Chrysler RAM TRUCK 2500 1994 Service Manual page 971

14 - 30 FUEL SYSTEM
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A sequential multi-port fuel injection (MFI) system
is used on all 3.9L V-6 or 5.2/5.9L V-8 gas powered
engines. On the 8.0L V-10 engine, a multi-port fuel
injection system is also used, although the fuel injec­
tors are simultaneously fired i n pairs.
The MFI system provides precise air/fuel ratios for
all driving conditions.
POWERTRAIN CONTROL MODULE (PCM)
The powertrain control module (PCM) is located i n
the right-rear side of the engine compartment (Fig.
1). I t is mounted to the dash panel cowl with three
bolts. The PCM was formerly referred to as the
SBEC or engine controller.
POWERTRAIN CONTROL
MODULE (PCM) J9414-11
Fig. 1 PCM Location—Typical
The PCM operates the fuel system. 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, A/C compressor clutch engagement, overdrive
operation, torque convertor clutch and idle speed.
The PCM can adapt its programming to meet chang­
ing 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 P C M Outputs. The sensors
and switches that provide inputs to the PCM are con­
sidered P C M Inputs.
The PCM adjusts ignition timing based upon in­
puts i t receives from sensors that react to: engine
crankshaft position, manifold absolute pressure, en­
gine coolant temperature, throttle position, transmis­
sion gear selection (automatic transmission), vehicle
speed, intake manifold air temperature and the
brake switch.
The PCM adjusts idle speed based on inputs i t re­
ceives from sensors that react to: throttle position,
vehicle speed, transmission gear selection, engine
coolant temperature and from inputs i t receives from
the air conditioning clutch switch and brake switch.
Based on inputs that i t receives, the PCM adjusts
ignition coil(s) dwell. The PCM also adjusts the gen­
erator charge rate through control of the generator
field and provides speed control operation.
The PCM contains a voltage convertor. This con­
verts battery voltage to a regulated 8.0 volts. I t is
used to power the crankshaft position sensor, cam­
shaft position sensor and vehicle speed sensor. The
PCM also provides a five (5) volt supply for the man­
ifold absolute pressure (MAP) sensor and throttle po­
sition sensor (TPS).
The P C M Inputs represent the instantaneous en­
gine operating conditions. Air-fuel mixture and igni­
tion timing calibrations for various driving and
atmospheric conditions are pre-programmed into the
PCM. The PCM monitors and analyzes various in­
puts. I t then computes engine fuel and ignition tim­
ing requirements based on these inputs. Fuel
delivery control and ignition timing will then be ad­
justed accordingly.
Other inputs to the PCM are provided by the brake
switch, air conditioning select switch and the speed
control switches. All inputs to the PCM are converted
into signals.
Electrically operated fuel injectors spray fuel i n
precise metered amounts into the intake port directly
above the intake valve. The injectors are fired i n a
specific sequence by the PCM. I n the Closed Loop
mode, the PCM maintains an air/fuel ratio of 14.7 to
1 by constantly adjusting injector pulse width. Injec­
tor 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 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.
The PCM adjusts ignition timing by controlling ig­
nition coil operation. The ignition coil(s) receives bat­
tery voltage when the ignition key is i n the run or
start (crank) position. The PCM provides a ground
for the ignition coil. The coil(s) discharges when the
PCM removes a ground. By switching the ground
path on and off, the PCM regulates ignition timing.
Refer to Group 8D, Ignition System for more infor­
mation.
P C M Inputs That Effect Fuel Injector Pulse
Widths
• Battery voltage
• EGR
• Engine coolant temperature
• Engine speed (rpm)
• Evaporative emissions purge
• Exhaust gas oxygen content from oxygen (02S)
sensor