Alternator interface function

Function requirements for the alternator interface, general points

 Circuit 87 (engine timing ON)

 Engine runs

Alternator interface, general points

Using the alternator interface, messages are exchanged via the drive-train LIN (LIN C1) between the alternator (G2) and the CDI control unit (N3/9) and the control response of the alternator is controlled. To do this, the CDI control unit reads in information from the following signals:

- crankshaft Hall sensor (B70), engine speed

– on-board electrical system battery voltage (G1) via "circuit 30"

– Alternator, status

Function sequence for alternator interface

The CDI control unit regulates the alternator control response, for example in order to reduce the regulation voltage (charging voltage) when the on-board electrical system battery is sufficiently charged.

This reduces the engine load, thus also reducing fuel consumption and improving exhaust emissions.

The CDI control unit controls the following functions:

– Switching on of the alternator, dependent on the condition of the on-board electrical system and on the requirements from the engine.

- regulation of the alternator based on characteristics maps stored in the CDI control unit. To do this, the regulation voltage is preset by the CDI control unit.

– protecting the alternator against overheating.

– adapting the regulation voltage with a delay in the event of frequent load change at the alternator when idling to stabilize the idling speed.

– Reporting recognized faults to the instrument cluster (A1) for actuation of the corresponding warning symbols and display messages via drive train CAN (CAN C1), powertrain control unit (N127), via chassis FlexRay (Flex E), the electronic ignition lock control unit (N73) and user interface CAN (CAN HMI).

– Keeping on-board electrical system voltage at a low value and charging the on-board electrical system battery more strongly in the deceleration phases (fuel saving)

– Generating circuit 61 CAN signal (alternator in operation).

The alternator constantly performs a self-diagnosis and sends the results when requested to the CDI 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 CDI control unit is defective (the alternator regulates to 14.3 V in this case)

– Electrical fault at alternator:

- possible causes: alternator regulator or diode defective, stator defective or excitation circuit open

- possible consequences: charging voltage or charging current not adequate or too high

– Mechanical fault at alternator:

- possible causes: a cracked or loose poly-V belt

- Possible consequences: no or poor charging of the on-board electrical system battery, no or poor on-board electrical system supply

 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 information received by the CDI control unit over the alternator interface, the "circuit 61 signal" is simulated in the CDI control unit and issued over the CAN network (e.g. for the instrument cluster).