Dodge ram truck 3500 1996 Service Manual page 1262

1 4 - 2 8 FUEL SYSTEM
B R
GENERAL INFORMATION (Continued)
ACCELERATION
MODE
This is an Open Loop mode. The powertrain control
module (PCM) recognizes an abrupt Increase in
throttle position or MAP pressure as a demand for
increased engine output and vehicle acceleration. The
PCM increases injector pulse width in response to
increased throttle opening.
DECELERATION
MODE
When the engine is at operating temperature, this
is an Open Loop mode. During hard deceleration, the
powertrain control module (PCM) receives the follow­
ing inputs.
• Air conditioning select signal (if equipped)
• Air conditioning request signal (if equipped)
• Battery voltage
• Engine coolant temperature sensor
• Crankshaft position sensor
• Intake manifold air temperature sensor
• Manifold absolute pressure (MAP) sensor
• Throttle position sensor (TPS)
• Camshaft position sensor signal (in the distribu­
tor)
• Park/neutral switch (gear indicator signal—auto,
trans, only)
I f the vehicle is under hard deceleration with the
proper rpm and closed throttle conditions, the PCM
will ignore the oxygen sensor input signal. The PCM
will enter a fuel cut-off strategy in which i t will not
supply battery voltage to the injectors. I f a hard
deceleration does not exist, the PCM will determine
the proper injector pulse width and continue injec­
tion.
Based on the above inputs, the PCM will adjust
engine idle speed through the idle air control (IAC)
motor.
The PCM adjusts ignition timing by turning the
ground path to the coil on and off.
The PCM opens the ground circuit to the A/C
clutch relay to disengage the A/C compressor clutch.
This is done until the vehicle is no longer under
deceleration (if the A/C system is operating).
WIDE OPEN THROTTLE MODE
This is an Open Loop mode. During wide open
throttle operation, the powertrain control module
(PCM) receives the following inputs.
• Battery voltage
• Crankshaft position sensor
• Engine coolant temperature sensor
• Intake manifold air temperature sensor
• Manifold absolute pressure (MAP) sensor
• Throttle position sensor (TPS)
• Camshaft position sensor signal (in the distribu­
tor)
During wide open throttle conditions, the following
occurs:
• Voltage is applied to the fuel injectors with the
powertrain control module (PCM). The PCM will
then control the injection sequence and injector pulse
width by turning the ground circuit to each individ­
ual injector on and off. The PCM ignores the oxygen
sensor input signal and provides a predetermined
amount of additional fuel. This is done by adjusting
injector pulse width.
• The PCM adjusts ignition timing by turning the
ground path to the coil on and off.
• The PCM opens the ground circuit to the A/C
clutch relay to disengage the A/C compressor clutch.
This will be done for approximately 15 seconds (if the
air conditioning system is operating).
IGNITION SWITCH OFF MODE
When ignition switch is turned to OFF position,
the PCM stops operating the injectors, ignition coil,
ASD relay and fuel pump relay.
D E S C R I P T I O N A N D O P E R A T I O N
SYSTEM DIAGNOSIS
The PCM can test many of its own input and out­
put circuits. I f the PCM senses a fault in a major
system, the PCM stores a Diagnostic Trouble Code
(DTC) in memory.
Technicians can display stored DTCs by two differ­
ent methods. The first is to cycle the ignition switch
On - Off - On - Off - On within 5 seconds. Then count
the number of times the malfunction indicator (check
engine) lamp on the instrument panel flashes on and
off. The number of flashes represents the DTC. There
is a slight pause between the flashes representing
the first and second digits of the code. Longer pauses
separate individual trouble codes.
The second method of reading DTCs uses the DRB
scan tool. For DTC information, refer to Group 25,
Emission Control Systems. See On-Board Diagnos­
tics.
POWERTRAIN CONTROL MODULE (PCM)
The powertrain control module (PCM) (Fig. 1) oper­
ates the fuel system. The PCM was formerly referred
to as the SBEC or engine controller. The PCM is a
pre-programmed, dual microprocessor digital com­
puter. I t regulates ignition timing, air-fuel ratio,
emission control devices, charging system, speed con­
trol, air conditioning compressor clutch engagement
and idle speed. The PCM can adapt its programming
to meet changing operating conditions.
The PCM receives input signals from various
switches and sensors. Based on these inputs, the
PCM regulates various engine and vehicle operations
through different system components. These compo­
nents are referred to as Powertrain Control Module