Engine 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:
^ Ignition switch ON
^ Engine start-up (crank)
^ Engine warm-up
^ Idle
^ Cruise
^ Deceleration
^ Wide Open Throttle (WOT)
^ 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:
^ The system power relay is energized.
^ 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).
^ The B+ latch relay is energized for 1 to 3 seconds.
^ The oxygen sensor heater element is activated (Input is used only to heat the sensor, not by the ECU to calibrate air/fuel ratio).
^ The EGR valve solenoid is energized by the ECU preventing EGR valve operation.
^ The upshift light is illuminated (manual transmission only).
The following engine sensors are activated and begin providing input to the ECU:
^ Coolant temperature sensor
^ Manifold air temperature (MAT) sensor
^ Manifold absolute pressure (MAP) sensor
^ 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:
^ Battery voltage
^ Coolant temperature sensor
^ Manifold air temperature sensor
^ Manifold absolute pressure (MAP) sensor
^ Starter motor relay
^ Speed sensor
^ Sync signal
^ 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
This is an OPEN LOOP mode. During engine warm-up the ECU receives inputs from:
^ Air conditioning select signal (if equipped)
^ Air conditioning request signal (if equipped)
^ Battery voltage
^ Coolant temperature sensor
^ Knock sensor
^ Manifold air temperature (MAT) sensor
^ Manifold absolute pressure (MAP) sensor
^ Neutral Safety Switch (Gear indicator signal-automatic transmission only)
^ Speed sensor
^ Sync signal
^ Throttle position sensor (TPS)
Based on these inputs the following occurs:
^ Voltage is applied to the fuel injectors with the ECU controlling the injection sequence and injector pulse width.
^ The ECU adjusts engine idle speed through the idle speed stepper motor and ignition timing through the ignition control module.
^ The ECU energizes the EGR valve solenoid to restrict EGR operation, and operates the A/C compressor clutch through the clutch relay (this only occurs if A/C has been selected by the vehicle operator and requested by the A/C thermostat).
^ If the vehicle has a manual transmission, the upshift light is operated by the ECU.
^ When the engine has reached operating temperature the ECU will begin monitoring the oxygen sensor input, the system will leave the warm-up mode and go into closed loop operation.
Idle
This is a CLOSED LOOP mode. At idle speed with the engine at operating temperature, the ECU receives inputs from:
^ Air conditioning select signal (if equipped)
^ Air conditioning request signal (if equipped)
^ Battery voltage
^ Coolant temperature sensor
^ Knock sensor
^ Manifold air temperature (MAT) sensor
^ Manifold absolute pressure (MAP) sensor
^ Neutral Safety Switch (Gear-indicator signal automatic transmission only)
^ Oxygen sensor
^ Speed sensor
^ Sync signal
^ Throttle position sensor (TPS)
Based on these inputs the following occurs:
^ Voltage is applied to the fuel injectors with the ECU controlling the injection sequence and injector pulse width (the length of injection time).
^ The ECU monitors the 02 sensor input and uses that information to adjust air/fuel ratio, engine idle speed, and ignition timing. The ECU 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.
^ The ECU energizes the EGR valve solenoid to restrict EGR operation.
^ The ECU operates the A/C compressor clutch through the compressor clutch relay (the A/C has to have been previously selected by the vehicle operator and requested by the A/C thermostat).
Cruise
This is a CLOSED LOOP mode. At cruising speed with the engine at operating temperature, the ECU receives inputs from:
^ Air conditioning select signal (if equipped)
^ Air conditioning request signal (if equipped)
^ Battery voltage
^ Coolant temperature sensor
^ Gear indicator signal (auto. trans. only)
^ Knock sensor
^ Manifold air temperature (MAT) sensor
^ Manifold absolute pressure (MAP) sensor
^ Oxygen sensor
^ Speed sensor
^ Sync signal
^ Throttle position sensor (TPS)
Based on these inputs the following occurs:
^ Voltage is applied to the fuel injectors with the ECU controlling the injection sequence and injector pulse width.
^ The ECU monitors the oxygen sensor input and adjusts air/fuel ratio, adjusts engine idle speed, and controls ignition timing. The ECU 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.
^ The ECU operates the A/C compressor clutch through the compressor clutch relay (the A/C has to have been previously selected by the vehicle operator and requested by the A/C thermostat).
^ The ECU breaks the ground path for the EGR valve solenoid allowing vacuum to reach the EGR valve transducer in anticipation of EGR operation.
Deceleration
This is an OPEN LOOP mode. During hard deceleration, when the engine is at operating temperature, the ECU receives the following inputs:
^ Air conditioning select signal (if equipped)
^ Air conditioning request signal (if equipped)
^ Battery voltage
^ Coolant temperature sensor
^ Gear indicator signal (auto. trans. only)
^ Manifold air temperature (MAT) sensor
^ Manifold absolute pressure (MAP) sensor
^ Speed sensor
^ Sync signal
^ Throttle position sensor (TPS)
If the vehicle is under hard deceleration with the proper RPM and closed throttle conditions, the ECU will ignore the oxygen sensor input signal and enter a fuel cut-off strategy in which it will not supply battery voltage to the injectors (this is done to ensure that an exhaust back-fire doesn't occur, and that emission levels are kept down). If a hard deceleration does not exist, the ECU will determine the proper injector pulse width and continue injection. Based on the above inputs the ECU will adjust engine idle speed through the idle speed stepper motor, ignition timing through the ignition control module, and energize the EGR valve solenoid to prevent EGR operation.
Wide Open Throttle
This is an OPEN LOOP mode. During wide open throttle operation the ECU receives the following inputs:
^ Battery voltage
^ Coolant temperature sensor
^ Knock sensor
^ Manifold air temperature (MAT) sensor
^ Manifold absolute pressure (MAP) sensor
^ Throttle position sensor (TPS)
^ Speed sensor
^ Sync signal
During wide open throttle conditions the following occurs:
^ The fuel pump relay and ballast resistor are bypassed and battery voltage is applied to the fuel pump through the oxygen sensor heater relay.
^ Voltage is applied to the fuel injectors with the ECU controlling the injection sequence and injector pulse width.
^ The ECU ignores the oxygen sensor input signal and provides a predetermined amount of additional fuel by adjusting injector pulse width.
^ The oxygen sensor heater relay is energized by the ECU causing the relay switch to open and stop power to the oxygen sensor heating element.
^ The ECU adjusts ignition timing through the ignition control module, energizes the EGR valve solenoid to restrict EGR operation, and disengages the A/C compressor clutch through the clutch relay for approximately 15 seconds.
^ If the vehicle has a manual transmission, the upshift light is operated by the ECU.
Ignition Switch OFF
The ECU performs a number of functions after the ignition switch is turned OFF, and the following is a list of those functions:
^ The ECU stops supplying power to the injectors.
^ Ignition system power is shut off.
^ Fuel pump relay ground path is broken by the ECU which causes the fuel pump operation to cease.
^ The ECU positions the idle speed stepper motor in anticipation of the next start-up and de-energizes the B + latch relay.