10 The voltage regulator plug has four terminals—I, A, S, and F from left to right. “I” stands alone on the left, which only applies to Mustangs with charge indicator (ALT) lights. It has no application if you have an ammeter. The “A” terminal (yellow wires) is battery power for field supply voltage, which is live all the time (test light is on). The “S” field relay terminal is energized only when the ignition switch is turned on. “F” is the field coil terminal.