Immobilisation System The ECM plays a major role in the immobilisation of the vehicle. The ECM prevents engine fuelling until it receives a valid coded signal from the immobilisation ECU. The coded signal from the immobilisation ECU is supplied in the form of a rolling code, preventing the code from being copied or by-passed. When new, the immobilisation ECU is blank and is programmed with a starting code known as a 'seed. The seed is then used as abase point for the rolling code when the immobilisation ECU is synchronised to the ECM during manufacture. Once synchronised, the ECM and the immobilisation ECU are not interchangeable and operate as a matching pair. When anew ECM is fitted to a vehicle during service, the new immobilisation ECU must be supplied with a seed which matches the vehicle. The rolling codes in the new immobilisation ECU and the existing ECM must be synchronised using TestBook/T4. The immobilisation ECU receives an engine speed signal from the ECM to inhibit starter motor operation when the engine is running, to prevent damage to the starter motor and ring gear. Engine speed information is broadcast on the CAN by the ECM. The instrument pack passes the engine speed signal on the K bus to other systems, including the immobilisation ECU. Cruise Control The ECM incorporates a cruise control program which controls all functions of the cruise control system. During cruise control operation, the ECM controls vehicle speed by adjusting fuel injection duration and timing. When the accelerator pedal is pressed with cruise control active, the ECM outputs a calculated throttle angle signal in place of the actual throttle angle signals produced by the APP sensor. The calculated throttle angle is derived from fuel demand.