Cherokee 2WD L4-150 2.5L VIN P MFI (1991)
Powertrain Control Module: Description and Operation
MODES OF OPERATION
As input signals to the SBEC II change, the SBEC II adjusts the responses sent to the output devices. For example, the SBEC II 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 SBEC II 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 SBEC II receives input signals and responds only to preset SBEC II programming. Input from the oxygen (02)
sensor is not monitored during OPEN LOOP modes. During CLOSED LOOP modes the SBEC II monitors the oxygen sensor input. This input
indicates to the SBEC II 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 SBEC II 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:
^
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, 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 MPFI System is activated by the ignition switch, the following actions occur:
^
The Automatic Idle Speed (AIS) motor is pre-positioned by the engine controller for engine start-up.
^
The Manifold Absolute Pressure (MAP) sensor sends a signal to the engine controller, indicating the atmospheric pressure. The engine controller
uses this input to set the basic fuel strategy.
^
The Coolant Temperature Sensor sends a signal to the engine controller, indicating the current coolant temperature. The engine controller uses
this input to set the basic fuel strategy.
^
The Manifold Air Temperature (MAT) sensor and Throttle Position Sensor (TPS) inputs are monitored by the engine controller.
^
When the key is turned to the ON position (engine not running), the Auto ShutDown (ASD) relay is ACTIVATED for approximately 3 seconds,
and if no crankshaft position signal is received by the engine controller, the ground will be removed from the ASD circuit DE-ACTIVATING the
ASD relay. If the ASD relay is not ACTIVATED, battery voltage is not supplied to the ignition coil or the injectors.
^
The fuel pump is ACTIVATED by the fuel pump relay, via the ballast resistor, by the engine controller. The fuel pump will operate for
approximately 1 second, and if no crankshaft position signal or starter motor engagement signal is received by the engine controller, the ground
will be removed from the fuel pump relay circuit, DE-ACTIVATING the fuel pump. If the fuel pump relay is not ACTIVATED, battery voltage is
not supplied to the fuel pump or the oxygen sensor heating element.
^
The oxygen sensor heating element is ENERGIZED via the fuel pump relay, by the engine controller. It will remain ENERGIZED for
approximately 1 second, and if no crankshaft position signal or starter motor engagement signal is received by the engine controller, the ground
will be removed from the fuel pump relay circuit, DE-ENERGIZING the oxygen sensor heating element. If the fuel pump relay is not
ACTIVATED, battery voltage is not supplied to the fuel pump or the oxygen sensor heating element
^
The upshift indicator lamp is illuminated (Manual transmission).
ENGINE START-UP
This is an OPEN LOOP mode. The following inputs are received by the SBEC II when the starter motor is engaged:
^
Battery voltage
^
Coolant Temperature Sensor (CTS)
^
Crankshaft Position Sensor (CPS)
^
Manifold Air Temperature (MAT) sensor
^
Manifold Absolute Pressure (MAP) sensor
^
Starter motor relay
^
Sync signal
^
Throttle Position Sensor (TPS)
If the engine controller receives an engine rotation signal from the Crankshaft Position Sensor (CPS) it will ENERGIZE the Auto ShutDown (ASD) and
fuel pump relays. The auto shutdown relay supplies voltage to the fuel injectors and the ignition coil, and the fuel pump relay supplies voltage to the fuel
pump and the oxygen sensor heater element. If the engine controller does not receive an engine rotation signal from the Crankshaft Position Sensor
(CPS), within approximately 3 seconds of ignition switch ON signal, the fuel injection system will become DE-ACTIVATED.
