Sensor Return-Pcm Input; Throttle Position Sensor (Tps)-Pcm Input; Vehicle Speed And Distance Sensor- Pcm Input; Auto Shutdown (Asd) Relay-Pcm Output - Jeep cherokee 2000 Service Manual

14 - 34 FUEL SYSTEM
DESCRIPTION AND OPERATION (Continued)
When pump pressure drops to approximately 1379
kPa (200 psi), the switch circuit will re-close and
engine idle speed will return to its previous setting.
SENSOR RETURN—PCM INPUT
OPERATION
The Sensor Return circuits are internal to the Pow-
ertrain Control Module (PCM).
Sensor Return provides a low–noise ground refer-
ence for all engine control system sensors. Refer to
Power Grounds for more information.
THROTTLE POSITION SENSOR (TPS)—PCM
INPUT
DESCRIPTION
The 3–wire Throttle Position Sensor (TPS) is
mounted on the throttle body and is connected to the
throttle blade.
OPERATION
The TPS is a 3–wire variable resistor that provides
the Powertrain Control Module (PCM) with an input
signal (voltage) that represents the throttle blade
position of the throttle body. The sensor is connected
to the throttle blade shaft. As the position of the
throttle blade changes, the resistance (output volt-
age) of the TPS changes.
The PCM supplies approximately 5 volts to the
TPS. The TPS output voltage (input signal to the
PCM) represents the throttle blade position. The
PCM receives an input signal voltage from the TPS.
This will vary in an approximate range of from.26
volts at minimum throttle opening (idle), to 4.49 volts
at wide open throttle. Along with inputs from other
sensors, the PCM uses the TPS input to determine
current engine operating conditions. In response to
engine operating conditions, the PCM will adjust fuel
injector pulse width and ignition timing.
The PCM needs to identify the actions and position
of the throttle blade at all times. This information is
needed to assist in performing the following calcula-
tions:
• Ignition timing advance
• Fuel injection pulse-width
• Idle (learned value or minimum TPS)
• Off-idle (0.06 volt)
• Wide Open Throttle (WOT) open loop (2.608
volts above learned idle voltage)
• Deceleration fuel lean out
• Fuel cutoff during cranking at WOT (2.608 volts
above learned idle voltage)
• A/C WOT cutoff (certain automatic transmis-
sions only)
VEHICLE SPEED AND DISTANCE SENSOR—
PCM INPUT
DESCRIPTION
The 3–wire Vehicle Speed Sensor (VSS) is located
on the speedometer pinion gear adapter. If equipped
with 4WD, this adapter is located on the extension
housing of the transfer case (drivers side). If
equipped with 2WD, this adapter is located on the
left side of the transmission extension housing.
OPERATION
The VSS is a 3–circuit (3–wire), magnetic, hall-ef-
fect sensor.
The 3 circuits are:
• A 5–volt power supply from the Powertrain Con-
trol Module (PCM).
• A ground is provided for the sensor though a
low-noise sensor return circuit in the PCM.
• An input to the PCM is used to determine vehi-
cle speed and distance traveled.
The speed sensor generates 8 pulses per sensor
revolution. These signals, in conjunction with a
closed throttle signal from the throttle position sen-
sor, indicate a closed throttle deceleration to the
PCM. When the vehicle is stopped at idle, a closed
throttle signal is received by the PCM (but a speed
sensor signal is not received).
Under deceleration conditions, the PCM adjusts
the Idle Air Control (IAC) motor to maintain a
desired MAP value. Under idle conditions, the PCM
adjusts the IAC motor to maintain a desired engine
speed.
AUTO SHUTDOWN (ASD) RELAY—PCM
OUTPUT
DESCRIPTION
The 5–pin, 12–volt, Automatic Shutdown (ASD)
relay is located in the Power Distribution Center
(PDC). Refer to label on PDC cover for relay location.
OPERATION
The ASD relay supplies battery voltage (12+ volts)
to the fuel injectors and ignition coil(s). With certain
emissions packages it also supplies 12–volts to the
oxygen sensor heating elements.
The ground circuit for the coil within the ASD
relay is controlled by the Powertrain Control Module
(PCM). The PCM operates the ASD relay by switch-
ing its ground circuit on and off.
The ASD relay will be shut–down, meaning the
12–volt power supply to the ASD relay will be de-ac-
tivated by the PCM if:
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