Throttle Position Sensor

A throttle position sensor photo.

In order for the engine control unit to tell what is going on with the throttle body, a throttle position sensor is attached to the butterfly valve shaft. As the throttle opens, the sensor, a variable resistor, allows varying amounts of electricity to flow back to the computer. This information is vital in automatic transmission vehicles, which tries to determine the driver's intentions; suddenly stepping on the throttle will cause some automatic transmission vehicles to shift down a gear, assuming the driver needs the speed to pass a car or avoid a collision. A slow increase in throttle tells the computer speed isn't urgent, and stays in the same gear, or shifts early for fuel economy reasons.