GF15.40-P-0001RC On-Board Electrical System Power Supply, Function

GF15.40-P-0001RC On-board electrical system power supply, function

General function requirements
^ Circuit 61

On-board electrical system power supply - general
(G2) When the engine is running, the alternator supplies the on-board electrical system with electrical energy and charges the battery (G1) For this purpose, the alternator generates three-phase current which is rectified.

There are two types of alternator interface between the ME-SFI control unit (N3/10) and the alternator.

- Alternator with bit synchronous interface (for engine 271)
- Alternator with local interconnect network (LIN) interface (for engine 272)

The bit synchronous interface is a single-wire interface with a maximum transfer rate of 1200 bit/s.

The LIN interface is a two-way single-wire interface with a maximum transfer rate of 19.2 kbit/s.

Function and task
Data is exchanged between the ME-SFI control unit and the alternator over the alternator interface.

The master-slave principle is applied.

The ME-SFI control unit is the master and the alternator is the slave. The alternator only sends when it has received a request from the ME-SFI control unit.

The following tasks are performed:

- The alternator is switched on after engine start and the alternator is regulated according to the performance maps stored in the ME-SFI control unit. The ME-SFI control unit sets the alternator's regulation voltage.
- Delayed adaptation of the regulation voltage in the case of a major change in alternator load.
- The alternator is protected from overheating.
- Terminal 61 signal ("alternator turning" message) is generated in the ME-SFI control unit.

The alternator performs continuous self-diagnosis and sends the results to the ME-SFI [ME] control unit when requested. The ME-SFI control unit compares these with the other signals (for example, engine rpm, battery voltage and time since engine start), and identifies the following errors:

- Electrical and mechanical faults in the alternator (regulator or diode fault, stator disconnected or stator short circuit, excitation circuit open, regulation voltage and charging current not reached, regulation voltage too high, poly-V belt cracked or loose).
- Interface between ME-SFI control unit and alternator lost. In this case, the alternator continues to operate using its fixed parameters. A regulation voltage of about U = 14.3 V is set.
- Short circuit or discontinuity in ME-SFI control unit output circuit 61.

Networking
Through the alternator interface, the ME-SFI control unit sets the control response of the alternator, for example, when idling, in order to reduce the alternator torque that occurs when power output is high. Because engine load is reduced, less fuel is injected and exhaust characteristics are optimized.

Terminal 61 in the ME-SFI control unit is simulated using information received by ME-SFI control unit over the alternator interface.

The upper limit of the regulation voltage can be up to U = 15 V.

Engine speed
The ME-SFI control unit reads the signals from the engine rpm sensors, evaluates them and makes the engine speed information (in rpm) available to other control units over the engine compartment CAN. The central gateway control unit (N93) sends the signal over the interior CAN.

Diagnosis
The following errors are identified by the ME-SFI control unit after "Ignition On", and stored in the internal fault memory:

- Alternator interface defective
- Line interruption (tml. 30)

The following errors are identified by the ME-SFI control unit after "Engine Start", and stored in the internal fault memory:

- Electrical fault at alternator
- Mechanical fault at alternator
- Line interruption (tml. 30)
- Undervoltage and defective alternator interface

The corresponding error messages are indicated by the battery symbol being lit on the multifunction display at the top (A1p13) of the instrument cluster (A1), or by the plain text message "Service Required".

The "Service Required" plain text message can also be set by the EIS control unit (N73).

Emergency running
If communication is interrupted between the alternator and the ME-SFI control unit during normal engine operation, the alternator continues to work using its fixed default parameters. The alternator also has an emergency start function. If communication is interrupted between the alternator and the ME-SFI control unit, the alternator can still generate a voltage from the residual magnetism if the engine is running.

If the regulator detects from voltage and frequency that the alternator is turning, it automatically begins to excite the magnetic field and goes into regulation mode.

Limitations:

- The "Regulation Off" command from the ME-SFI control unit interrupts limp-home mode.
- Precise specification of the starting speed for the emergency start function is not possible.