carlo’s technical page
Carlo du Plessis
Understanding
Electronic Throttle
Control (ETC) or EGAS
Carlo is the director of
Bosch Cencar and the
chairperson of the Bosch
Service Marketing Forum.
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.
W
hen 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.
driveable, but will develop significantly
reduced power and the engine light inside
the car will come on to warn the driver of a
problem.
➲ A throttle valve assembly for a VW MP9 system,
which is operated by an accelerator cable
In response, the throttle valve motor
determines how wide the throttle valve
must open. The Potentiometer once again
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.
➲ A Volvo’s accelerator pedal, incorporating
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
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.
| words in action
66
may 2014
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.