Charging System: Description and Operation

GF07.10-P-1009MG Alternator Interface, Function
ENGINES 271.8 in MODEL 204.0 /2 /3, 207.3 /4, 212.0 /2

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

Alternator interface, general points
Using the alternator interface messages (e.g. the dynamo field monitor signal) are exchanged via the drivetrain LIN (LIN C1) between the alternator (G2) and the ME-SFI control unit (N3/10) and the control response of the alternator is controlled. To do this there is delayed adaptation of the regulation voltage for a larger change in load of the alternator. The current operating rate of the alternator is transmitted by the dynamo field monitor signal. The maximum data transmission rate between the ME-SFI control unit and the LIN control for the alternator is 19.2 kbit/s. The LIN control is not fitted with any direct outlets. The ME-SFI [ME] control unit reads in the following sensor and signals:

- 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

Function sequence for alternator interface
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 the engine load, thus also reducing fuel consumption and improving exhaust gas emissions.

The ME-SFI [ME] control unit controls the following functions:

- Switching on of the ALT dependent on the condition of the on board electrical system and the requirements from the engine.
- 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.
- Generation of a "circuit 61 signal" (alternator in operation).
- Recognized faults via the chassis CAN (CAN E) to the instrument cluster (A1) for actuation of the respective warning lamp and display messages.
- Keeps on-board voltage at a low value and charges the on- board electrical system battery more strongly in the deceleration phases (fuel saving)

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:

- Line (drivetrain LIN) for the alternator interface has an open circuit or the interface driver in the ME-SFI [ME] control unit is defective (the ALT regulates in this case at 14.3 V).
- Electrical and mechanical faults at alternator:
- Controller or diodes defective
- Stator open 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, a battery tester must be used to place a load on the on-board electrical system battery, as the regulation voltage can be significantly reduced when the on-board electrical system battery is fully charged and under 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 ME-SFI [ME] control unit and sent through an output stage connected to ground (e.g. for the instrument cluster).