A dual mode electrically operated servo-actuator with a motorized speed reducer rotating a pair of spaced discs each having a pair of common spiral cam tracks. A sliding carrier block disposed between the discs has cam followers separately engaging the track. A bell crank is pivoted on the carrier block for attachment to a load function. In the rotary mid-position, the carrier block is centered and the servo motor is not used to enable a fixed position of the bell crank pivot. In a first mode function the motor is driven to rotate the cam for modulation about positions between the mid-position and fully clockwise position. In a second mode function the motor is driven to rotate the cam for modulation about positions between the mid-position and the fully counter-clockwise position. In a throttle actuator application the first mode is cruise control and the second mode is traction control, with the mid-position comprising normal driver operation of the throttle.