COMPUTERIZED ENGINE CONTROLS (GASOLINE) DESCRIPTION
The computerized engine control system monitors and controls a variety of engine/vehicle functions. The computerized engine control system is primarily an emission control system which is designed to maintain a 14.7:1 air/fuel ratio under most operating conditions. When the ideal air/fuel ratio is maintained, the 3-way catalytic converter can control oxides of nitrogen (NOx), hydrocarbon (HC) and carbon monoxide (CO) emissions.
The computerized engine control system consists of the following sub-systems: Electronic Control Module (ECM), Input Devices (sensor and switch input signals) and Output Signals.
ELECTRONIC CONTROL MODULE (ECM) DESCRIPTION
ECM is located in passenger compartment. For exact location of ECM, see ECM LOCATION in appropriate SELF-DIAGNOSTICS article. The ECM consists of the Arithmetic Logic Unit (ALU), Central Processing Unit (CPU), power supply and system memories.
The ECM has a "learning" ability which allows it to make minor corrections for fuel system variations. If battery power is interrupted, a vehicle performance change may be noticed. This will correct itself and normal performance will return if vehicle is allowed to "relearn" optimum control conditions. This is accomplished by driving vehicle at normal operating temperature, under part throttle, moderate acceleration and idle conditions.
ESC DETONATION RETARD OPERATION
In conjunction with the HEI-EST system, an Electronic Spark Control (ESC) retard system is used on all except 2.5L engine. System consists of the following: a detonation (knock) sensor, a high energy ignition system, an ESC controller, and the ECM.
When detonation (engine knock) occurs, detonation sensor produces a low voltage AC signal. This signal goes to the ESC controller. Controller supplies the ECM with a 12-volt signal. When detonation occurs, controller grounds the 12-volt signal to the ECM, pulling the signal down to zero volts. The ECM interprets this as a need to retard timing. The ECM then retards spark timing until the ESC controller returns the 12-volt signal.
If ESC signal wire were to become open or grounded, ECM would continuously provide full ignition timing retard. A malfunction in the ESC circuit should set a related trouble code. If a code is not present and ESC system is suspected as the cause of driveability problems, perform functional check of ESC system. See SYSTEM/COMP TESTS article.