THROTTLE POSITION SENSOR (TPS)

General description

      A throttle position sensor (TPS) is used to monitor the throttle valve position in internal combustion engines. TPS is usually located on the throttle valve spindle so that it can directly monitor its position.
      The TPS sensor is a potentiometer, providing a variable resistance depending on the position of the throttle valve (and hence throttle position sensor).
      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 on the position of the throttle valve, and also depending on the rate of change of that position.
      Some throttle valve modifications have built-in end switches. They are closed throttle position sensor (CTPS) and often include a wide open throttle (WOT) sensor which is mounted on the accelerator pedal.
      Throttle position signal can be produced from a simple contact (TS) or a potentiometer (TPS), and also of combined TS/TPS sensor. Some systems use both types as separate elements.

Appearance
Fig. 1 shows a typical TPS.


Fig. 1

Types of TPS sensors
According to its construction are:

  • with end switches
  • potentiometer type
  • combination of both above

Working principle of the TPS
Throttle potentiometer sensor (TPS)
      TPS gives to the onboard controller information about the idling, deceleration, rate of acceleration and the fully open throttle valve state (WOT). TPS is a three wire potentiometer. On the first wire a voltage of +5V is applied to the sensor resistive layer and the second wire closes the sensor circuit to the ground. The third wire is connected to the potentiometer wiper, thereby the changing the resistance and hence the voltage of the signal returned to the onboard computer.
      Based on the received voltage, the onboard computer can calculate the idling (below 0.7V), the full load (about 4.5V), and the opening speed of the throttle valve. At full load state the onboard computer provides further enrichment of the fuel mixture. In deceleration mode (closed throttle valve and engine speed above certain RPM) the onboard computer shuts-off the fuel injection. Fuel supply is resumed after the engine speed reaches its idle value or when the throttle valve is open. Some cars allow adjustment of these values.

THROTTLE SENSOR (TS)

      TS informs onboard computer about the idling state. Usually it has a second contact for the fully open throttle valve state (WOT). In most cases the onboard computer provides extra enrichment of the fuel mixture in the idle state and in the fully open throttle valve state. Each TS contact has two positions - open and closed - by which the onboard computer detects three different states of engine:

  • The throttle valve is closed (the idle speed contact is closed)
  • The throttle valve is opened (the idle speed contact and WOT are open)
  • The throttle valve is fully open (the idle speed contact is open and the WOT contact is closed)

Some cars allow regulation of TS.

Procedure for verification of TPS functionality 
 THROTTLE SENSOR (TS)  

NOTE: The following operations are applied in a typical three state throttle valve switch. In some cases the idle switch and the full load switch can be connected separately. Also there are separate idle and full load switches. In some Rover models the throttle valve switch is located on the accelerator pedal. Regardless of the location of the switch, the verification procedure is performed similarly for all types of sensors.
-- Check TS voltage

  • The three wires entering the throttle switch coupling are grounding, idle mode signal and full load signal.
  • Connect the negative terminal of a voltmeter to the engine ground.
  • Determine the ground, idling and full load terminals of the sensor.
  • Turn on the ignition, but do not start the engine.
  • Connect the positive terminal of a voltmeter to the wire, connected to the idle signal contact of the throttle switch.
  • Voltmeter must read voltage of 0V. If it reads voltage 5.0V, loosen the screws and adjust the switch so that the voltmeter reads zero voltage.

NOTE: In some cars the throttle valve switch could not be adjusted.
-- Check TS resistance

  • Disconnected the throttle connector.
  • Connect ohmmeter between the ground and the idle mode terminals.
  • When the throttle valve switch is turned on, the ohmmeter should read resistance around 0 Ω.
  • Slowly open the throttle valve and when the switch opens the resistance should be equal to infinity and remain the same even if the throttle is fully opened.
  • Connect ohmmeter between the ground and the full load mode terminals.
  • When the throttle switch is closed, ohmmeter must read circuit break (infinite resistance).
  • Slowly open the throttle valve. When the switch opens it should click and the resistance should remain equal to infinity. When the throttle valve opening angle becomes greater than 72 degrees, the resistance will be equal to 0 Ω.
  • If the switch does not work in the described way, and the turning on and turning off cannot be regulated by bending the levers driving the throttle valve, most likely the throttle switch is defective.

-- Possible damages in TS:
      1) Can not get voltage 0V (closed throttle valve)

  • Check the throttle valve state.
  • Check the switch connection to the ground.
  • Take measurements of the switch resistance.
  • If the voltage is normal with closed throttle valve, sharply open the throttle valve, switch must click, and the voltage should rise up to 5.0V.

