Mercedes-Benz OM 471 Introduction Into Service Manual page 164

Function
The frictional connection between the belt pulley (9) and the
drive shaft is made by an electromagnetic viscous coupling, which
is actuated by the engine management (MCM) control unit (A4).
i The power is transmitted between the belt pulley (9) and the
drive shaft through the silicone oil shearing in the working
chamber (4). Shearing is understood to mean the displacement of
silicone oil between the basic body's (2) drive slats and the drive
slats of the viscous coupling (5).
The engine management (MCM) control unit (A4) requires the
speed of the coupling drive (rotational speed of pump impeller
(13)) and the drive speed of the belt pulley (9) to regulate the
electromagnetic viscous coupling.
The belt pulley (9) drive speed is determined using the engine rpm
and the diameter of the belt pulley (9).
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The armature (7) is closed through actuating the coolant pump
solenoid valve (Y631). The silicone oil is then pumped by the
centrifugal force out of the working chamber (4) between the lid
(3) and viscous disk (5), thereby reducing the pump impeller's (13)
mechanical power.
The constant balancing between drive and output speed enables
the engine management (MCM) control unit (A4) to detect the
current delivery rate at any given time. The engine management
(MCM) control unit (A4) raises or lowers the delivery rate of the
coolant pump (1) depending on the respective cooling
requirement.
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The coolant pump (1) is equipped with the following emergency
running characteristics:
f In the event of electrical faults (e.g. cable breakage, sensor
system malfunction) the coolant pump (1) automatically
supplies the maximum delivery rate for design reasons.
f Mechanical defects (e.g. rpm drop because of an oil loss at
the viscous coupling or a cracked poly>V belt) are detected
by the coolant pump rpm sensor (B640) and they result in
engine output limitation.
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 step>up ratio through the poly>V belt is also taken into
consideration.
This information is filed in the engine management (MCM)
control unit (A4).
The speed of the coupling output shaft is recorded by the coolant
pump rpm sensor (B640) and evaluated by the engine
management (MCM) control unit (A4).
This information is used by the engine management (MCM)
control unit (A4) to determine the current speed and therefore
the coolant pump's delivery rate (1).
For design reasons the coolant pump (1) always supplies the full
delivery rate when it is not actuated. In other words, the belt
pulley (9) and the pump impeller (13) are 95% nonpositive. If the
coolant pump (1) delivery rate is to be reduced, the engine
management (MCM) control unit (A4) actuates the coolant pump
solenoid valve (Y631) through a pulse width modulated signal
(PWM signal).
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i The coolant pump (1) is configured as a fail>safe component.
This means that the working chamber (4) of the electromagnetic
viscous coupling is filled with silicone oil when in an idle condition.
If the electronic actuation of the electromagnetic viscous coupling
is missing because of a defect, then 100 % of the cooling output
available is always used. This means however, that the working
principle of the viscous coupling may be extremely restricted at
very low temperatures or when the engine is cold started. The
extremely sluggish and cold silicone oil (particularly at low speeds)
cannot be pumped out of the working chamber (4) and therefore it
results in full activation of the coolant pump (1).
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f A mechanical defect in the coolant pump (1) with
simultaneous failure of the coolant pump rpm sensor (B640)
can be detected through the coolant temperature trend,
and it also results in an engine output limitation.
i In the event of a fault, the driver is notified by means of a
corresponding warning message shown on the multifunction
display (A1 p1).
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