Engine Control Module: Description and Operation

MODES OF OPERATION

As input signals to the engine controller change, the engine controller adjusts its response that it sends 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 (CRANK)
- Engine warm-up
- Acceleration
- Deceleration
- Cruise
- Wide Open Throttle (WOT)
- 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 acceleration, 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 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 the throttle position sensor and engine coolant temperature sensor input. The engine controller uses these inputs to modify fuel strategy.
When the key is turned to the ON position, the Auto ShutDown (ASD) relay is not ACTIVATED. For this reason, battery voltage is not supplied to the ignition coil, fuel pump, injector, or oxygen sensor heater.

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 injector, ignition coil, and oxygen sensor heater with battery voltage. If the engine controller does not receive an engine rotation signal from the crankshaft position sensor, the ASD relay will not be ENERGIZED.
Once the ASD relay is ENERGIZED, the engine controller:

- Will supply the injectors with a ground path, firing them in pairs (1,6; 2,3; 4,5).
- 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, crankshaft position, camshaft position, detonation sensor signals, 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.
- Vehicle is stationary.
- 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
- Manifold Absolute Pressure (MAP) sensor
- Engine speed sensor (crankshaft position sensor)
- Throttle Position Sensor (TPS)

Voltage is applied to the fuel injectors with the engine controller controlling the ground path to control injection sequence and injector pulse width. The engine controller determines the proper ignition timing according to input received from the speed sensor. The engine controller controls the idle speed by adjusting the automatic idle speed motor.

ACCELERATION

This is a CLOSED LOOP mode. When the engine controller receives a sudden increase in throttle position or MAP pressure, it recognizes it as a demand for increased engine output. The engine controller responds by increasing injector pulse width to supply the necessary fuel for acceleration.

DECELERATION

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

- Air conditioning switch
- Battery voltage
- Coolant temperature sensor
- Engine speed sensor (crankshaft position 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 and the evaporative purge solenoid to prevent EGR flow and canister purge from occuring.

CRUISE OR IDLE

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

- Air Conditioning Control Module (ACM)
- Battery voltage
- Coolant temperature sensor
- Manifold Absolute Pressure (MAP) sensor
- Oxygen sensor
- Engine speed (crankshaft position sensor)
- 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, 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 automatic idle speed motor, and adjusts the ignition timing through the ignition control module.

WIDE OPEN THROTTLE

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

- Coolant temperature sensor
- Manifold absolute pressure (MAP) sensor
- Throttle position sensor (TPS)
- Engine speed sensor (crankshaft position sensor)

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 restrict EGR operation, and disengages the A/C compressor clutch through the clutch relay.

IGNITION SWITCH OFF

The engine controller performs a number of functions after the ignition switch is turned OFF, and the following is a list of those functions:

- No inputs are monitored.
- All outputs are DE-ENERGIZED.
- The engine controller is shut down.