Engine management system - siemens (all except NAS) (Operation): Description and operation

Operation

General

When the ignition is switched on, the interface ECU and the master switch receive a power feed from the passenger compartment fusebox. When the master switch is pressed, the LED in the master switch illuminates and further power feeds are connected to the interface ECU and the cruise control ECU to enable the system.

Cruise control is enabled provided the interface ECU is in receipt of a cruise enable signal from the ECM and the system inputs indicate that: l The brakes are off. l The vehicle is moving at a road speed between 22 and 125 mph (35 and 200 km/h).

The ECM outputs the cruise enable signal to the interface ECU, when: l The engine is running and vehicle speed is above 3 mph (5 km/h). l Engine speed does not exceed 6496 rpm. l The gearbox is not in Park, Neutral or Reverse. l Electronic Traction Control (ETC) is not active.

While the system is enabled, the interface ECU outputs the power supply to the cruise control ECU for operation of the vacuum pump assembly.

Engagement

When the system is enabled, cruise control is engaged by pressing the SET+ steering wheel switch. On receipt of the input from the SET+ switch, the cruise control ECU stores the current vehicle speed as the set speed and outputs the pump and valve power supply to the vacuum pump assembly. The cruise control ECU also outputs signals to operate the vacuum pump and control valves in the vacuum pump assembly. The vacuum pump and control valves regulate the vacuum sensed by the vacuum actuator, to adjust the throttle as necessary to maintain the vehicle at the set speed.

When cruise control is engaged, the pump and valve power supply is also connected to the EAT ECU and the instrument pack. On receipt of the power supply, the EAT ECU adopts the cruise control mode, which uses a gear change map less sensitive to changes of throttle opening to prevent unnecessary gear changes, which improves operating refinement for a minor loss of performance. In the instrument pack, the power supply illuminates the cruise control warning lamp.

Acceleration

While cruise control is engaged, the vehicle can be accelerated using either the SET+ switch or the accelerator pedal. A momentary press (less than 0.5 second) of the SET+ switch causes the cruise control ECU to increment the stored set speed by 1 mph (1.6 km/h) and operate the vacuum pump assembly to accelerate the vehicle to the new set speed. If the switch is held on, the cruise control ECU keeps incrementing the stored set speed and accelerating the vehicle until the switch is released. When the switch is released, the cruise control ECU adopts the increased vehicle speed as the new set speed.

If the accelerator pedal is used to accelerate the vehicle, the cable from the accelerator pedal overrides the vacuum actuator and increases the throttle opening to meet driver demand. Cruise control remains engaged and the set speed is resumed once the accelerator pedal is released. If the SET+ switch is pressed before the accelerator pedal is released, the cruise control ECU adopts the higher speed as the new set speed.

Suspend/Resume

Cruise control can be manually suspended and resumed (at the previous set speed) using the RES steering wheel switch. The interface ECU, the cruise control ECU and the ECM automatically suspend cruise control if one of the conditions required to enable the system is no longer present, e.g. the brakes are applied or ETC activates. Cruise control is also automatically suspended by the cruise control ECU if the vehicle speed decreases to less than 75% of the set speed, e.g. when travelling up a steep hill.