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

- Battery voltage (G1) through "tml. 30"

Alternator interface function sequence

The ME-SFI control unit regulates the alternator control response, for example in order to reduce the regulation voltage (charging voltage) when the battery is sufficiently charged. This reduces engine load, thus also reducing fuel consumption and improving 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. This compares the results with other signals (for example, engine rpm, 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

In order to check the regulation voltage, the battery must be placed under load using a battery testing device since the regulation voltage can be reduced significantly in a fully charged battery under no load.

"Terminal 61 signal"

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