Electric Fan Relay Wiring Diagram Sketch - 1991 Ford Bronco (On/Off/Auto Switch)

This is a hand-drawn wiring diagram sketched in ink on lined notebook paper, uploaded to the gallery of a 1991 Ford Bronco. It illustrates a proposed electric cooling fan control circuit. The fan (drawn at upper left) has one lead going to ground and another lead feeding into a primary relay. That primary relay is grounded and receives power from the positive battery terminal ('Pos Battery'). The relay's control (trigger) side splits into two paths: one goes to a temperature 'Sensor' and one goes to a three-position switch ('3 Pos Switch') labeled with positions On, Off, and Auto, which is fed from the battery. The sensor's output routes through a second, smaller relay that is grounded and triggered by a 12V key-on (ignition-switched) source ('12 V Key'), so the automatic fan operation only functions with the ignition on. A reader can learn the general architecture of a manually overridable, thermostatically controlled electric fan setup: battery power through a main relay to the fan, with the relay triggered either manually (On), disabled (Off), or automatically (Auto) via a temperature sensor and an ignition-gated secondary relay. No wire gauges, wire colors, fuse ratings, or part numbers are given.