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by Bob O’Neill and Bob Lincoln
The position of the throttle plate is used by the logic module for many engine functions. The position is ‘reported’ to the logic module by the ‘Throttle Position Sensor (TPS).
Throttle Position Sensor (TPS) is a potentiometer (variable resistor) that reports how far open the throttle is, to the computer. It converts electrical resistance (zero to 5,000 ohms scale, ±25%) to a voltage that the computer interprets when setting ignition timing, fuel mixture and idle speed. It's mounted on the end of the throttle arm that's opposite where the throttle cable hooks up, and has 3 wires (5V, signal and ground).
The TPS uses a three pin connection to the wiring harness. To detect the position of the throttle plate, 5 volts DC is applied to pin one, which is at one end of the potentiometer; the other end is grounded, and the wiper of the potentiometer reads the voltage resulting from the resistance.
When the logic module senses the throttle position is closed it can set the AIS motor to adjust the idle speed to the target idle speed. When it senses wide open throttle, the logic module exits closed loop and richens the fuel mixture for turbo engines and turns off the A/C compressor to avoid diverting needed power to accessories. This will provide more power at wide open throttle while protecting the engine at higher boost levels by providing more fuel.
While at partial throttle positions, the logic module uses the TPS sensor signals to improve reaction times of air/fuel mixture adjustments. On turbo engines, the information is used to help adjust the wastegate for smoother performance. If the throttle is changed very quickly, the logic module can richen the mixture momentarily which provides better engine performance. This behavior mimics the accelerator pump action in carburetor engines.
Should the signal to the computer from the TPS fall outside the range of 0.2v and 4.7v, fault code 24 is triggered, the power limited is lit, the system enters limp mode, and the MAP sensor signal is used as a proxy for throttle position. MAP sensor voltage less than 2.0v indicates to the logic module that the throttle is closed and a voltage above 2.0v is considered by the logic module as wide open throttle.
This sensor tends to get "rough spots" in the middle, which can cause problems that do not set a code; these can be tested by measuring its voltage as the TPS is slowly moved up and down. Sudden spikes or dropouts show a problem. (This is most easily tested on an analog voltmeter without a damped needle.)
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