. This causes the O₂ molecules to be split into double charged oxygen ions and then pumped out of or into the first chamber (depending on whether rich or lean 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 NO_x sensors are transmitted to the respective NO_x sensor control unit and processed there. Via the drive train sensor CAN, the NO_x sensor control units transmit the NO_x and O₂ concentrations to the CDI control unit.

Schematic function diagram of the NO _x  sensor (double chamber sensors) 

Courtesy of MERCEDES-BENZ USA

	Wiring diagram of common rail diesel injection system control unit (CDI)	Engine 656.9 in model 213 up to model year 2021	PE07.16-P-2101-97DBH
		Engine 656.9 in model 222	PE07.16-P-2101-97SEH
		Engine 656.9 in model 238 up to model year 2021	PE07.16-P-2101-97DCH
	Wiring diagram for NOx sensor	Engine 656.9 in model 257 up to model year 2021	PE07.04-P-2102-97XBA