Response When A Temperature Sensor Fails; Response When An Exhaust Gas Recirculation (Egr) Component Fails; Response When Engine Management (Mcm) Control Unit (A4) Fails - Mercedes-Benz OM 471 Introduction Into Service Manual

4

Response when a temperature sensor fails

If the values from a temperature sensor are implausible, or if a
temperature sensor has an open circuit or a short circuit, the
engine management (MCM) control unit (A4) continues to
operate with substitute values.
Own replacement values are stored for each temperature sensor
since the measurement values of the individual temperature
sensors can deviate from each other. It is not feasible for these to
be suitable for all operating conditions, however, which is why a
mild reduction in the maximum possible torque available takes
place.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
6 Response when atmospheric pressure sensor in engine
management (MCM) control unit (A4) fails
If the atmospheric pressure sensor in the engine management
(MCM) control unit (A4) has a malfunction or a fault, then >
depending on the particular situation > it uses substitute values:
• If the pressure sensor in the charge air pressure and
temperature sensor in the charge air pipe (B608) is intact,
the engine management (MCM) control unit (A4) uses the
air pressure in the charge air housing during the start
procedure and the values stored in a substitute characteristic
when the engine is running.
The reason why the engine management (MCM) control unit
(A4) falls back on values in a substitute characteristic while
the engine is running is because the charge pressure in the
charge air housing can differ according to the engine
loading and cannot therefore be used as the atmospheric
pressure.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
7 Response when engine management (MCM) control unit
(A4) fails
The engine management (MCM) control unit (A4) has only one
host CPU, and no limp>home mode CPU.
If the host CPU fails the engine stops and cannot be started
anymore.
The drive control (CPC) control unit (A3) however is capable of
diagnosing the failure of the engine management (MCM) control
unit (A4) by reading out the fault memory, so that diagnosis is still
possible.
i While the engine management (MCM) control unit (A4)
switches to the limp>home mode when a severe fault occurs or if a
less serious fault occurs it operates using substitute values, it saves
every fault that occurs and notifies the driver of them by means of
an appropriate display in the instrument cluster (ICUC) control
unit (A1).
i Introduction of engine OM 471 and exhaust aftertreatment > 09/2011 >
– This printout will not be recorded by the update service. Status: 09 / 2011 –
5 Response when an exhaust gas recirculation (EGR)
component fails
If the values of the charge air pressure and temperature sensor in
the charge air pipe (B608), the charge air temperature sensor in
the charge air housing (B617), the exhaust gas recirculation (EGR)
differential pressure sensor (B621) or the exhaust gas recirculation
positioner (Y621) are implausible, or if one of them suffers an
open circuit or a short circuit, the engine management (MCM)
control unit (A4) adjusts the limp>home mode speed and the
engine's maximum torque is limited.
i The fact that the limp>home strategy is alterable by the
manufacturer means that it is possible that the engine speed
remains unchanged and only the maximum torque of the engine
is limited.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
• If the pressure sensor in the charge air pressure and
temperature sensor in the charge air pipe (B608) is also
defective, the engine management (MCM) control unit (A4)
then uses the values of the previously>mentioned substitute
characteristic for all operating conditions.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
In addition to information about the type of fault and possibly
also a measurement value, the engine control (MR) also stores
information about whether the fault is current or just transient or
occurs sporadically. The fault memory can also be automatically
deleted again, of course, for less serious faults if the stored fault
does not occur again within the next 50 hours. In order to avoid
coming to wrong conclusions when troubleshooting, please
observe that certain operating conditions can lead to limitation of
the maximum torque of the engine.
e. example, a radiator contaminated with dirt and dust will have a
reduced cooling output. The engine management (MCM) control
unit (A4) determines an increase in coolant temperature and
limits the engine's maximum torque so that the coolant
temperature limit is not exceeded.
Functions
41