The back end of main generators and traction alternators on EMD road units have a separate alternator that provides three-phase AC for the cooling fans. EMD cooling fans started out as 36", and 48" showed up in late GP9s. A mentioned, a combination of 36" and 48" fans were used on GP/SD/DD35 units. There are many variations in fans, horsepower of the motor and number of blades in addition to the obvious size and shape differences. By 50 series and late Dash-2 production, a two speed fan became an option. Fan application varies according to the amount of horsepower of the engine, and the thickness of the radiator cores also vary by model.

Each fan has its own temperature switch, and as engine coolant temperature increases typically another fan turns on every 5-10 degrees until all are running. Most locos are wired so the fans don't turn off until the lowest temperature switch opens.

The radiator shutters will normally open when the first fan turns on, and close when all the fans are off.

There are exceptions. GP7s have a switch that turns off the #1 fan in winter, and the front half of the radiator shutters are manually opened in summer and closed in winter because of their cab heater arrangement. GP15s have a split circuit that initially open half the shutters. Modern units and retrofit rebuilds have electronic fan controllers that control the cooling fans.