Mercedes-Benz OM 471 Introduction Into Service Manual page 130

Function
3 Measuring probe (ceramic body)
5 Chamber
6 Heating element
7 Reference air duct
8 Chamber
Ip0 Pump current (main pump electrode)
Ip1 Pump current (auxiliary pump
electrode)
Ip2 Pump current (measuring electrode)
The exhaust aftertreatment unit inlet NOx sensor (A70 b1)
functions according to the principle of the so>called oxygen ions
line according to which the wideband oxygen sensor also
functions.
Part of the exhaust flowing past the measuring probe (3) passes
through a diffusion barrier into the first chamber (8).
In this case the O concentration is regulated at a defined value by
2
means of the so>called pump voltage applied to an electrode until
the oxygen content differs at both sides of the ceramic body.
The special properties of the ceramic body create the signal
voltage (Nernst voltage) at its boundary surfaces. This is the
measure for the residual oxygen content in the exhaust.
The traces of HC, CO and H in the exhaust oxygenate at the
2
electrode made out of platinum.
i Introduction of engine OM 471 and exhaust aftertreatment > 09/2011 >
– This printout will not be recorded by the update service. Status: 09 / 2011 –
System components
The gas then passes over a further diffusion barrier and arrives in
the second chamber (5) where it is broken down with the aid of a
second NOx electrode into NO and O
also regulated to close to zero at the same time with the aid of a
further electrode. The nitrous oxide concentration is deduced
from the size of the pump current (Ip2) required.
The exhaust aftertreatment unit inlet NOx sensor control unit
(A70) serves to compute the required pump voltages, to regulate
the complex system and to compute the NOx raw values.It receives
the analogue sensor signals and digitizes them. It forwards these
as digital CAN signals at a defined transfer rate to the exhaust
aftertreatment control unit (ACM) (A60), which then evaluates
them.
W14.40>1350>76
. The O concentration is
2 2
127