Achieva L4-144 2.4L DOHC VIN T SFI (1996)
Powertrain Control Module: Description and Operation
Modes of Operation
The PCM uses voltage inputs from several sensors to determine how much fuel to give the engine. The fuel is delivered under one of several
conditions called modes. All modes are controlled by the PCM.
STARTING MODE
When the ignition is first turned ON (before engaging the starter), the PCM energizes the fuel pump relay for a calibrated time (approximately 2
seconds) allowing the fuel pump to build up pressure. The PCM also checks the Engine Coolant Temperature (ECT) and Throttle Position (TP)
sensors to determine the proper air/fuel ratio for starting. This ranges from approximately 0.8:1 at -40° C (-40 F) to 16.8:1 at 110° C (230° F )
engine coolant temperature. The PCM controls the amount of fuel delivered by changing how long the injectors are energized. This is done by
pulsing the injectors for very short times.
CLEAR FLOOD MODE
Whenever the engine floods, the engine can be cleared by depressing the accelerator pedal to the floor while cranking the engine. The PCM
de-energizes the fuel injectors and holds them de-energized as long as the throttle stays wide open. Whenever the throttle is released to less than
70, the PCM returns to the starting mode.
ACCELERATION MODE
When the driver depresses on the accelerator pedal, air flow into the cylinders increases rapidly, while fuel flow tends to lag behind. To prevent
possible hesitation, the PCM increases the pulse width to the injectors to provide extra fuel during acceleration. The amount of fuel required is
based on throttle position, manifold air pressure, and engine speed. A restricted fuel filter or fuel line may cause a DTC Pi 171 to set during heavy
acceleration.
FUEL CUTOFF MODE
To prevent possible engine damage from over-rewing, the PCM cuts off fuel from the injectors at a calibrated engine speed, or vehicle speed. On
vehicles equipped with automatic transmission, fuel will be shut off if the engine exceeds 4000 RPM in PARK or NEUTRAL. The PCM also cuts
off fuel to the fuel injectors at vehicle speeds above 108 mph to prevent tire damage.
DECELERATION MODE
When the driver releases the accelerator pedal, air flow into the engine is reduced. The corresponding changes in throttle position and manifold air
pressure are relayed to the PCM, which reduces the injector pulse width to reduce fuel flow. Whenever the deceleration is very rapid, or for long
periods (such as long closed throttle coast-down), the PCM shuts OFF fuel completely to protect the catalytic converter.
BATTERY VOLTAGE CORRECTION MODE
When battery voltage is low, a DTC P0562 may be set and the PCM will compensates for the weak spark delivered by the electronic ignition
system by:
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Increasing the injector pulse width.
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Increasing the idle RPM.
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Increasing ignition dwell time,
RUN MODE
The run mode has two conditions called Open Loop and Closed Loop.
OPEN LOOP
When the engine is first started and engine speed is above 637 RPM, the system goes into Open Loop operation. Also, when the engine is first
started and the throttle position has not changed, the system remains in Open Loop, until the throttle is moved. The PCM ignores the signal from
the O2S and calculates the air/fuel ratio based on inputs from the ECT and Manifold Absolute Pressure (MAP) sensors. The system stays in Open
Loop until the following conditions are met:
1. The O2S has varying voltage output, showing that it is hot enough to operate properly. (This depends on temperature.)
2. The ECT sensor is above a specified temperature.
CLOSED LOOP
Specific values for the above conditions exist for each different engine, and are stored in the Electronically Erasable Programmable Read Only
Memory (EEPROM). When these values are met, the system goes into Closed Loop operation. The PCM calculates the air/fuel ratio (injector
on-time) based on the signal from the 02S. This allows the air/fuel ratio to stay very close to 14.6:1.
