Further guidance
Throttle servomotors have been fitted to throttle butterflies for several years and are now commonplace on drive-by-wire engine management systems.
The throttle servomotor activates the throttle butterfly in varying increments dependent upon information coming from the vehicle's Electronic Control Module (ECM). The servomotor works in a similar way to a stepper motor but instead of just governing the engines idle speed, the servomotor activates the throttle from idle to the full throttle position.
The system may still use a throttle cable, but this now mainly operates a dual-output throttle position sensor (TPS) that is usually located inside the vehicle's engine compartment with no physical connection to the throttle body. The accelerator pedal sensor is used to communicate the driver's throttle movements to the vehicle's ECM. The component is connected to the throttle pedal by the auxiliary throttle cable (when fitted) which actuates the potentiometer. Systems that do not operate through a throttle cable have a dual-output TPS located on the throttle pedal assembly.
The twin TPS outputs are constantly monitored against each other and any discrepancies are logged as a fault code, causing the Malfunction Indicator Light (MIL) to illuminate. The vehicle's ECM digests all the information from the sensors and determines the angle of throttle opening. This information is sent to the throttle servomotor in the form of a square wave whose duty cycle depends on the angle of throttle butterfly opening. The signal sent to the servomotor has what is called a low duty cycle, which is a measure of the off time or the percentage of time the actuator receives zero volts. (A system that operates in the opposite direction is measured in terms of its high duty cycle, which is the percentage of time that the component receives a supply voltage compared to the off time.)
The example waveform in Figure 2 shows a lower low duty cycle of 15.65% while, in Figure 3, the waveform has been altered to increase its low duty cycle to 71.34%. These figures are for the vehicle being tested and will differ between motor vehicle manufacturers. The relevant technical data should be sought if the vehicle being tested has an engine speed-related problem.
Throttle servomotors are an ideal systems application when used in conjunction with both cruise control to maintain a predefined road speed and traction control systems to limit the power available to the road wheels.
To prevent a faulty sensor from causing an incorrect throttle opening for a given set of inputs, which would cause a safety problem, failsafe settings are installed in the vehicle's ECM.
If the engine fails to rev, the output(s) from the throttle position sensor should be monitored and should rise as the throttle is opened. A constant voltage of 5 volts means that the sensor is either signalling full throttle or has lost its earth connection. Should the low duty cycle fail to increase as the throttle is opened, the ECM is suspect and should be either tested by a recognised specialist or replaced with a new unit.
Our test vehicle was a 2002 Volkswagen Golf with a 10-pin multi-plug. No pin data was available.
