General System Description

The Electronic Control Module (ECM), located in the passenger compartment, is the control center of the fuel injection system. The ECM continually monitors and processes the input information and generates output commands to the various systems affecting vehicle performance.
The Throttle Body Injection system (TBI) uses three basic types of electronic control modules. Most ECM's have a removable calibration unit called a Programmable Read Only Memory (PROM) that enables the ECM to recognize and adjust for vehicle variations (vehicle weight, axle ratio, etc.). There are specific ECM/PROM combinations for each specific vehicle which are not interchangeable. Some PROM equipped ECM's also have a CALPAK unit that allows fuel delivery if other parts of the ECM are damaged. Some vehicles use a MEM-CAL unit that functions as a combined PROM/CALPAK assembly. The GMP4 ECM system used on 4-121 and 4-151 engines is new for 1987-88 but consists of the two basic parts described previously: a controller (an ECM without a calibrator) and a calibrator (PROM).
The ECM also performs the diagnostic function of the fuel injection system. When the ECM senses an operational problem, it will illuminate the ``Check Engine'' or ``Service Engine Soon'' light and store the appropriate code(s) to identify the problem area.