2. OBJECTIVES After studying Chapter 12, 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. THROTTLE POSITION SENSORS
Most computer-equipped engines use a throttle position (TP) sensor to signal to the computer the position of the throttle.
FIGURE 12-1 A typical TP sensor mounted on the throttle plate of this port-injected engine.

4. THROTTLE POSITION SENSORS
Potentiometers
A potentiometer is a variable-resistance sensor with three terminals.
FIGURE 12-2 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.

5. 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.

6. 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

7. 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.
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.

8. TESTING THE THROTTLE POSITION SENSOR
FIGURE 12-3 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.

9. TESTING THE THROTTLE POSITION SENSOR
The procedure for testing the sensor using a digital multimeter is as follows:
Turn the ignition switch on (engine off).
Measure the voltage between the signal wire and ground (reference low) wire. The voltage should be about 0.5 volt.
With the engine still not running (but with the ignition still on), slowly increase the throttle opening. The voltage signal from the TP sensor should also increase.
With the voltmeter still connected, slowly return the throttle down to the idle position. The voltage from the TP sensor should also decrease evenly on the return to idle.

10. TESTING THE THROTTLE POSITION SENSOR
FIGURE 12-4 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)

11. CHECK POWER AND GROUND BEFORE CONDEMNING A BAD SENSOR
FIGURE 12-5 Checking the 5-volt reference from the computer being applied to the TP sensor with the ignition switch on (engine off).

12. CHECK POWER AND GROUND BEFORE CONDEMNING A BAD SENSOR
FIGURE 12-6 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.

13. 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.
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.
With the ignition on, engine off, slowly depress and release the accelerator pedal from inside the vehicle.
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.

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

15. THROTTLE POSITION SENSOR DIAGNOSIS

16. THROTTLE POSITION SENSOR DIAGNOSIS

17. THROTTLE POSITION SENSOR DIAGNOSIS

18. THROTTLE POSITION SENSOR DIAGNOSIS

19. THROTTLE POSITION SENSOR DIAGNOSIS

20. THROTTLE POSITION SENSOR DIAGNOSIS

21. THROTTLE POSITION SENSOR DIAGNOSIS

22. THROTTLE POSITION SENSOR DIAGNOSIS

23. THROTTLE POSITION SENSOR DIAGNOSIS

24. 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 sensor.
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.

25. REVIEW QUESTIONS
What is the purpose of each of the three wires on a typical TP sensor?
What all does the PCM do 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?

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

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

28. 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

29. 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

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

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

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

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

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

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

36. 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

37. 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

38. 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.

39. 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.

40. 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.

41. 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.

42. 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

43. 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

44. 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.

45. 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.