Actron GM Code Scanner CP9001 - Manual (Page 37)

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Inputs
Electrical signals running
into the ECM. These
signals come from
sensors, switches or other
electronic modules. They
give the ECM information
about vehicle operation.
ISC
Idle Speed Control. This
refers to a small electric
motor mounted on the
throttle body and controlled
by the ECM. The ISC motor
moves a spindle back and
forth. When the throttle is
released during idle, it rests
on this spindle. The ECM
can control idle speed by
adjusting this spindle
position. The ECM
determines the desired idle
speed by looking at battery
voltage, coolant
temperature, engine load
and RPM.
Knock sensor
This sensor is used to
detect engine detonation
(knocking). When spark
knock occurs, the sensor
sends a pulsing signal.
Depending upon vehicle,
this signal goes either to the
ECM or a separate ESC
(Electronic Spark Control)
module. Then the spark
advance is retarded to
eliminate detonation. The
sensor contains a
piezoelectric element and is
threaded into the engine
block. Vibrating the element
generates the signal.
Special construction makes
the element only sensitive to
the engine vibrations
associated with knocking.
MAF
Mass Air Flow sensor. This
sensor measures the
amount of air entering the
engine and sends a
frequency or voltage signal
(depends upon sensor type)
to the ECM. The signal
voltage or frequency
increases when the amount
of incoming air goes up.
This gives the ECM
information required for
control of fuel delivery and
spark advance.
MAP
Manifold Absolute
Pressure sensor. This
sensor measures manifold
vacuum and sends a
frequency or voltage signal
(depends upon sensor
type) to the ECM. This
gives the ECM information
on engine load for control
of fuel delivery, spark
advance and EGR flow.
MAT
Manifold Air Temperature
sensor. This sensor is a
thermistor - a resistor
whose resistance
decreases with
temperature. It is threaded
into the intake manifold so
the ECM can determine
the temperature of the
incoming air. This is used
for fuel delivery
calculations.
MCS
Mixture Control Solenoid.
Used on computer
controlled vehicles having
carburetors. Built into the
carburetor, it allows the
ECM to “fine tune” fuel
delivery during warm
engine cruise.
MEMCAL
A small electronic
assembly containing the
the functions of both the
PROM and CALPAK. It is
hidden behind an access
door on the ECM and is
replaceable. Only some
ECM’s have MEMCAL.
MFI
Multi-Port Fuel Injection.
(See MPFI definition.)
Mode
A type of operating
condition, such as “idle
mode” or “cruise mode.”
MPFI
Multi-Port Fuel Injection. A
fuel injection system using
one injector for each
cylinder. The injectors are
mounted in the intake
manifold. The injectors are
fired in groups rather than
individually.
Open (circuit)
A break in the continuity of
a circuit such that no
current can flow.
Open Loop (O/L)
This is when the control
system performs an action
(expecting a certain result),
but has no way of verifying
if the desired results were
achieved. Example: the
ECM operates a fuel
injector expecting a certain
amount of fuel to be
delivered. (The ECM
assumes everything in the
fuel system is performing
as expected.) In open loop
operation, the ECM has no
way of checking the actual
amount of fuel delivered.
Thus, a faulty fuel injector
or incorrect fuel pressure
can change the amount of
fuel delivered and the ECM
would not know it.
In general, a control
system operates “open
loop” only when there is no
practical way to monitor the
results of an action.
Example: Fuel delivery
during cold engine warm-
up. The computer runs
“open loop”because the
oxygen sensor is not ready
to send a signal. Without
the sensor signal, the
computer can not check
the actual amount of fuel
delivered.
O
2
Oxygen sensor. The
oxygen sensor is threaded
into the exhaust manifold,
directly into the stream of
the exhaust gases. The
ECM uses the sensor to
“fine tune” fuel delivery.
The sensor generates a
voltage of 0.6 to 1.1 volts
when the exhaust gas is
rich (low oxygen content).
The voltage changes to 0.4
volts or less when the
exhaust gas is lean (high
oxygen content). The
sensor only operates after
it reaches a temperature of
349°C (660°F).
Outputs
Electrical signals sent from
the ECM. These signals
may activate relays or
other actuators for control
purposes around the
vehicle. The signals can
also send information from
the ECM to other electronic
modules, such as ignition
or trip computer.