Jeep Cherokee 2000 Service Manual - page 857
If 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 it will not
supply a ground to the injectors. If a hard decelera-
tion does not exist, the PCM will determine the
proper injector pulse width and continue injection.
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.
WIDE OPEN THROTTLE MODE
This is an Open Loop mode. During wide open
throttle operation, the 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
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 ignores the oxygen sensor input
signal and provides a predetermined amount of addi-
tional 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.
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.
AUTOMATIC SHUTDOWN (ASD) RELAY
SENSE—PCM INPUT
DESCRIPTION
The ASD relay is located in the Power Distribution
Center (PDC). The PDC is located in the engine com-
partment. Refer to label on PDC cover for relay loca-
tion.
OPERATION
A 12 volt signal at this input indicates to the PCM
that the ASD has been activated. The relay is used to
connect the oxygen sensor heater element, ignition
coil and fuel injectors to 12 volt + power supply.
This input is used only to sense that the ASD relay
is energized. If the powertrain control module (PCM)
does not see 12 volts at this input when the ASD
should be activated, it will set a diagnostic trouble
code (DTC).
BATTERY VOLTAGE—PCM INPUT
OPERATION
The battery voltage input provides power to the
Powertrain Control Module (PCM). It also informs
the PCM what voltage level is supplied to the igni-
tion coil and fuel injectors.
If battery voltage is low, the PCM will increase
injector pulse width (period of time that the injector
is energized). This is done to compensate for the
reduced flow through injector caused by the lowered
voltage.
BRAKE SWITCH—PCM INPUT
OPERATION
When the brake light switch is activated, the Pow-
ertrain Control Module (PCM) receives an input indi-
cating that the brakes are being applied. After
receiving this input, the PCM maintains idle speed to
a scheduled rpm through control of the Idle Air Con-
trol (IAC) motor. The brake switch input is also used
to disable vent and vacuum solenoid output signals
to the speed control servo.
FIVE VOLT SENSOR SUPPLIES—PRIMARY
AND SECONDARY
DESCRIPTION
Two different Powertrain Control Module (PCM)
five volt supply circuits are used; primary and sec-
ondary.
OPERATION
These 2 circuits will:
• supply the required 5 volt power source to the
Crankshaft Position (CKP) sensor.
• supply the required 5 volt power source to the
Camshaft Position (CMP) sensor.
• supply a reference voltage for the Manifold Abso-
lute Pressure (MAP) sensor.
• supply a reference voltage for the Throttle Posi-
tion Sensor (TPS) sensor.
• supply the required 5 volt power source to the
oil pressure sensor.
• supply the required 5 volt power source for the
Vehicle Speed Sensor (VSS) (if equipped).
XJ
FUEL SYSTEM
14 - 29
DESCRIPTION AND OPERATION (Continued)