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delivered (The PCM assumes everything
in the fuel system is performing properly).
In open loop operation, the PCM does not
check the actual amount of fuel delivered.
Thus, a faulty fuel injector or incorrect fuel
pressure can change the amount of fuel
delivered and the PCM would not know it.
In general, a control system operates in
open loop mode only when there is no prac-
tical way to monitor the results of an action;
i.e. Fuel delivery during cold engine warm-
up. The computer runs in open loop mode
because the oxygen sensors are ready to
send a signal. Without the sensor signal,
the computer cannot check the actual
amount of fuel delivered.
Outputs:
Electrical signals sent from the PCM. These
signals may activate relays or other actua-
tors for control purposes throughout the ve-
hicle. The signals can also send informa-
tion from the PCM to other electronic mod-
ules, such as the ignition or trip computer.
P/N:
Park/Neutral Switch. This switch tells the
PCM when the gear shift lever is in the Park
or Neutral position. Then the PCM will op-
erate the engine in an idle mode.
PCM:
Powertrain Control Module. The brains of
the engine control system. It is a computer
housed in a metal box with a number of
sensors and actuators connected with a
wiring harness. Its job is to control fuel de-
livery, idle speed, spark advance timing, and
emission systems. The PCM receives in-
formation from sensors, then energizes
various actuators to control the engine. The
PCM is frequently called the ECM (Engine
Control Module).
PROM:
Programmable Read-Only Memory. The
PROM contains programming information
the PCM needs to operate a specific ve-
hicle model.
Pending Codes:
Also referred to as Continuous Memory
codes and Maturing Diagnostic Trouble
codes. These codes are set when intermit-
tent faults occur while driving. If the fault does
not occur after a certain number of drive
cycles, the code is erased from memory.
Purge Solenoid:
This device controls the flow of fuel vapors
from the carbon canister to the intake mani-
fold. The canister collects vapors evaporat-
ing from the fuel tank, preventing them from
escaping to the atmosphere and causing
pollution. During warm engine cruise con-
ditions, the PCM energizes the Purge Sole-
noid so the trapped vapors are drawn into
the engine and burned.
Relay:
An electric/mechanical device for switching
high current circuits on and off. It is elec-
tronically controlled by a low current circuit.
Relays allow a low power PCM signal to
control a high power device such as an elec-
tric cooling fan.
Reluctance Sensor:
Crankshaft or Camshaft - Speed, position
(for spark timing or fuel injector control).
Driveshaft - Vehicle speed (transmission or
torque converter control, cooling fan use,
variable assist power steering, and cruise
control). Wheel Speed - Anti-lock brakes or
traction control systems
ROM:
Read-Only Memory. Permanent program-
ming information stored inside the PCM,
containing the information to operate a spe-
cific vehicle model.
SAE:
Society of Automotive Engineers.
Sensor:
Devices which give the PCM information. The
PCM can only work with electrical signals.
The job of the sensor is to take something
the PCM needs to know, such as engine
temperature, and convert it to an electrical
signal that the PCM can understand. The