Throttle Modulator: Operation
Two types of speed sensing units are used; on heavy duty vehicles an electronic speed sensor unit controls solenoid vacuum valve, on light duty vehicles a speedometer driven speed sensor controls vacuum to throttle modulator (see Fig 1). Engine speed sensors are calibrated to activate or deactivate throttle modulator at approximately 1850 RPM (heavy duty) or 24 MPH (light duty). When engine speed exceeds cut-in speed, vacuum is applied to throttle modulator. Throttle modulator then holds throttle lever in high idle position. When engine speed drops below cut-in speed, vacuum to throttle modulator vacuum diaphragm is allowed to bleed off. Throttle modulator shaft retracts and engine returns to curb idle.