CHAPTER 21 Throttle Position (TP) Sensors

1. CHAPTER 21 Throttle Position (TP) Sensors

2. OBJECTIVES After studying Chapter 21, the reader will be able to: • Prepare for ASE Engine Performance (A8) certification test content area “E” (Computerized Engine Controls Diagnosis and Repair). • Discuss how throttle position sensors work. • List the methods that can be used to test TP sensors. • Describe the symptoms of a failed TP sensor. • List how the operation of the TP sensor affects vehicle operation. • Discuss TP sensor rationality tests.

3. Potentiometer Skewed Throttle position (TP) sensor KEY TERMS

4. FIGURE 21–1 A typical TP sensor mounted on the throttle plate of this port-injected engine. THROTTLE POSITION SENSOR CONSTRUCTION • Most computer-equipped engines use a throttle position (TP) sensor to signal to the computer the position of the throttle. • The TP sensor consists of a potentiometer, a type of variable resistor.

5. THROTTLE POSITION SENSOR CONSTRUCTIONPOTENTIOMETERS • A potentiometer is a variable-resistance sensor with three terminals. • One end of the resistor receives reference voltage, while the other end is grounded. • The third terminal is attached to a movable contact that slides across the resistor to vary its resistance. • Depending on whether the contact is near the supply end or the ground end of the resistor, return voltage is high or low.

6. FIGURE 21–2 The signal voltage from a throttle position increases as the throttle is opened because the wiper arm is closer to the 5-volt reference. At idle, the resistance of the sensor winding effectively reduces the signal voltage output to the computer. THROTTLE POSITION SENSOR CONSTRUCTIONPOTENTIOMETERS

7. TP SENSOR COMPUTER INPUT FUNCTIONS • The computer senses any change in throttle position and changes the fuel mixture and ignition timing. • The throttle position (TP) sensor used on fuel-injected vehicles acts as an “electronic accelerator pump.” • The PCM supplies the TP sensor with a regulated voltage that ranges from 4.8 to 5.1 volts. • This reference voltage is usually referred to as a 5-volt reference or “Vref.”

8. PCM USES FOR THE TP SENSOR • The TP sensor is used by the Powertrain Control Module (PCM) for the following reasons. • Clear Flood Mode • Torque Converter Clutch Engagement And Release • Rationality Testing For Map And Maf Sensors • Automatic Transmission Shift Points • Target Idle Speed (Idle Control Strategy) • Air-conditioning Compressor Operation • Backs Up Other Sensors

9. TESTING THE THROTTLE POSITION SENSOR • A TP sensor can be tested using one or more of the following tools: • A digital voltmeter with three test leads connected in series between the sensor and the wiring harness connector or back probing using T-pins or other recommended tool that will not cause harm to the connector or wiring. • A scan tool or a specific tool recommended by the vehicle manufacturer. • A breakout box that is connected in series between the computer and the wiring harness connector(s). • An oscilloscope.

10. FIGURE 21–3 A meter lead connected to a T-pin that was gently pushed along the signal wire of the TP sensor until the point of the pin touched the metal terminal inside the plastic connector. TESTING THE THROTTLE POSITION SENSOR

11. FIGURE 21–4 A typical waveform of a TP sensor signal as recorded on a DSO when the accelerator pedal was depressed with the ignition switch on (engine off). Clean transitions and the lack of any glitches in this waveform indicate a good sensor. (Courtesy of Fluke Corporation) TESTING THE THROTTLE POSITION SENSOR

12. Check Power and Ground Before Condemning a Bad Sensor • Most engine sensors use a 5-volt reference and a ground. If the 5 volts to the sensor is too high (shorted to voltage) or too low (high resistance), then the sensor output will be skewed or out of range. Before replacing the sensor that did not read correctly, measure both the 5-volt reference and ground. To measure the ground, simply turn the ignition on (engine off) and touch one test lead of a DMM set to read DC volts to the sensor ground and the other to the negative terminal of the battery. Any reading higher than 0.2 volt (200 mV) represents a poor ground.

13. FIGURE 21–5 Checking the 5-volt reference from the computer being applied to the TP sensor with the ignition switch on (engine off). Check Power and Ground Before Condemning a Bad Sensor

14. FIGURE 21–6 Checking the voltage drop between the TP sensor ground and a good engine ground with the ignition on (engine off). A reading of greater than 0.2 volt (200 mV) represents a bad computer ground. Check Power and Ground Before Condemning a Bad Sensor

15. TESTING A TP SENSOR USING THE MIN/MAX FUNCTION • Many digital multimeters are capable of recording voltage readings over time and then displaying the minimum, maximum, and average readings. • To perform a MIN/MAX test of the TP sensor, manually set the meter to read higher than 4 volts. • STEP 1 Connect the red meter lead to the signal wire and the black meter lead to a good ground on the ground return wire at the TP sensor. • STEP 2 With the ignition on, engine off, slowly depress and release the accelerator pedal from inside the vehicle. • STEP 3 Check the minimum and maximum voltage reading on the meter display. Any 0- or 5-volt reading would indicate a fault or short in the TP sensor.

