Mercedes-Benz OM 471 Introduction Into Service Manual page 155

System components
GF09.41>W>4130H
ENGINE 471.9 in MODEL 963
Location
B608 Charge air pressure and temperature sensor in charge air
pipe
The charge air pressure and temperature sensor in the charge air
pipe (B608) is located on the charge air pipe on the left side of the
engine.
Task
The charge air pressure and temperature sensor in the charge air
pipe (B608) enables the engine management (MCM) control unit
(A4) to determine the air pressure and the air temperature of the
aspirated or supercharged combustion air.
These two values and the values of the differential pressure sensor
exhaust gas recirculation (EGR) (B621), the charge air temperature
sensor in the charge air housing (B617), as well as the values of the
temperature sensor downstream of the air filter (B611) with code
(M5Z) Euro VI engine version, are used to deduce the air mass that
is routed to the engine for combustion.
Design
The charge air pressure and temperature sensor (B608) consists of
two electrically separate components for measuring the boost
pressure and the charge air temperature.
Pressure sensor for determining the charge air temperature
The pressure sensor is a so>called semiconductor pressure sensor
which detects the air pressure in the charge air pipe by means of
the piezoelectric principle. This consists of four pressure>
dependent resistors (strain measuring resistors) which are
arranged in a silicon membrane as well as an electronic analysis
system which is supplied with a 5 V DC voltage via the engine
management (MCM) control unit (A4).
Temperature sensor for measurement of the charge air
temperature
There is an NTC resistor at the cone point of the charge air
pressure and temperature sensor in the charge air pipe (B608).
NTC stands for "Negative Temperature Coefficient", which means
that electrical resistance falls as temperature rises.
152
Charge air pressure and temperature sensor in charge air pipe, component
description
– This printout will not be recorded by the update service. Status: 09 / 2011 –
Function
Determining the boost pressure
The intake air or supercharged combustion air in the charge air
housing passes over a bore in the silicon diaphragm with the four
pressure>dependent resistors and distorts it.
The resistors in the silicon membrane are arranged in such a way
that, on deformation of the membrane, the transverse stress of
the resistance bridge is altered.
The electronic analysis system amplifies this transverse stress and
compensates in this way for possible temperature fluctuations or
compensates for any manufacturing tolerances of the resistors. It
subsequently routes the filtered measurement voltage to the
engine management (MCM) control unit (A4) which then deduces
the air pressure in the charge air housing.
Measuring the charge air temperature
Depending on its temperature, the air flowing by influences the
temperature of the measuring element on the tip of the sensor
and therefore the electrical resistance value. The engine
management (MCM) control unit (A4) deduces the associated
temperature from the electrical resistance.
20.7.11
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