OBJECTIVES

2. OBJECTIVES After studying Chapter 11, the reader will be able to: • Prepare for ASE Engine Performance (A8) certification test content area “E” (Computerized Engine Controls Diagnosis and Repair). • Explain the purpose and function of the ECT and IAT temperature sensors. • Describe how to test temperature sensors. • Discuss how automatic fluid temperature sensor valves can affect transmission operation.

3. ENGINE COOLANT TEMPERATURE SENSORS • Computer-equipped vehicles use an engine coolant temperature (ECT) sensor. • When the engine is cold, the fuel mixture must be richer to prevent stalling and engine stumble. FIGURE 11-1 A typical engine coolant temperature (ECT) sensor. ECT sensors are located near the thermostat housing on most engines.

4. ENGINE COOLANT TEMPERATURE SENSORS FIGURE 11-2 A typical ECT sensor temperature versus voltage curve.

5. STEPPED ECT CIRCUITS • Some vehicle manufacturers use a step-up resistor to effectively broaden the range of the ECT sensor. FIGURE 11-3 A typical two-step ECT circuit showing that when the coolant temperature is low, the PCM applies a 5-volt reference voltage to the ECT sensor through a higher resistance compared to when the temperature is higher.

6. STEPPED ECT CIRCUITS FIGURE 11-4 The transition between steps usually occurs at a temperature that would not interfere with cold engine starts or the cooling fan operation. In this example, the transition occurs when the sensor voltage is about 1 volt and rises to about 3.6 volts.

7. TESTING THE ENGINE COOLANT TEMPERATURE SENSOR • Testing the Engine Coolant Temperature by Visual Inspection • Properly filled cooling system • Proper pressure maintained by the radiator cap • Proper antifreeze-water mixture • Proper operation of the cooling fan • Testing the ECT Using a Multimeter • Testing the ECT Sensor Using a Scan Tool

8. TESTING THE ENGINE COOLANT TEMPERATURE SENSOR FIGURE 11-5 Measuring the resistance of the ECT sensor. The resistance measurement can then be compared with specifications. (Courtesy of Fluke Corporation)

9. TESTING THE ENGINE COOLANT TEMPERATURE SENSOR FIGURE 11-6 When the voltage drop reaches approximately 1.20 volts, the PCM turns on a transistor. The transistor connects a 1-kΩresistor in parallel with the 10-kΩresistor. Total circuit resistance now drops to around 909 ohms. This function allows the PCM to have full binary control at cold temperatures up to approximately 122°F, and a second full binary control at temperatures greater than 122°F.

10. TESTING THE ENGINE COOLANT TEMPERATURE SENSOR FIGURE 11-7 A chart showing the voltage decrease of the ECT sensor as the temperature increases from a cold start. The bumps at the bottom of the waveform represent temperature decreases when the thermostat opens and is controlling coolant temperature.

11. INTAKE AIR TEMPERATURE SENSOR • The intake air temperature (IAT) sensor is a negative temperature coefficient (NTC) thermistor that decreases in resistance as the temperature of the sensor increases. • The IAT sensor can be located in one of the following locations: • In the air cleaner housing • In the air duct between the air filter and the throttle body • Built into the mass air flow (MAF) or air flow sensor • Screwed into the intake manifold where it senses the temperature of the air entering the cylinders

12. INTAKE AIR TEMPERATURE SENSOR FIGURE 11-8 The IAT sensor on this General Motors 3800 V-6 engine is in the air passage duct between the air cleaner housing and the throttle plate.

13. TESTING THE INTAKE AIR TEMPERATURE SENSOR • If the intake air temperature sensor circuit is damaged or faulty, a diagnostic trouble code (DTC) is set and the malfunction indicator lamp (MIL) may or may not turn on depending on the condition and the type and model of the vehicle. To diagnose the IAT sensor follow these steps: • After the vehicle has been allowed to cool for several hours, use a scan tool, observe the IAT, and compare it to the engine coolant temperature (ECT). The two temperatures should be within 5°F of each other. • Perform a thorough visual inspection of the sensor and the wiring. • Check the voltage and compare to the following chart.

14. WHAT EXACTLY IS AN NTC SENSOR? FIGURE 11-9 A typical temperature sensor circuit.

15. TRANSMISSION FLUID TEMPERATURE SENSOR • The transmission fluid temperature (TFT), also called transmission oil temperature (TOT), sensor is an important sensor for the proper operation of the automatic transmission. • Cylinder Head Temperature Sensor • Engine Fuel Temperature (EFT) Sensor • Exhaust Gas Recirculation (EGR) Temperature Sensor • Engine Oil Temperature Sensor

16. TEMPERATURE SENSOR DIAGNOSTIC TROUBLE CODES

17. SUMMARY • The ECT sensor is a high-authority sensor at engine startup and is used for closed loop control, as well as idle speed. • All temperature sensors decrease in resistance as the temperature increases. This is called negative temperature coefficient (NTC). • The ECT and IAT sensors can be tested visually, as well as by using a digital multimeter or a scan tool. • Some vehicle manufacturers use a stepped ECT circuit inside the PCM to broaden the accuracy of the sensor. • Other temperature sensors include transmission fluid temperature (TFT), engine fuel temperature (EFT), exhaust gas recirculation temperature (EGR), and engine oil temperature.

