Wagoneer L6-242 4.0L VIN M FI (1988)
Powertrain Control Module: Description and Operation
ELECTRONIC CONTROL UNIT (ECU)
Modes of Operation
As input signals to the ECU change, the ECU adjusts its response that it sends to the output devices. For example, the ECU must calculate a different
injector pulse width and ignition timing for idle than it does for wide open throttle (WOT). There are seven modes of operation that determine how the
ECU responds to the various input signals. Modes of operation are separated into two different types, OPEN LOOP and CLOSED LOOP. During
OPEN LOOP modes the ECU receives input signals and responds only to preset ECU programming. Input from the oxygen (02) sensor is not monitored
during OPEN LOOP modes. During CLOSED LOOP modes the ECU monitors the oxygen sensor input. This input indicates to the ECU whether or not
the calculated injector pulse width results in the ideal air/fuel ratio of 14.7 parts of air to 1 part of fuel. By monitoring the exhaust oxygen content, using
the oxygen sensor, the ECU can "fine tune" the injector pulse width to achieve optimum fuel economy and performance combined with low emission
levels. The TBI 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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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, deceleration, wide open throttle, and ignition switch OFF modes are OPEN LOOP
modes. The idle and cruise modes, with the engine at operating temperature, are CLOSED LOOP modes.
Ignition Switch ON
This is an OPEN LOOP mode. When the MPI system is activated by the ignition switch, the following actions occur:
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The system power relay is energized.
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The fuel pump is activated by the ECU.
(The fuel pump will operate for approximately one second unless the engine is operating or the starter motor is engaged).
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The B+ latch relay is energized for 1 to 3 seconds.
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The oxygen sensor heater element is activated (Input is used only to heat the sensor, not by the ECU to calibrate air/fuel ratio).
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The EGR valve solenoid is energized by the ECU preventing EGR valve operation.
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The upshift light is illuminated (manual transmission only).
The following engine sensors are activated and begin providing input to the ECU:
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Coolant temperature sensor
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Manifold air temperature (MAT) sensor
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Manifold absolute pressure (MAP) sensor
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Throttle position sensor (TPS)
Engine Start-Up
This is an OPEN LOOP mode. The following actions occur when the starter motor is engaged. The ECU receives inputs from:
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Battery voltage
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Coolant temperature sensor
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Manifold air temperature sensor
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Manifold absolute pressure (MAP) sensor
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Starter motor relay
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Speed sensor
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Sync signal
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Throttle position sensor
The fuel pump is activated by the ECU. Voltage is applied to the fuel injectors with the ECU controlling the injection sequence and injector pulse width.
The ECU determines the proper ignition timing according to input received from the speed sensor. The EGR valve solenoid is energized by the ECU
preventing EGR operation. If the ECU detects a wide open throttle condition it assumes that a flooding condition exists. The ECU will then discontinue
injector operation until engine speed reaches approximately 400 rpm.
Engine Warm-Up
