Engine Control Module: Description and Operation

MODES OF OPERATION

As input signals to the engine controller change, the engine controller adjusts the responses sent to the output devices. For example, the engine controller must calculate a different injector pulse width and ignition timing for idle than it does for Wide Open Throttle (WOT). There are eight modes of operation that determine how the engine controller 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 engine controller receives input signals and responds only to preset engine controller programming. Input from the oxygen (02) sensor is not monitored during OPEN LOOP modes. During CLOSED LOOP modes the engine controller monitors the oxygen sensor input. This input indicates to the engine controller 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 engine controller can "fine tune" the injector pulse width to achieve optimum fuel economy and performance combined with low emission levels. The Multi-Point Fuel Injection (MPFI) system has the following modes of operation:

- Ignition switch ON
- Engine start-up
- Engine warm-up
- Cruise or Idle
- Acceleration
- Deceleration
- Wide Open Throttle (WOT)
- Ignition switch OFF

The ignition switch ON, engine start-up, engine warm-up, wide open throttle, and ignition switch OFF modes are OPEN LOOP modes. The acceleration, deceleration and cruise or idle modes, with the engine at operating temperature, are CLOSED LOOP modes.



IGNITION SWITCH ON

This is an OPEN LOOP mode. When the Multi-Point Fuel Injection (MPFI) system is ACTIVATED by the ignition switch, the following actions occur:

- The engine controller reads atmospheric air pressure from the Manifold Absolute Pressure (MAP) sensor and calculates the basic fuel strategy.
- The engine controller monitors coolant temperature sensor and throttle position sensor inputs. The engine controller uses these inputs to modify fuel strategy.
The Automatic Shutdown (ASD) relay is not located inside the SBEC, but it is turned OFF and ON by the SBEC (controls the ground). The Distributor (Hall Effect) Signal goes directly to the SBEC, and in the event that the SBEC sees no distributor signal, the ground is removed from the ASD circuit. If the ignition switch is ON and the engine is OFF, the SBEC will allow the ground to be applied to the ASD for approximately 2 seconds, then if no distributor signal is sensed the SBEC removes the ground from the circuit. If the ground is removed from the ASD relay circuit, then the ASD relay acts as an OPEN switch in the power circuits to the fuel injector, fuel pump, ignition coil, and oxygen sensor heating element causing a no start condition.

ENGINE START-UP

This is an OPEN LOOP mode. The following actions occur when the starter motor is engaged:

- If the engine controller receives an engine rotation signal from the crankshaft position sensor it will ENERGIZE the Auto Shutdown (ASD) relay. The auto shutdown relay will supply the fuel pump, fuel injectors, ignition coils, and oxygen sensor heater with battery voltage. If the engine controller does not receive an engine rotation signal, the ASD relay will not be ENERGIZED.

When the engine is operating normally and idling within +/-64 rpm of its specified rpm, the engine controller will compare the current MAP value with the atmospheric pressure reading it received during KEY-ON MODE. If a substantial difference is not detected between the two readings, then a pneumatic fault code is set in memory.
Once the ASD relay is ENERGIZED, the engine controller:

- Will supply the injectors with a ground path, firing two injectors at a time (1 and 6, 2 and 3, 4 and 5).
- Will monitor coolant temperature, barometric pressure (at engine CRANKING only), and the number of engine revolutions since the cranking was initiated. The engine controller uses these inputs to calculate the injector pulse width.
- Will monitor coolant temperature, manifold absolute pressure, and throttle position. The engine controller uses these inputs to calculate the correct ignition timing.

The following conditions must be met for the engine controller to set a pneumatic fault code into the memory:

- Engine running within 64 rpm of the desired rpm.
- Current operating rpm is greater than the START FUEL rpm.
- The difference between atmospheric pressure readings taken at start-up and current reading is not great enough.

ENGINE WARM-UP

This is an OPEN LOOP mode. The engine controller receives inputs from:

- Air conditioning switch
- Battery voltage
- Coolant temperature sensor
- Engine speed sensor (crankshaft position)
- Manifold Absolute Pressure (MAP) sensor
- Throttle Position Sensor (TPS)

The fuel injectors are controlled by the engine controller (switching the ground path on and off), and it fires the injectors two times for every engine revolution. The engine controller determines the proper ignition timing according to input received from the speed sensor, and controls the idle speed throttle stop angle. The engine controller controls the idle speed by adjusting the automatic idle speed motor and the ignition timing.


CRUISE OR IDLE

This is a CLOSED LOOP mode, when the engine is at normal operating temperature. At cruising speed or idle the engine controller receives inputs from:

- Air conditioning control positions
- Battery voltage
- Coolant temperature sensor
- Manifold Absolute Pressure (MAP) sensor
- Oxygen sensor
- Engine speed sensor (crankshaft position)
- Throttle Position Sensor (TPS)

Based on these inputs the following occurs:

- Voltage is applied to the fuel injectors with the engine controller controlling the ground path to control the injection sequence and injector pulse width.
- The engine controller monitors the oxygen sensor input and adjusts air/fuel ratio, adjusts engine idle speed, controls the idle speed throttle stop angle, and controls ignition timing. The engine controller adjusts the air/fuel ratio by controlling the injector pulse width, adjusts the engine idle speed by controlling the idle speed stepper motor, and adjusts the ignition timing through the ignition control module.


ACCELERATION

This is a CLOSED LOOP mode. When the engine controller receives a sudden increase in throttle position or MAP pressure, it recognizes these signals as a demand for increased engine output. The engine controller responds by increasing injector pulse width to supply the necessary fuel for acceleration. If the air conditioning is ON, and WOT is sensed by the engine controller, the engine controller will de-energize the A/C clutch relay so that all engine power is made available.


DECELERATION

This is a CLOSED LOOP mode. During deceleration the engine controller receives inputs from:

- Air conditioning control positions
- Battery voltage
- Coolant temperature sensor
- Engine speed sensor
- Manifold Absolute Pressure (MAP) sensor
- Oxygen sensor
- Throttle Position Sensor (TPS)

If the engine controller receives a closed throttle input from the TPS and a sudden decrease in manifold pressure from the MAP sensor, it determines that hard deceleration is occurring. The engine controller may reduce injector firing to once per engine revolution to maintain better control of the air fuel mixture. The engine controller also ACTIVATES the Electric EGR Transducer (EET) and the evaporative purge solenoid to prevent EGR flow and canister purge from occuring.


WIDE OPEN THROTTLE

This is an OPEN LOOP mode. During wide open throttle operation the engine controller receives the following inputs:

- Coolant temperature sensor
- Engine speed sensor
- Manifold Absolute Pressure (MAP) sensor
- Throttle Position Sensor (TPS)

During wide open throttle conditions the following occurs:

- The engine controller ignores the oxygen sensor input signal and provides a predetermined amount of additional fuel by adjusting injector pulse width.
- The engine controller energizes the Electric EGR Transducer (EET) to prevent EGR operation, and disengages the A/C compressor clutch through the clutch relay.

IGNITION SWITCH OFF

This is an OPEN LOOP mode. When the key is placed in the OFF position, the engine controller removes the ground from the Auto Shutdown (ASD) Relay. When the ASD relay is DE-ENERGIZED, no battery voltage is allowed to flow to the fuel pump, fuel injectors, ignition coil, or oxygen sensor heating element. When this occurs, all fuel injection, and other systems controlled by the SBEC, cease to function.