Mercedes-Benz OM 471 Introduction Into Service Manual page 117

System components
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 outlet NOx sensor (A57 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.
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– This printout will not be recorded by the update service. Status: 09 / 2011 –
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 outlet NOx sensor (A57) serves to
compute the required pump voltages, to regulate the complex
system and to compute the NOx raw values.It receives the analog
signal from the exhaust aftertreatment unit outlet NOx sensor
(A57 b1) and digitalizes it. 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