Main Relay (Computer/Fuel System): Description and Operation

The Engine Control Assembly (ECA) receives its operating voltage from the Electronic Engine Control (EEC) power relay.
When the ignition switch is turned ON, the normally OPEN contact points of the relay CLOSE and supply power to the ECA. The control coil circuit of the relay contains a diode, wired in series, to prevent relay operation if the battery polarity is reversed, preventing damage to the ECA.