Alternator: Description and Operation

GF07.10-P-1009MI Alternator Interface Function

Function requirements for the alternator interface, general points
^ Circuit 87M ON (engine control ON)
^ Engine running

Alternator interface, general points
Through the alternator interface, messages are exchanged over the drive LIN (LIN C1) between the alternator (G2) and the ME-SFI [ME] control unit (N3/10), in order to regulate the alternator control response. The ME-SFI [ME] control unit reads in the following sensor and signals for this purpose:
- Crankshaft Hall sensor (B70), engine speed
- On-board electrical system battery voltage (G1) over "circuit 30"
- Alternator, status and circuit 61 via the drive train LIN

Alternator interface function sequence
The ME-SFI [ME] control unit controls the alternator control response, for example, in order to reduce the regulation voltage (charging voltage) when the engine is at idle and the on-board electrical system battery is sufficiently charged

This reduces engine load, thus lowering the fuel consumption and improving the exhaust gas emissions.

The ME-SFI [ME] control unit controls the following functions:
- Switching on the alternator after engine start.
- Regulation of the alternator according to performance maps stored in the ME-SFI [ME] control unit. To do this, the control voltage is preset by the ME-SFI [ME] control unit.
- Adapting the control voltage with a delay in the event of frequent load changes at the alternator to stabilize the idling speed.
- Protecting the alternator against overheating.
- Generating of a "circuit 61 signal" (alternator in function).
- Recognized faults via the chassis CAN (CAN E) to the instrument cluster (A1) for actuation of the respective warning lamp and display messages.

The alternator performs continuous self-diagnosis and sends the results when requested to the ME-SFI [ME] control unit. The control unit compares the results with other signals (for example, engine rpm, on-board electrical system battery voltage, time since engine start) and thus detects any alternator faults.

The following faults are identified:
- Short circuit or open circuit on output "circuit 61" of the ME-SFI [ME] control unit
- Line for alternator interface (drive-LIN) is interrupted or interface driver in ME-SFI [ME] control unit defective (A regulation voltage of 13.45 to 14.15 V is prescribed for this fault).
- Electrical and mechanical faults at alternator:
- controller or diodes defective
- stator interruption or short circuit
- excitation circuit open
- regulation voltage and charging current not reached
- regulation voltage too high
- cracked or loose poly-V belt

Diagnosis
To check the regulation voltage, the on-board electrical system battery must be loaded with a battery tester, as the regulation voltage can be significantly reduced when the on-board electrical system battery is under full or no load.

Circuit 61 signal
For alternators with an alternator interface the "circuit 61" line is not needed. Using the information received by the ME-SFI [ME] control unit through the alternator interface, the "circuit 61 signal" is simulated in the CDI control unit and sent through an output stage connected to ground (e.g. for the instrument cluster).