Throttle Body Fuel Injection

The Electronic Control Module (ECM), located in the passenger compartment, continually monitors and processes sensor and switch inputs and uses these to generate output commands to the various systems affecting vehicle performance.

The Throttle Body Injection (TBI) system uses three types of ECMs. Most ECMs have a removable calibration unit called a Programmable Read Only Memory (PROM) that enables the ECM to recognize variations in vehicle weight, axle ratio, etc..

- There are specific ECM/PROM combinations for each vehicle which are not interchangeable.

- Some PROM equipped ECMs 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 (ECM) and a calibrator (PROM).

The ECM also compares system parameters to a programmed set of limits. If the ECM senses a parameter outside of limits, it illuminates the "Check Engine" or "Service Engine Soon" light and stores the appropriate code(s) to identify the .