Understanding Electronic Throttle Control (ETC) or EGAS

There are two types of throttle devices. The first type is where the driver accelerates by cable, while the other is where the driver accelerates by wire. 

When a driver accelerates by cable, Potentiometers connected directly to the throttle valve, provide signals to the ECU (Electronic Control Unit). The ECU calculates to what degree the throttle valve is open. These signals are converted by the ECU, taking into consideration the engine’s operating status, in conjunction with other signals provided to the ECU. These other signals include information about engine temperature, engine speed and camshaft angle.

The ECU calculates the throttle valve opening and converts it into a triggering signal, which is sent to the throttle valve motor. With this signal, the throttle valve motor can adjust precise settings depending on the load required. The Potentiometer works between zero and five volts. The voltage changes as two sliding “pins” move over a PC-board resistor plate. This movement changes the voltage as the angle of the throttle valve alters. The one sliding “pin” is used by the ECU as an angle signal, while the second “pin” performs a plausibility test. While the first “pin” moves from zero volts up to a maximum of five volts, the second “pin”, which is simultaneously performing the plausibility test, moves in the opposite direction i.e. from five volts down to zero volts.

A Volvo’s accelerator pedal, which demonstrates acceleration by wire

Potentiometers are duplicated for redundancy reasons and in case of a malfunction. If malfunctioning takes place, the engine will shift into a “limp home” mode. In this state, the vehicle is still driveable, but will develop significantly reduced power and the engine light inside the car will come on to warn the driver of a problem.

The EGAS system does not work with a cable, but rather, with wires or input signals that are sent to the ECU. The ECU calculates these signals, which are demanded by the driver, and then sends signals to the throttle valve motor. In response, the throttle valve motor determines how wide the throttle valve must open. The Potentiometer then sends the information back to the ECU, telling it how wide the throttle valve has been opened by the throttle valve motor.

There are also two types of accelerator pedals that are linked to the accelerate by wire system. The first one also works with a Potentiometer where “pins” move over a resistor plate/PC board, sending signals to the ECU. The Potentiometer here also performs a plausibility test, ensuring that correct signals are received by the ECU. These signals also work between zero and five volts. The second type of accelerator pedal operates with a contact-free angle-sensor. The contact-free angle sensor is a Hall sensor, operating from zero to five volts but, the plausibility test that it performs is done at half of the voltage of the main signal. In theory, the plausibility signal is always half of what the main signal, sent to the ECU, is. For instance, if the main signal measures 4.6 volts, then the plausibility signal should measure 2.3 volts.

If there is a malfunction with the signals received by the ECU, the vehicle will be rendered into “limp home” mode. Limp home mode is a clever system, designed for driver safety.


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