Alternator: Description and Operation

GF15.40-P-0001A Power supply for on-board electrical system, function

ENGINE 112, 113 in MODEL 203, 209, 211, 220, 230
only valid with engine control unit variant ME 2.8
ENGINE 113 in MODEL 164, 171, 215, 219, 251
only valid with engine control unit variant ME 2.8
ENGINE 155 in MODEL 199
only valid with engine control unit variant ME 2.8
ENGINE 648 in MODEL 211, 220
only valid with engine control unit variant CDI 3

Component Identification:

General
When the engine is running, the alternator supplies the on-board electrical system with electrical energy and charges the battery. For this purpose, the alternator generates three-phase current which is rectified.

Function
The alternator is driven by a poly-V-belt. When the engine is started, the driver-side SAM control unit with fuse and relay module energizes the alternator. The alternator produces its pre-excitation current or excitation current automatically and supplies current via terminal B+ of the alternator to the battery and to the on-board electrical system.

The alternator is regulated by the multifunction regulator according to:
^ the alternator speed (determined internally)
^ the on-board power supply voltage

The on-board power supply voltage is measured and evaluated internally by the electronics of the alternator.

The DFM connection outputs a signal which indicates the capacity utilization of the alternator.

This signal is used for electrical control of the engine's heater booster. In model series 211, the AAC [KLA] control and operating unit evaluates the signal and regulates the heater booster according to the alternator load condition.

Networking
The driver-side SAM control unit with fuse and relay module energizes the alternator. If the charging function is working properly, the alternator supplies a voltage signal along the same line.

This signal is evaluated by the driver-side SAM control unit with fuse and relay module and is sent as a message on the Controller Area Network bus class B to the instrument cluster.

Engine speed
Depending on the variant, the pulses from the crankshaft position sensor are read in by the ME-SFI [ME] control unit or CDI control unit.

The control unit places the engine speed message (in rpm) on the Controller Area Network bus class C.

Alternator monitor
If there is an alternator fault, the "battery symbol" and the "Visit workshop" message are shown in the multifunction display in the instrument cluster.

The message/symbol appears if the alternator is not operating correctly although the engine speed is higher than 200 rpm. There is no message when the engine is OFF.

This fault message in the multifunction display of the instrument cluster can also be triggered by information from the following control units:
^ battery control unit (N82)
^ EIS [EZS] control unit (N73)

Diagnosis
The alternator with multifunction regulator without interface cannot be checked using Star Diagnosis (DAS).

The appropriate work instructions in the workshop information system (WIS) apply.

Depending on the vehicle variant, the alternators have a load response.

There are two types:
^ Starting load response

This function prevents the alternator from supplying current while and immediately after the engine is started. As a result, the starting procedure is not impeded or prolonged by the retarding torque of the alternator.

^ Driving load response

This function prevents the rise in torque produced by the alternator in the case of a positive change in electrical load from being passed directly on to the engine. The power output is increased continuously by means of a ramp function. The load is connected smoothly.