Starter relay

The BCU checks the status of the following inputs and internal BCU conditions to determine whether or not to provide an output to enable the starter relay:

• AutoStart inhibit (vehicles with automatic gearbox only).
• Security start inhibit (immobilization check).
• Engine running (link to instrument pack).
• EEPROM locked (internal check).
• IDM and BCU matched.

When the BCU receives a crank signal from the ignition switch, an earth path is completed to the starter relay coil, provided that the security system has been de-activated. If the ECM has not received a valid unlock/ re-mobilize signal, the starter relay will be disengaged and the engine stopped. The BCU also receives an engine running signal from the instrument pack, so that if the ignition key is turned to the crank position while the engine is running, the starter motor relay will not be engaged.

If the logic conditions are correct to allow starter operation, the completion of the earth path from the starter relay coil to the BCU energizes the coil and the relay contacts close to supply battery power to the starter motor.

When the ignition switch is released from the crank position, the power supply feed from the ignition switch to the starter relay coil is interrupted and the relay contacts open to prevent further battery feed to the starter motor.

If a communications link failure is experienced, the BCU will be prevented from detecting the 'engine running' condition and the BCU will default to assume that the engine is not running.