      2) Voltage is low or missing (the throttle valve is opened)

  • Check whether the idle mode switch terminal is not connected to ground.
  • Disconnect the switch connector and check for presence of 5.0V voltage in the idle mode contact. If there is no voltage, carry out the following checks:
    • check the integrity of the idle mode signal wire between the switch and the onboard controller;
    • If the switch wires are good, check all supply and ground connections of the onboard controller. If they are correct, the fault may be in the onboard controller.

      3) The voltage is normal (throttle valve is opened)

  • Connect the positive terminal of a voltmeter to the wire, connected to the full load mode switch contact.
  • When the throttle valve is in idle state or slightly open, the voltmeter should read voltage of 5.0V.

      4) Voltage is low or missing (the throttle valve is closed or slightly open)

  • Check the ground connection.
  • Check whether the full load mode contact of the throttle switch is not connected to ground.
  • Disconnect the switch connector. Check the presence 5.0V voltage in the full load mode contact of the connector. If there is no voltage carry out the following checks:
    • check the integrity of the idle mode signal wire between the switch and the onboard controller;
    • If the switch wires are good, check all supply and ground connections of the onboard controller. If they are correct, the fault may be in the onboard controller.

      5) The voltage is normal (the throttle valve is closed or slightly open)

  • Fully open the throttle valve. When the opening angle becomes more than 72 degrees, the voltage should drop to zero. If the voltage does not drop most likely the throttle switch is malfunctioning.

— Throttle Position Sensor (TPS) —

-- Check TPS voltage

  1. Connect the negative terminal of a voltmeter to the engine ground.
  2. Determine the ground, idling and full load terminals.

NOTE: Most throttle potentiometers have three terminals, but some may have and extra contacts, which function as throttle switches. If there is such a contact, it must be checked as described above for throttle switch.

  1. Connect the voltmeter positive terminal to the wire connected to the contact signal from the throttle valve potentiometer.
  2. Turn on the ignition, but do not start the engine. In most systems the voltage reading should be less than 0.7V.
  3. Open and close the throttle valve several times, by checking the smoothness of the rising voltage.

-- Check the resistance of the TPS

  1. Connect an ohmmeter between the potentiometer wiper and the reference voltage terminal or between the potentiometer wiper and the ground.
  2. Open and close the throttle valve several times and check the smoothness of resistance variation. If the potentiometer resistance is infinite or zero, this indicates a malfunction.
  3. Exact values of the throttle potentiometer resistance are not shown. One of the reasons is that many manufacturers do not publish control data. The fact that the resistance of the potentiometer is kept within limits is less important than the proper operation of the potentiometer, i.e. resistance smooth change when moving the throttle valve.
  4. Connect an ohmmeter between the ground and the reference voltage terminals. The resistance must be constant.
  5. If the resistance is infinite or is low, the potentiometer must be replaced.

-- Possible damages in TSP
      Chaotic output signal

  • Chaotic output signal is observed when the voltage signal changes rapidly, drops to zero and disappears.
  • When the throttle valve potentiometer output signal is chaotic, a defective potentiometer is usually the reason for this. In this case, the potentiometer must be replaced.

      Missing voltage signal

  • Check the presence of reference voltage (5.0V) on the throttle potentiometer power terminal.
  • Check the condition of the potentiometer grounding contact.
  • Check the signal wire connecting the throttle potentiometer to the onboard controller.
  • If the power supply and ground are bad, check the wires integrity between the potentiometer and the onboard controller.
  • If the potentiometer wires are good, check all supply and ground connections of the onboard controller. If they are correct, most likely reason is the onboard controller itself.

      The output signal or the reference voltage is equal to the battery voltage

  • Check for short-circuit in the wire connected to the positive terminal of the car battery or the power supply wire.

Check the throttle valve potentiometer by using oscilloscope

  • Best way to obtain changes of potentiometer signal is using an oscilloscope.
  • Connect the oscilloscope active probe to the potentiometer signal terminal, and the GND probe - to the engine ground.
  • Start the engine.
  • Smoothly operate the accelerator pedal and then sharply release the pedal. You must see a signal as in fig. 2.


Fig. 2

This is a properly working throttle valve potentiometer waveform - a smooth voltage rising and fast collapse.
Figures 3, 4 and 5 indicate defective potentiometers waveforms.


Fig. 3


Fig. 4


Fig. 5

      You can clearly see the signal cuts, which means that there are breaks in throttle valve potentiometer resistive layer and it has to be replaced.

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