Jeep Cherokee 2000, Service Manual

Page: 858 / 1528

•
supply the 5 volt power source to the transmis-
sion pressure sensor (if equipped with an RE auto-
matic transmission).
FUEL LEVEL SENSOR—PCM INPUT
DESCRIPTION
The fuel level sensor (fuel gauge sending unit) is
located on the fuel pump module.
OPERATION
Refer to Fuel Gauge Sending Unit in the Fuel
Delivery section for information.
ENGINE COOLANT TEMPERATURE SENSOR—
PCM INPUT
DESCRIPTION
The Engine Coolant Temperature (ECT) sensor is
used to sense engine coolant temperature. The sensor
protrudes into an engine water jacket.
The ECT sensor is a two-wire Negative Thermal
Coefficient (NTC) sensor. Meaning, as engine coolant
temperature increases, resistance (voltage) in the
sensor decreases. As temperature decreases, resis-
tance (voltage) in the sensor increases.
OPERATION
At key-on, the Powertrain Control Module (PCM)
sends out a regulated 5 volt signal to the ECT sensor.
The PCM then monitors the signal as it passes
through the ECT sensor to the sensor ground (sensor
return).
When the engine is cold, the PCM will operate in
Open Loop cycle. It will demand slightly richer air-
fuel mixtures and higher idle speeds. This is done
until normal operating temperatures are reached.
The PCM uses inputs from the ECT sensor for the
following calculations:
•
for engine coolant temperature gauge operation
through CCD or PCI (J1850) communications
•
Injector pulse-width
•
Spark-advance curves
•
ASD relay shut-down times
•
Idle Air Control (IAC) motor key-on steps
•
Pulse-width prime-shot during cranking
•
O2 sensor closed loop times
•
Purge solenoid on/off times
•
EGR solenoid on/off times (if equipped)
•
Leak Detection Pump operation (if equipped)
•
Radiator fan relay on/off times (if equipped)
•
Target idle speed
EXTENDED IDLE SWITCH—PCM INPUT
DESCRIPTION
USED ONLY WITH OPTIONAL POLICE
PACKAGE WHEN EQUIPPED WITH A 4.0L
ENGINE: The extended idle switch is a rocker-type
switch mounted to the instrument panel.
OPERATION
The extended idle switch is used to raise the
engine idle speed to approximately 1000 rpm by sup-
plying a ground circuit to the Powertrain Control
Module (PCM). This idle speed control can only be
operated when the shifter is in either the Park or
Neutral position.
OXYGEN SENSOR—PCM INPUT
DESCRIPTION
The Oxygen Sensors (O2S) are attached to, and
protrude into the vehicle exhaust system. Depending
on the emission package, the vehicle may contain
either 2 or 4 sensors. On non-California emissions
packages, 2 sensors are used: upstream (referred to
as 1/1) and downstream (referred to as 1/2). On Cal-
ifornia emissions packages, 4 sensors are used: 2
upstream (referred to as 1/1 and 2/1) and 2 down-
stream (referred to as 1/2 and 2/2).
OPERATION
An O2 sensor is a galvanic battery that provides
the PCM with a voltage signal (0-1 volt) inversely
proportional to the amount of oxygen in the exhaust.
In other words, if the oxygen content is low, the volt-
age output is high; if the oxygen content is high the
output voltage is low. The PCM uses this information
to
adjust injector pulse-width to achieve the
14.7–to–1 air/fuel ratio necessary for proper engine
operation and to control emissions.
An O2 sensor must have a source of oxygen from
outside of the exhaust stream for comparison. Cur-
rent O2 sensors receive their fresh oxygen (outside
air) supply through the wire harness. This is why it
is important to never solder an O2 sensor connector,
or pack the connector with grease.
Four wires (circuits) are used on each O2 sensor: a
12–volt feed circuit for the sensor heating element; a
ground circuit for the heater element; a low-noise
sensor return circuit to the PCM, and an input cir-
cuit from the sensor back to the PCM to detect sen-
sor operation.
Oxygen Sensor Heaters/Heater Relays: On a
certain non-California emission package, the heaters
on both sensors are fed battery voltage from the ASD
relay which is controlled by the PCM. Refer to ASD
relay for more information. On another non-Califor-
14 - 30 FUEL SYSTEM
XJ
DESCRIPTION AND OPERATION (Continued)