RAM 1500 Truck 4WD V8-5.9L VIN Z (2002)
Engine Control Module: Description and Operation
Modes of Operation
MODES OF 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 (O(2)S)
sensors is not monitored during Open Loop modes.
During Closed Loop modes, the PCM will monitor the Oxygen (O(2)S) 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
O(2)S 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:
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Ignition switch ON
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Engine start-up (crank)
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Engine warm-up
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Idle
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Cruise
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Acceleration
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Deceleration
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Wide open throttle (WOT)
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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:
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The PCM pre-positions the idle air control (IAC) motor.
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The PCM determines atmospheric air pressure from the MAP sensor input to determine basic fuel strategy.
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The PCM monitors the engine coolant temperature sensor input. The PCM modifies fuel strategy based on this input.
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Intake manifold air temperature sensor input is monitored.
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Throttle Position Sensor (TPS) is monitored.
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The auto shutdown (ASD) relay is energized by the PCM for approximately three seconds.
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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.
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The O(2)S sensor heater element is energized via the ASD or O(2)S heater relay. The O(2)S 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:
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Battery voltage
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Engine coolant temperature sensor
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Crankshaft position sensor
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Intake manifold air temperature sensor
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Manifold absolute pressure (MAP) sensor
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Throttle position sensor (TPS)
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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.
