Jeep Cherokee (XJ): Modes of operation

OPERATION

As input signals to the Powertrain Control Module (PCM) change, the PCM adjusts its response to the output devices. For example, the PCM must calculate different injector pulse width and ignition timing for idle than it does for wide open throttle (WOT).

The PCM will operate in two different modes: Open Loop and Closed Loop.

During Open Loop modes, the PCM receives input signals and responds only according to preset PCM programming. Input from the oxygen (O2S) sensors is not monitored during Open Loop modes.

During Closed Loop modes, the PCM will monitor the oxygen (O2S) sensors input. This input indicates 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 temperature) 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 temperature 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 position sensor.

ENGINE WARM-UP MODE

This is an Open Loop mode. During engine warmup, 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 distributor)
•  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 timing.
•  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-pressure A/C switches. Refer to Group 24, Heating and Air Conditioning for additional information.
•  When engine has reached operating temperature, the PCM will begin monitoring O2S sensor input. The system will then leave the warm-up mode and go into closed loop operation.

IDLE MODE

When the engine is at operating temperature, this is a Closed Loop mode. At idle speed, the PCM receives inputs from:

•  Air conditioning select signal (if equipped)
•  Air conditioning request signal (if equipped)
•  Battery voltage
•  Crankshaft position sensor
•  Engine coolant temperature sensor
•  Extended idle switch (4.0L engine with police package only)
•  Intake manifold air temperature sensor
•  Manifold absolute pressure (MAP) sensor
•  Throttle position sensor (TPS)
•  Camshaft position sensor signal (in the distributor)
•  Battery voltage
•  Park/neutral switch (gear indicator signal-auto. trans. only)
•  Oxygen sensors
•  Power steering pressure switch (2.5L engine only)

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 injection sequence and injector pulse width by turning the ground circuit to each individual injector on and off.
•  The PCM monitors the O2S sensor input and adjusts air-fuel ratio by varying injector pulse width. It also adjusts engine idle speed through the idle air control (IAC) motor.
•  The PCM adjusts ignition timing by increasing and decreasing spark advance.
•  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-pressure A/C switches. Refer to Group 24, Heating and Air Conditioning for additional information.

The optional Extended Idle Switch is used to raise and hold the engine idle speed to approximately 1000 rpm. This is when the shifter is in either the Park or Neutral position and throttle pedal is not used. A rocker-type switch (extended idle switch) is mounted to the instrument panel. This switch will supply a ground circuit (input) to the PCM. The switch is available only with 4.0L engine when supplied with optional police package.

On 2.5L 4-cylinder engines, a power steering pressure switch is used to supply an input to the PCM when steering pump pressure is high. This will raise engine speed. Refer to Power Steering Pressure Switch in this group for additional information. The 4.0L 6-cylinder engine does not use this switch.

CRUISE MODE

When the engine is at operating temperature, this is a Closed Loop mode. At cruising speed, the PCM receives inputs from:

•  Air conditioning select signal (if equipped)
•  Air conditioning request signal (if equipped)
•  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 (in the distributor)
•  Park/neutral switch (gear indicator signal-auto. trans. only)
•  Oxygen (O2S) sensors

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 adjust the injector pulse width by turning the ground circuit to each individual injector on and off.
•  The PCM monitors the O2S sensor input and adjusts air-fuel ratio. It also adjusts 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.
•  The PCM operates the A/C compressor clutch through the clutch relay. This happens if A/C has been selected by the vehicle operator and requested by the A/C thermostat.

ACCELERATION MODE

This is an Open Loop mode. The PCM recognizes an abrupt increase in throttle position or MAP pressure as a demand for increased engine output and vehicle acceleration. The PCM increases injector pulse width in response to increased throttle opening.

DECELERATION MODE

When the engine is at operating temperature, this is an Open Loop mode. During hard deceleration, the PCM receives the following inputs.

•  Air conditioning select signal (if equipped)
•  Air conditioning request signal (if equipped)
•  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 (in the distributor)
•  Park/neutral switch (gear indicator signal-auto. trans. only)
•  Vehicle speed sensor

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 deceleration 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 distributor)

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 additional 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.