A Throttle Body Injection (TBI) system is used on all 1989-91, as well as some 1992 model engines. The TBI system is controlled by a pre-programmed digital computer known as the Engine Control Module (ECM). The ECM controls ignition timing, air/fuel ratio, emission control devices, charging system and idle speed. The ECM constantly varies timing, fuel delivery and idle speed to meet changing engine operating conditions.
Various sensors provide the input necessary for the ECM to correctly regulate the fuel flow at the fuel injector. These include the manifold absolute pressure, throttle position, oxygen sensor, coolant temperature, charge temperature, vehicle speed (distance) sensors and throttle body temperature. In addition to the sensors, various switches also provide important information. These include the neutral safety, heated backlite, air conditioning, air conditioning clutch switches, and an electronic idle switch.
All inputs to the ECM are converted into signals which are used to calculate and adjust the fuel flow at the injector or ignition timing or both. The ECM accomplishes this by sending signals to the power module.
The ECM tests many of its own input and output circuits. If a fault is found in a major system this information is stored in the ECM as a Diagnostic Trouble Code (DTC). Information on this fault can be displayed to a technician by means of the grounding a terminal and reading the check engine lamp flashes or by connecting a scan tool and reading the DTCs ( see Driveability & Emission Controls for a more complete procedure).