Vehicle speed sensor (Engine management system)

When the intake air pressure changes, pressure in the detection chamber causes the diaphragms to deflect. The deflection of the diaphragms alters the length of each resistor, changing their resistance. This change in resistance is converted into a signal within the pressure sensor. The change in output from the pressure sensor is detected by the engine control module where it is converted into a pressure value.

Manifold absolute pressure when used in conjunction with the signal from the intake air temperature sensor allows the engine control module to accurately calculate the volume of oxygen in the air and adjust the quantity of fuel being injected to achieve optimum fuelling of the engine.

Vehicle speed sensor

The vehicle speed sensor is located on the top of the differential housing. The vehicle speed sensor is driven by a shaft from the final drive gear where it is converted into an electrical signal proportional to road speed. The output from the vehicle speed sensor is used to drive the instrument pack speedometer in addition to providing a signal to the engine control module. The engine control module uses this signal to provide active surge damping and adjust idle stabilisation and the quantity of fuel being delivered to injectors.

Throttle position sensor

The engine management system is a ’Drive by Wire’ system. The throttle pedal is not physically connected to the fuel injection pump by a mechanical linkage as in a traditional injection system, but instead the amount of fuel injected is controlled by the control unit. The throttle pedal is connected by a linkage to a throttle position sensor. Movements of the throttle pedal are sensed by the throttle position sensor and a signal sent to the engine control module.

The throttle position sensor consists of a thick film potentiometer together with a sender switch. With the throttle pedal at rest the switch is open. When the throttle pedal has been moved enough to rotate the potentiometer more than 9 degrees the switch closes. The sender switch signal from the potentiometer is used by the engine control module to check the operation of the circuit, implement idle speed control and over-run fuel shut-off.

When the potentiometer is moved the sender switch signals a logic input to the engine control module. The engine control module then checks the voltage

signal from the potentiometer. This voltage is compared with a pre-programmed value to check the potentiometer is working correctly.

Throttle pedal movement causes voltage across the potentiometer to vary. The control unit calculates the rate of change of the voltage signal in positive (acceleration) or negative (deceleration) directions. From this the engine control module can determine the required engine speed, rate of acceleration or rate of deceleration and apply acceleration enrichment, deceleration fuel metering or over-run fuel cut-off.

The engine control module calculates the ’maximum allowable fuel quantity’ from the input signal according to strategies such as smoke limitation, active surge damping, fuel reduction to calculate the final figure. If the signal from the throttle potentiometer is smaller than the maximum allowable quantity then the requested quantity will be injected. However, if the requested quantity is greater than the maximum allowable quantity, then the maximum allowable quantity will be injected rather than that demanded.

Brake pedal switch

The brake pedal switch informs the engine control module when the vehicle is braking and allows it to implement active surge damping and over-run fuel cut-off.

The engine control module has two brake inputs, each of opposite polarity. Comparison of the two polarity states provides the engine control module with brake sense. If both switches are the same polarity the engine control module senses a fault.