Dodge ram truck 3500 1996 Service Manual page 1260

14 - 26 FUEL SYSTEM — ^ ^
GENERAL INFORMATION (Continued)
Fig. 1 Powertrain Control Module (PCM)
speed control switches. A l l Inputs to the PCM are
converted Into signals.
Electrically operated fuel Injectors spray fuel in
precise metered amounts Into the intake port directly
above the Intake valve. The Injectors are fired in a
specific sequence.by the PCM. The PCM maintains
an air/fuel ratio of 14.7 to 1 by constantly adjusting
injector pulse width. Injector pulse width is the
length of time that the injector opens and sprays fuel
into the chamber. The PCM adjusts injector pulse
width by opening and closing the ground path to the
Injector.
Manifold absolute pressure (air density) and
engine rpm (speed) are the primary inputs that
determine fuel injector pulse width. The PCM also
monitors other inputs when adjusting air-fuel ratio.
MOtJES OF OPERATION
As input signals to the powertrain control module
(PCM) change, the PCM adjusts Its response to the
output devices. For example, the PCM must calculate
different Injector pulse width and ignition timing for
idle than i t does for wide open throttle (WOT).
The PCM will operate In two different modes:
Open Loop and Closed Loop.
During Open Loop modes, the powertrain control
module (PCM) receives Input signals and responds
only according to preset PCM programming. Input
from the oxygen (02S) sensors is not monitored dur­
ing Open Loop modes.
During Closed Loop modes, the PCM will monitor
the oxygen (02S) sensors Input. This Input indicates
to the PCM whether or not the calculated Injector
pulse width results in the ideal air-fuel ratio. This
ratio Is 14.7 parts air-to-1 part fuel. By monitoring
the exhaust oxygen content through the 02S sensor,
the PCM can fine tune the injector pulse width. This
is done to achieve optimum fuel economy combined
with low emission engine performance.
BR
The fuel injection system has the following modes
of operation:
© Ignition switch ON
• Engine start-up (crank)
© Engine warm-up
• Idle
• Cruise
• Acceleration
• Deceleration
• Wide open throttle (WOT)
• Ignition switch OFF
The ignition switch On, engine start-up (crank),
engine warm-up, acceleration, deceleration and wide
open throttle modes are Open Loop modes. The idle
and cruise modes, (with the engine at operating tem­
perature) are Closed Loop modes.
mrnmom SWITCH ffccF-onJ MODE
This is an Open Loop mode. When the fuel system
is activated by the Ignition switch, the following
actions occur:
• The powertrain control module (PCM) pre-posi-
tions the Idle air control (IAC) motor.
• The PCM determines atmospheric air pressure
from the MAP sensor Input to determine basic fuel
strategy.
• The PCM monitors the engine coolant tempera­
ture sensor input. The PCM modifies fuel strategy
based on this input.
• Intake manifold air temperature sensor Input Is
monitored.
• Throttle position sensor (TPS) is monitored.
• The auto shutdown (ASD) relay Is energized by
the PCM for approximately three seconds.
• The fuel pump is energized through the fuel
pump relay by the PCM. The fuel pump will operate
for approximately three seconds unless the engine is
operating or the starter motor is engaged.
• The 02S sensor heater element is energized
through the fuel pump relay. The 02S sensor Input Is
not used by the PCM to calibrate air-fuel ratio dur­
ing this mode of operation.
ENGINE START-UP MODE
This is an Open Loop mode. The following actions
occur when the starter motor Is engaged.
The powertrain control module (PCM) receives
inputs from:
• Battery voltage
• Engine coolant temperature sensor
• Crankshaft position sensor
• Intake manifold air temperature sensor
• Manifold absolute pressure (MAP) sensor
• Throttle position sensor (TPS)
® Starter motor relay
• Camshaft position sensor signal