Editing Throttle Position Sensor (section)

==What it does==

The TPS tells the engine ECU when to take over idle control.  When you take your foot off the accelerator, the throttle butterfly comes to the idle position, the TPS idle switch closes and the ECU starts operating the AAC valve to keep the engine idling properly.  This is the only influence the TPS has on the engine ECU.  If it is not closing properly, the engine will idle very poorly, or may not idle at all.

The TPS also has a switch that activates at about half throttle, and the TPS on an automatic car also has a potentiometer under the switch mechanism.  The additional switch and potentiometer are used exclusively by the automatic transmission ECU.