Cherokee 2WD L4-150 2.5L VIN H TBI (1988)
Powertrain Control Module: Description and Operation
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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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, wide open throttle, and ignition switch OFF modes are OPEN LOOP modes. The
deceleration and cruise modes, with the engine at operating temperature, are CLOSED LOOP modes.
IGNITION SWITCH ON
This is an OPEN LOOP mode. When the TBI 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 via the relay.
(The fuel pump will operate for approximately one second unless the engine is operating or the starter motor is engaged)
The following engine sensors are activated and begin providing input to the ECU:
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Coolant temperature sensor (CTS)
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Manifold air/fuel temperature sensor
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Manifold absolute pressure (MAP) sensor
(The up-shift indicator lamp is illuminated)
ENGINE START-UP
This is an OPEN LOOP mode. The following inputs are received by the ECU:
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Coolant temperature sensor (CTS)
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Starter motor relay
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Speed sensor
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WOT switch
The following actions occur when the starter motor is engaged:
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The fuel pump is activated by the ECU.
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Voltage is applied to the fuel injector and the ECU controls the injector pulse width.
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The ECU determines the proper ignition timing according to the input from the speed sensor.
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If a wide open throttle condition exists, the ECU will deactivate the injector because a possible flooding condition could occur.
ENGINE WARM-UP
This is an OPEN LOOP mode. The following inputs are received by the ECU:
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A/C control positions
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Coolant temperature
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Engine speed
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Gear position (automatic transmission)
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Manifold air/fuel temperature
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Manifold absolute pressure
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Throttle position
