ME-SFI 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 control unit (N3/10), in order to regulate the alternator control response. For this purpose, the ME-SFI control unit reads the following sensor and signal:

- Crankshaft Hall sensor (B70), motor speed

- On-board electrical system battery voltage (G1) through "circuit 30"

Alternator interface function sequence

The ME-SFI 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 the engine load, thus also reducing fuel consumption and improving the exhaust gas emissions.

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The ME-SFI 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 a "circuit 61 signal" (alternator in function).

- Reporting detected faults to the instrument cluster (A1) for actuation of the respective warning lamps and display messages.

The alternator constantly performs 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.

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The following faults are identified:

- Short circuit or discontinuity at output "terminal 61" from ME-SFI [ME] control unit.

- Open circuit on alternator interface line (drive LIN) or interface driver fault in the ME-SFI control unit. For this type of fault, a regulation voltage of 13.45 up to 14.15 V is given.

- Electrical and mechanical faults at alternator:
- controller or diodes defective
- stator interruption or short circuit
- exciter interruption
- 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.

"Terminal 61 signal"

For alternators with an alternator interface, line
"terminal 61" is not needed. From information which the ME-SFI [ME] control unit receives via the alternator interface, the "terminal 61 signal" is simulated in the ME-SFI [ME] control unit and is emitted via an output stage switching to ground (e.g. for the instrument cluster).