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Throttle Position Sensor

Throttle Position sensor

A throttle position sensor (TPS) is a sensor used to monitor the position of the throttle in an internal combustion engine. The sensor is usually located on the butterfly spindle so that it can directly monitor the position of the throttle valve butterfly.

The sensor is usually a potentiometer, and therefore provides a variable resistance dependent upon the position of the valve (and hence throttle position).

The sensor signal is used by the engine control unit (ECU) as an input to its control system. The ignition timing and fuel injection timing (and potentially other parameters) are altered depending upon the position of the throttle, and also depending on the rate of change of that position. For example, in fuel injected engines, in order to avoid stalling, extra fuel may be injected if the throttle is opened rapidly (mimicking the accelerator pump of carburetor systems).

More advanced forms of the sensor are also used, for example an extra closed throttle position sensor (CTPS) may be employed to indicate that the throttle is completely closed.

Some ECUs also control the throttle position and if that is done the position sensor is utilized in a feedback loop to enable that control.

Related to the TPS are accelerator pedal sensors, which often include a wide open throttle (WOT) sensor. The accelerator pedal sensors are used in "drive by wire" systems, and the most common use of a wide open throttle sensor is for the kick down function on automatic transmissions.

Modern day sensors are Non Contact type, wherein a Magnet and a Hall Sensor is used. In the potentiometer type sensors, two metal parts are in contact with each other, while the butterfly valve is turned from zero to WOT, there is a change in the resistance and this change in resistance is given as the input to the ECU.

Non Contact type TPS work on the principle of Hall Effect, wherein the magnet is the dynamic part which mounted on the butterfly valve spindle and the hall sensor is mounted with the body and is stationary. When the magnet mounted on the spindle which is rotated from zero to WOT, there is a change in the magnetic field for the hall sensor. The change in the magnetic field is sensed by the hall sensor and the hall voltage generated is given as the input to the ECU. Normally a two pole magnet is used for TPS and the magnet may be of diametrical type or Ring type or segment type; however the magnet is defined to have a certain magnetic field.


Inconsistent Idle


The first and most telling symptom of a failing TPS is an idle that rises and falls sporadically or maintains a very high or low RPM. This condition can be extreme enough to cause stalling or to cause the tires to chirp when the transmission is engaged. It can be difficult to tell the difference between a failing TPS and a malfunctioning or clogged idle air control (IAC) valve, so electrical testing and cleaning of the IAC is recommended.


Engine Stalling


As mentioned, a failing TPS can result in random engine stalling at idle, but this can also happen under driving conditions. Most often this will occur under hard braking, since the computer assumes normal operation and continues to deliver cruise-amounts of fuel when the throttle butterfly is shut. If deceleration is slow enough, the computer may be able to maintain proper air-fuel ratio by using data from the oxygen sensor(s), but efficacy of this approach varies by vehicle.


Erratic Shifting


Transmissions are programmed to increase shift firmness and the RPM at which the transmission shifts according to acceleration demands. A car that shifts very hard and at high RPM under normal driving may be experiencing a TPS failure, as may one that accelerates sluggishly and shifts low when the gas pedal is floored.


Maladjusted TPS


If GM TPS sensors have any inherent flaws, it is a tendency to slip out of adjustment. Though this can happen to any car, many GM owners have reported such a failure. The TPS must be adjusted (rotated) so that it reads idle and full-throttle positions properly, or it may appear to be malfunctioning. Try adjusting the TPS before replacement to save a few.