Engine Control Module: Description and Operation
The engine controller used on this vehicle is referred to as a Single Board Engine Controller (SBEC). This controller regulates the following:- Ignition Timing
- Air/Fuel Ratio
- Emission Control Devices
- Cooling Fan
- Charging System
- Idle Speed
The SBEC receives a variety of input signals from different switches and sensors. From these inputs, the SBEC determines the injector pulse width, ignition coil dwell, idle speed, spark advance, canister purge, cooling fan operation, and alternator charge rate.
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 (Optical) 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.
The SBEC constantly tests many of its input and output circuits to verify that all of the circuits are operating within their parameters. If a fault is found in one of its major systems, it is stored in its memory. Information concerning any faults that are stored can be displayed to the technician using an appropriate scan tool or the instrument panel's CHECK ENGINE lamp. For more information concerning on-board diagnosis and extracting fault codes refer to DIAGNOSIS AND TESTING PROCEDURES/TESTING PROCEDURES.
The SBEC contains a voltage regulator and converter that converts an input voltage of 12 volts to a regulated output voltage of 8 volts and 5 volts. The 8 volt output is sent to power the distributor pick-up and the 5 volt output is used to provide power to the Throttle Position Sensor (TPS), the Manifold Absolute Pressure (MAP) Sensor, and logic circuits.
LIMP IN MODE
This mode is a mode that is entered by the SBEC if a component or a system should fail. If the SBEC senses no data or incorrect data from the Throttle Position Sensor (TPS), Manifold Absolute Pressure (MAP) Sensor, or Coolant Temperature Sensor, then this mode is entered and the engine controller uses standard information instead of using information from its input and output devices. When this mode is entered the Check Engine Lamp on the instrument panel is illuminated. The purpose of this mode is to allow the driver to operate the vehicle until the problem is fixed.