Jeep cherokee 2000 Service Manual page 855

XJ
DESCRIPTION AND OPERATION (Continued)
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 O2S sensor,
the PCM can fine tune the injector pulse width. This
is done to achieve optimum fuel economy combined
with low emission engine performance.
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.
IGNITION SWITCH (KEY-ON) MODE
This is an Open Loop mode. When the fuel system
is activated by the ignition switch, the following
actions occur:
• The PCM pre-positions 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 O2S sensor heater element is energized via
the ASD relay. The O2S sensor input is not used by
the PCM to calibrate air-fuel ratio during 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 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)
• Camshaft position sensor signal
The PCM monitors the crankshaft position sensor.
If the PCM does not receive a crankshaft position
sensor signal within 3 seconds of cranking the
engine, it will shut down the fuel injection system.
The fuel pump is activated by the PCM through
the fuel pump relay.
Voltage is applied to the fuel injectors with the
ASD relay via the PCM. The PCM will then control
the injection sequence and injector pulse width by
turning the ground circuit to each individual injector
on and off.
The PCM determines the proper ignition timing
according to input received from the crankshaft posi-
tion sensor.
ENGINE WARM-UP MODE
This is an Open Loop mode. During engine warm-
up, the PCM receives inputs from:
• 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)
• Park/neutral switch (gear indicator signal—auto.
trans. only)
• Air conditioning select signal (if equipped)
• Air conditioning request signal (if equipped)
Based on these inputs the following occurs:
• Voltage is applied to the fuel injectors with the
ASD relay via the PCM. The PCM will then control
the injection sequence and injector pulse width by
turning the ground circuit to each individual injector
on and off.
• The PCM adjusts engine idle speed through the
idle air control (IAC) motor and adjusts ignition tim-
ing.
• The PCM operates the A/C compressor clutch
through the A/C compressor clutch relay. This is done
if A/C has been selected by the vehicle operator and
specified pressures are met at the high and low–pres-
sure A/C switches. Refer to Group 24, Heating and
Air Conditioning for additional information.
• When engine has reached operating tempera-
ture, the PCM will begin monitoring O2S sensor
input. The system will then leave the warm-up mode
and go into closed loop operation.
FUEL SYSTEM 14 - 27