Starter control - function

 The starter control function is described using engine 926.9 (6 cylinders) as an example.

G07.15-A037-79

General

Actuation of the starter takes place exclusively over the circuit 50 output on the engine management (MR) control unit.

This makes it possible to protect the starter from mechanical and thermal overload (start time limitation, overspeed limiting device).

 Since the starter end stage in the engine management (MR) control unit does not deliver sufficient power or is not designed for high currents, there is a front-end relay between the engine management (MR) control unit and the engaging relay of the starter.

Actuation of the starter takes place:

 If the circuit 50 (3) signal from the ignition lock is detected at the input of the engine management (MR) control unit

 If the engine start button is pressed

 If the engine start and engine stop buttons are pressed at the same time

Preconditions

 Power supply (circuit 30 and circuit 15 On (2)) available at engine management (MR) control unit.

 Electronic immobilizer from DAS deactivated

 Starter lockout canceled, i.e.:

 Transmission in neutral position (only for start request via the engine start button)

 Power take-off, where present, switched off

 Engine off or engine speed below 50 rpm

Function

If all preconditions are met and the ignition lock or the engine start button is actuated or a corresponding message with the start request is received via the drive train CAN, the engine management (MR) control unit actuates the engaging relay of the starter via the front-end relay.

The starter sets the engine in motion. With the aid of the rpm signals from the crankshaft position sensor and the camshaft position sensor, the engine management (MR) control unit checks whether the starter is turning the engine at a minimum speed of 50 rpm.

Actuation of the starter following a start request via the ignition lock

The drive control (CPC) control unit and the engine management (MR) control unit receive the signal for the start request directly once circuit 15 On (2) has been switched on and circuit 50 (3) has been activated at the ignition lock.

The drive control (CPC) control unit then sends a start request message via the drive train CAN to the engine management (MR) control unit which it compares with the circuit 50 (3) signal coming directly from the ignition lock (consistency check).

If the circuit 50 (3) signal is present at the engine management (MR) control unit without the message being sent by the drive control (CPC) control unit, then the error "Circuit 50 inconsistent" will be stored in the fault memory after 1 s and the start will be delayed by this period of time. This redundancy means that it is still possible to start the engine via circuit 50 (3) if the CAN bus system fails, for example.

If the engine management (MR) control unit receives the circuit 50 (3) signal simultaneously over the circuit 50 connection on the control unit input and via the drive train CAN, the engine management (MR) control unit will actuate the starter immediately (without delay).

The starter continues to be actuated until the start request is retracted, the engine has reached the starter break-off speed (engine speed of approx. 450 rpm for a warm engine and up to approx. 650 rpm for a cold engine) or the maximum start time of 90 s has been exceeded (start time limitation).

Actuation of the starter following a start request via the engine start button

The engine management (MR) control unit reads in the signals of the engine start button directly. If the engine start button is pressed, the engine management (MR) control unit sends a start request message to the drive control (CPC) control unit via the drive train CAN.

The drive control (CPC) control unit checks whether this is permissible, i.e. the preconditions that must be fulfilled to lift the starter lockout, and sends the start enable to the engine management (MR) control unit via the drive train CAN.

The engine management (MR) control unit then actuates the starter until the engine start button is released, the starter break-off speed is reached or the actuation is interrupted autonomously to protect against mechanical or thermal overload (start time limitation).

Actuation of the starter when the engine start and engine stop buttons are pressed simultaneously

If the engine start and engine stop buttons are pressed at the same time, the engine can be turned over for servicing without the engine starting as, in this case, actuation of the single injection pumps for cylinders 1 to 6 is interrupted by the engine management (MR) control unit.

 If the starter was actuated by pressing the engine start button or by pressing the engine start and engine stop buttons at the same time, for safety reasons it is possible to start the engine via the ignition lock only once the ignition has been switched off and back on again.

 The signal which initiated a start must always be retracted again for safety reasons before a new start is possible based on the same signal (start locking).