Idle Speed/Throttle Actuator - Mechanical: Description and Operation

The Throttle Positioner (TP) system uses a diaphragm type actuator, connected to the throttle linkage, to prevent complete closing of the throttle during high vacuum conditions such as deceleration and/or at idle. Actuator operation is controlled by ported vacuum signals, modified in a variety of ways.




4A-C & 4A-LC ENGINES COROLLA FRONT WHEEL DRIVE

These models use a dual diaphragm throttle positioner in order to open the throttle slightly during deceleration and at idle when vehicle electrical loads cause idle speed to drop. Manifold vacuum is applied directly to the positioner outer port while ported vacuum is applied to the inner diaphragm chamber. Vacuum must be applied to both diaphragm chambers to retract the positioner and allow the throttle to close fully.

During normal idle conditions, vacuum is applied to both diaphragm chambers and the throttle positioner is retracted. When heavy electrical loads are placed on the alternator the Vacuum Switching Valve (VSV) circuit is energized, the VSV opens venting the outer diaphragm chamber to atmosphere and spring tension extends the positioner to the first step raising the idle speed. When electrical loads are decreased, the VSV is de-energized and vacuum retracts the positioner.

During normal running, vacuum in the inner diaphragm chamber is nearly equal to atmospheric pressure and spring tension extends the positioner. When the throttle is suddenly closed, the resulting high vacuum is delayed from reaching the inner chamber by a Vacuum Transmitting Valve (VTV). This delay allows the positioner to remain extended briefly, then slowly retract allowing the throttle to close.