Acceleration Control

Acceleration Control

OVERVIEW
Acceleration control for the V8 petrol engine is achieved via an electronic throttle drive by wire system. The throttle is electronically connected to the ECM (engine control module). The ECM (engine control module) then controls the fueling in accordance to signals received from the APP (accelerator pedal position) sensor.

ELECTRONIC THROTTLE

Electronic Throttle Body





The electronic throttle controls the air flow into the engine. In addition to the normal engine power control function, the electric throttle allows the speed control, idle speed control and engine speed limiting functions to be performed without the need for additional hardware.

The electronic throttle consists of a throttle body which incorporates a throttle plate driven by a DC (direct current) motor via reduction gears. A return spring biases the throttle plate in the closed direction.

Operation of the DC (direct current) motor is controlled by the ECM (engine control module), which outputs two PWM (pulse width modulation) signals to an H bridge drive circuit in the motor. The ECM (engine control module) varies the speed and direction of the motor by varying the duty cycle of the PWM (pulse width modulation) signals.

To enable closed loop control, the position of the throttle plate is supplied to the ECM (engine control module) by two feedback Hall effect sensors in the throttle body. The feedback sensors have a common 5 volt supply and a common ground connection from the ECM (engine control module). The sensors produce separate linear signal voltages to the ECM (engine control module) proportional to the position of the throttle plate. The ECM (engine control module) uses the signal from feedback sensor 1 as the primary signal of throttle plate position, and the signal from feedback sensor 2 for plausibility checks.
^ The signal from feedback sensor 1 varies between 0.5 volt (0% throttle open) and 4.5 volts (100% throttle open)
^ The signal from feedback sensor 2 varies between 4.5 volts (0% throttle open) and 0.5 volt (100% throttle open)

While the ignition is on, the ECM (engine control module) continuously monitors the two feedback sensors for short and open circuits. The ECM (engine control module) checks the feedback sensor signals, against each other and the inputs from the APP (accelerator pedal position) sensor, for plausibility. If a fault is detected in the feedback sensor signals or the DC (direct current) motor, the ECM (engine control module) :
^ Stores a related DTC (diagnostic trouble code) in its memory
^ Illuminates the SERVICE ENGINE warning lamp in the instrument cluster
^ Adopts a throttle limp home mode or disables throttle control, depending on the nature of the fault.

The throttle limp home mode adopted depends on the nature of the fault:
^ If there is a fault with one feedback sensor, or the throttle position controller in the ECM (engine control module), the ECM (engine control module) limits vehicle acceleration by limiting throttle plate opening.
^ If there is a fault with both feedback sensors, the ECM (engine control module) uses fuel injection cut-off to limit engine speed to 1300 rev/min maximum.

ACCELERATOR PEDAL POSITION SENSOR





The APP (accelerator pedal position) sensor is located in plastic housing which is integral with the throttle pedal. The housing is injection molded and provides location for the APP (accelerator pedal position) sensor. The sensor is mounted externally on the housing and is secured with 2 Torx screws. The external body of the sensor has a 6 pin connector which accepts a connector on the vehicle wiring harness.

The sensor has a spigot which protrudes into the housing and provides the pivot point for the pedal mechanism. The spigot has a slot which allows for a pin, which is attached to the sensor potentiometers, to rotate through approximately 90°, which relates to pedal movement. The pedal is connected via a link to a drum, which engages with the sensor pin, changing the linear movement of the pedal into rotary movement of the drum. The drum has 2 steel cables attached to it. The cables are secured to two tension springs which are secured in the opposite end of the housing. The springs provide feel' on the pedal movement and require an effort from the driver similar to that of a cable controlled throttle. A detente mechanism is located at the forward end of the housing and is operated by a ball located on the drum. At near maximum throttle pedal movement, the ball contacts the detente mechanism. A spring in the mechanism is compressed and gives the driver the feeling of depressing a 'kickdown' switch when full pedal travel is achieved.

APP Sensor Output Graph





The APP (accelerator pedal position) sensor has 2 potentiometer tracks which each receive a 5V input voltage from the ECM (engine control module). Track 1 provides an output of 0.5V with the pedal at rest and 2.0V at 100% full throttle. Track 2 provides an output of 0.5V with the pedal at rest and 4.5V at 100% full throttle. The signals from the two tracks are used by the ECM (engine control module) to determine fueling for engine operation and also by the ECM (engine control module) and the TCM (transmission control module) to initiate a kickdown request for the automatic transmission.

The ECM (engine control module) monitors the outputs from each of the potentiometer tracks and can determine the position, rate of change and direction of movement of the throttle pedal. The 'closed throttle' position signal is used by the ECM (engine control module) to initiate idle speed control and also overrun fuel cut-off.