 It tells the engine's computer to slow the speed of the motors to prevent wheel slip.The GP49 is equipped with a 12-645F3B engine with an AR15 alternator rated at 4680 amperes and has four D87 traction motors.Externally the GP49 looks like a GP50 but has two 48" fans above the radiator instead of three as on the GP50, and the engine-room has eight access doors on each side under the Dynamic Brake blister for engine and turbocharger maintenance, while the GP50 has ten.