An ECM has several quad drivers. They are an integrated circuit that essentially acts as four small relays to control the solenoids and stepper-motors that the computer turns on and off--EGR solenoid, IAC motor, AIR solenoid, fuel injectors, etc. If the wire harness or the solenoid/motor shorts to ground, it'll pull too much current through one or more sections of the quad driver; damaging it.
In the TBI era, sometimes the quad drivers were "protected" and sometimes they weren't--depending on the application the computer was used on. I have no idea why GM wouldn't protect the quad drivers other than saving a few cents per car. Seems to me the V8 computers were not protected, but the Iron Duke four-poppers were. Go figure.
In the TBI era, sometimes the quad drivers were "protected" and sometimes they weren't--depending on the application the computer was used on. I have no idea why GM wouldn't protect the quad drivers other than saving a few cents per car. Seems to me the V8 computers were not protected, but the Iron Duke four-poppers were. Go figure.