What does the EEC power relay do on a 1990 F250 Ford truck?

On a 1990 Ford F250, the EEC (Electronic Engine Control) power relay provides power to the Powertrain Control Module (PCM), often referred to as the EEC-IV module in those years. This module controls various engine functions, including fuel injection, ignition timing, and other crucial aspects of engine operation. Without power from this relay, the engine won't run.