On-board electrical system
76
Networking
Introduction of the New E-Class, Model Series 212 Sedan
b
On-board electrical system management
The task of the on-board electrical system manage-
ment system is to guarantee that all the electrical
consumers are supplied and the engine can still be
started on demand in all situations.
The on-board electrical system control system is inte-
grated into the rear SAM control unit with fuse and
relay module.
The system consists of the following components:
• On-board electrical system battery for power
storage and supply when the engine is off
• Alternator for power generation
• Battery sensor
• Front prefuse box
• Rear prefuse box (in E 500)
• CDI control unit (with diesel engine)
• ME-SFI [ME] control unit (with gasoline engine)
• All the electrical consumers or components that
influence the performance of the on-board
electrical system
The load state of the on-board electrical system is
determined from the characteristic variables available
in the vehicle, and suitable measures are initiated as
and when necessary.
These include the following sub-functions:
• Alternator management
• No-load current shutoff
• Consumer shutoff
• Dynamic idle speed increase
Alternator management
The state of the on-board electrical system battery is
determined via the battery sensor. This uses voltage,
current and temperature measurements to define
characteristic values which serve as the basis for the
on-board electrical system control. From these values
the rear SAM control unit calculates a nominal voltage
for the alternator.
The CDI or ME-SFI control unit reads in this informa-
tion and, if necessary, other values (e.g. if the air
conditioner compressor is on), computes an optimum
nominal voltage from these input variables and
forwards this to the alternator. In addition the input
values are checked for plausibility in order to rule out
the following faults:
• Battery overcharged
• Insufficient battery charge
The CDI or ME-SFI control unit compares the values of
the alternator request with those of the alternator
output, and is thus able to recognize the energy state
of the on-board electrical system.
No-load current shutoff
The integrated no-load current management function
in the on-board electrical system control of the rear
SAM control unit is intended to preserve the starting
ability of the engine. It allows the vehicle to stand idle
for a longer time without the on-board electrical
system battery becoming completely discharged. To
do this, consumers or components are isolated from
the power supply by the no-load current shutoff relay.
The no-load current management function tolerates
an increased level of energy consumption for a certain
period of time. This is desirable, for example, when the
radio is left switched on while the car is being washed.
– This printout will not be recorded by the update service. Status: 01 / 2009 –
