Throttle Return Control System: Operation

The 1975-78 TRC system is not controlled electrically. On engine deceleration, increased manifold vacuum opens the vacuum control valve. As vacuum control valve opens, vacuum is applied to throttle lever actuator opening carburetor throttle valve slightly. When manifold vacuum returns to normal, vacuum control valve closes allowing throttle lever actuator to retract.

On 1979 TRC system, manifold vacuum is routed through the normally closed solenoid vacuum valve to the throttle lever actuator. On vehicle deceleration, electronic speed sensor signals solenoid vacuum valve to open when engine speed is above a preset RPM. When solenoid vacuum valve opens, manifold vacuum is directed to throttle lever actuator and throttle is opened slightly. When engine speed drops below the preset RPM, solenoid valve closes, retracting throttle lever actuator and returning throttle to curb idle position.