16. TESTING THE TP SENSOR USING A SCAN TOOL • A scan tool can be used to check for proper operation of the throttle position sensor using the following steps. • STEP 1 With the key on, engine off, the TP sensor voltage display should be about 0.5 volt, but can vary from as low as 0.3 volt to as high as 1.2 volts. • STEP 2 Check the scan tool display for the percentage of throttle opening. The reading should be zero and gradually increase in percentage as the throttle is depressed.

17. TESTING THE TP SENSOR USING A SCAN TOOL • STEP 3 The idle air control (IAC) counts should increase as the throttle is opened and decrease as the throttle is closed. Start the engine and observe the IAC counts as the throttle is depressed. • STEP 4 Start the engine and observe the TP sensor reading. Use a wedge or thin object to increase the throttle opening slightly. The throttle percentage reading should increase. Shut off and restart the engine. If the percentage of throttle opening returns to 0%, the PCM determines that the increased throttle opening is now the new minimum and resets the idle position of the TP sensor. Remove the wedge and cycle the ignition key. The throttle position sensor should again read zero percentage.

18. TP SENSOR DIAGNOSTIC TROUBLE CODES • The diagnostic trouble codes (DTCs) associated with the throttle position sensor include the following.

19. SUMMARY • A throttle position (TP) sensor is a three-wire variable resistor called a potentiometer. • The three wires on the TP sensor include a 5-volt reference voltage from the PCM, plus the signal wire to the PCM, and a ground, which also goes to the PCM. • The TP sensor is used by the PCM for clear flood mode, torque converter engagement and release, and automotive transmission shift points, as well as for rationality testing for the MAP and MAF sensors. • The TP sensor signal voltage should be about 0.5 volt at idle and increase to about 4.5 volts at wide-open throttle (WOT). • A TP sensor can be tested using a digital multimeter, a digital storage oscilloscope (DSO), or a scan tool.

20. REVIEW QUESTIONS • What is the purpose of each of the three wires on a typical TP sensor? • What all does thePCMdo with the TP sensor signal voltage? • What is the procedure to follow when checking the 5-volt reference and TP sensor ground? • How can a TP sensor be diagnosed using a scan tool?

21. CHAPTER QUIZ 1. Which sensor is generally considered to be the electronic accelerator pump of a fuel-injected engine? • O2S • ECT sensor • Engine MAP sensor • TP sensor

22. CHAPTER QUIZ 2. Typical TP sensor voltage at idle is about ________. • 2.50 to 2.80 volts • 0.5 volt or 10% of WOT TP sensor voltage • 1.5 to 2.8 volts • 13.5 to 15.0 volts

23. CHAPTER QUIZ 3. A TP sensor is what type of sensor? • Rheostat • Voltage generating • Potentiometer • Piezoelectric

24. CHAPTER QUIZ 4. Most TP sensors have how many wires? • 1 • 2 • 3 • 4

25. CHAPTER QUIZ 5. Which sensor does the TP sensor back up if the PCM determines that a failure has occurred? • Oxygen sensor • MAF sensor • MAP sensor • Either b or c

26. CHAPTER QUIZ 6. Which wire on a TP sensor should be back-probed to check the voltage signal to the PCM? • 5-volt reference (Vref) • Signal • Ground • Meter should be connected between the 5-volt reference and the ground

27. CHAPTER QUIZ 7. After a TP sensor has been tested using the MIN/MAX function on a DMM, a reading of zero volts is displayed. What does this reading indicate? • The TP sensor is open at one point during the test. • The TP sensor is shorted. • The TP sensor signal is shorted to 5-volt reference. • Both b and c are possible.

28. CHAPTER QUIZ 8. After a TP sensor has been tested using the MIN/MAX function on a DMM, a reading of 5 volts is displayed. What does this reading indicate? • The TP sensor is open at one point during the test. • The TP sensor is shorted. • The TP sensor signal is shorted to 5-volt reference. • Both b and c are possible.

29. CHAPTER QUIZ 9. A technician attaches one lead of a digital voltmeter to the ground terminal of the TP sensor and the other meter lead to the negative terminal of the battery. The ignition is switched to on, engine off and the meter displays 37.3 mV. Technician A says that this is the signal voltage and is a little low. Technician B says that the TP sensor ground circuit has excessive resistance. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

30. CHAPTER QUIZ 10. A P0122 DTC is retrieved using a scan tool. This DTC means ________. • The TP sensor voltage is low • The TP sensor could be shorted-to-ground • The TP sensor signal circuit could be shorted-to-ground • All of the above are correct.