Actron GM Code Scanner CP9001 - Manual (Page 36)

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delivery modified by an
electronic signal from the
ECM. The signal controls a
“mixture control solenoid”
(MCS) attached to the
carburetor body.
Frequency
The frequency of an
electronic signal is a
measure of how often the
signal repeats a voltage
pattern in a one second
time span. For example:
suppose a signal starts at
zero volts, goes to five
volts then returns to zero
again. If this pattern
repeats itself 100 times in
one second, then the
signal frequency is 100
cycles per second - or 100
Hertz.
Fuel Injector
An electronically controlled
flow valve. Fuel injectors
are connected to a
pressurized fuel supply.
(The pressure is created by
a fuel pump.) No flow
occurs when the injector is
off (not energized). When
the injector is powered, it
opens fully allowing the fuel
to flow. The ECM controls
fuel delivery by varying the
amount of time the injectors
are turned on.
Fuel Pump
Relay
The ECM energizes this
relay to apply power to the
vehicle fuel pump. For
safety reasons, the ECM
removes power from the
fuel pump when ignition
signals are not present.
Fuel Pump
signal
This is a wire between the
ECM and the fuel pump
motor power terminal. The
ECM uses this signal to
verify when voltage is at
the fuel pump (for
diagnosing fuel pump
problems).
Gear switches
These are switches (usually
two) located inside certain
automatic transmissions.
The ECM monitors the
switches to determine what
transmission gear is
engaged. The switches are
activated by hydraulic
pressure and may be
normally open or closed,
depending upon vehicle.
The ECM uses gear
information for control of the
torque convertor clutch,
some emission systems and
for transmission diagnostic
purposes.
Ground
The return path for current
to flow back to its source.
(Usually the negative
battery terminal.) It is also
the reference point from
which voltage measure-
ments are made. That is, it
is the connection place for
the minus (-) test lead from
the voltmeter.
Hall Effect
sensor
This sensor is a three wire
type containing electronic
circuitry. Two wires supply
power and ground. The
third wire carries the
sensor signal back to the
ECM. The sensor consists
of a permanent magnet
and a small module
containing a transistorized
Hall Effect switch. A small
air gap separates the
sensor and the magnet.
The magnetic field causes
the Hall switch to turn on
and send out a low voltage
signal. If a metal strip (iron
or steel) is placed in the
gap, it will block the
magnetic field from
reaching the Hall device.
This causes the Hall switch
to turn off and send a high
voltage signal out on the
signal wire.
The metal strips (blades)
are part of a cup or disk
attached to a rotating
component such as the
crankshaft or camshaft. As
the blades pass through
the sensor gap. the signal
voltage will switch high and
low creating a series of
pulses. The ECM
determines the speed of
rotation by measuring how
fast pulses appear. Hall
Effect type sensors may be
used to measure speed
and position of the
crankshaft or camshaft -
for spark timing or fuel
injector control.
Handwheel
sensor
This is a three wire sensor
(power, ground and signal
wires). It is used in some
Variable Assist Power
Steering systems. The
ECM uses the sensor
signal to determine how
fast the steering wheel is
being turned. Then the
ECM can apply the correct
amount of power steering
assist based on vehicle
speed. See “EVO”
(Electronic Variable
Orifice) actuator definition
for more information.
HEI
High Energy Ignition.
Ignition system which
pulses the spark coil by
using transistor switches
instead of mechanical
breaker points. The
electronics are in a module
which use a reference
signal coming from a
magnetic pick-up coil
driven by the camshaft.
Hertz (Hz)
A term for frequency -
cycles per second.
IAC
Idle Air Control. This is a
device mounted on the
throttle body. It adjusts the
amount of air bypassing a
closed throttle so that the
ECM can control idle
speed. The IAC is a
stepper motor which
moves a pintle within the
air bypass passage. When
the ECM wants to change
idle speed, it will move the
pintle backwards, for more
air and faster idle, or it will
move it forward for less air
and slower idle. (See
Stepper Motor definition.)
Idle switch
This is a switch built into
the tip of the ISC (Idle
Speed Control) motor
spindle. (See “ISC”
definition.) During idle, the
throttle rests against the
ISC spindle and activates
the switch. The ECM uses
this switch signal to
identify closed throttle
condition, then operates
the engine in an “idle” or
“deceleration” mode.