18. REVIEW QUESTIONS • How does a typical NTC temperature sensor work? • What is the difference between a stepped and a non-stepped ECT circuit? • What temperature should be displayed on a scan tool if the ECT sensor is unplugged with the key on, engine off? • What are the three ways that temperature sensors can be tested? • If the transmission fluid temperature (TFT) sensor were to fail open (as if it were unplugged), what would the PCM do to the transmission shifting points?

19. CHAPTER QUIZ • The sensor that most determines fuel delivery when a fuel-injected engine is first started is the ______. • O2S • ECT sensor • Engine MAP sensor • IAT sensor

20. CHAPTER QUIZ • The sensor that most determines fuel delivery when a fuel-injected engine is first started is the ______. • O2S • ECT sensor • Engine MAP sensor • IAT sensor

21. CHAPTER QUIZ 2. What happens to the voltage measured at the ECT sensor when the thermostat opens? • Increases slightly • Increases about 1 volt • Decreases slightly • Decreases about 1 volt

22. CHAPTER QUIZ 2. What happens to the voltage measured at the ECT sensor when the thermostat opens? • Increases slightly • Increases about 1 volt • Decreases slightly • Decreases about 1 volt

23. CHAPTER QUIZ 3. Two technicians are discussing a stepped ECT circuit. Technician A says that the sensor used for a stepped circuit is different than one used in a non-stepped circuit. Technician B says that a stepped ECT circuit uses different internal resistance inside the PCM. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

24. CHAPTER QUIZ 3. Two technicians are discussing a stepped ECT circuit. Technician A says that the sensor used for a stepped circuit is different than one used in a non-stepped circuit. Technician B says that a stepped ECT circuit uses different internal resistance inside the PCM. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

25. CHAPTER QUIZ 4. When testing an ECT sensor on a vehicle, a digital multimeter can be used and the signal wires back probed. What setting should the technician use to test the sensor? • AC volts • DC volts • Ohms • Hz (hertz)

26. CHAPTER QUIZ 4. When testing an ECT sensor on a vehicle, a digital multimeter can be used and the signal wires back probed. What setting should the technician use to test the sensor? • AC volts • DC volts • Ohms • Hz (hertz)

27. CHAPTER QUIZ 5. When testing the ECT sensor with the connector disconnected, the technician should select what position on the DMM? • AC volts • DC volts • Ohms • Hz (hertz)

28. CHAPTER QUIZ 5. When testing the ECT sensor with the connector disconnected, the technician should select what position on the DMM? • AC volts • DC volts • Ohms • Hz (hertz)

29. CHAPTER QUIZ 6. When checking the ECT sensor with a scan tool, about what temperature should be displayed if the connector is removed from the sensor with the key on, engine off? • 284°F (140°C) • 230°F (110°C) • 120°F (50°C) • –40°F (–40°C)

30. CHAPTER QUIZ 6. When checking the ECT sensor with a scan tool, about what temperature should be displayed if the connector is removed from the sensor with the key on, engine off? • 284°F (140°C) • 230°F (110°C) • 120°F (50°C) • –40°F (–40°C)

31. CHAPTER QUIZ 7. Two technicians are discussing the IAT sensor. Technician A says that the IAT sensor is more important to the operation of the engine (higher authority) than the ECT sensor. Technician B says that the PCM will add fuel if the IAT indicates that the incoming air temperature is cold. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

32. CHAPTER QUIZ 7. Two technicians are discussing the IAT sensor. Technician A says that the IAT sensor is more important to the operation of the engine (higher authority) than the ECT sensor. Technician B says that the PCM will add fuel if the IAT indicates that the incoming air temperature is cold. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

33. CHAPTER QUIZ 8. A typical IAT or ECT sensor reads about 3,000 ohms when tested using a DMM. This resistance represents a temperature of about _____. • –40°F (–40°C) • 70°F (20°C) • 120°F (50°C) • 284°F (140°C)

34. CHAPTER QUIZ 8. A typical IAT or ECT sensor reads about 3,000 ohms when tested using a DMM. This resistance represents a temperature of about _____. • –40°F (–40°C) • 70°F (20°C) • 120°F (50°C) • 284°F (140°C)

35. CHAPTER QUIZ 9. If the transmission fluid temperature (TFT) sensor were to fail electrically open (infinity resistance), what would the PCM do to the shifts? • Normal shifts and normal operation of the torque converter clutch • Disable torque converter clutch; normal shift points • Delayed shift points and torque converter clutch disabled • Normal shifts but overdrive will be disabled

36. CHAPTER QUIZ 9. If the transmission fluid temperature (TFT) sensor were to fail electrically open (infinity resistance), what would the PCM do to the shifts? • Normal shifts and normal operation of the torque converter clutch • Disable torque converter clutch; normal shift points • Delayed shift points and torque converter clutch disabled • Normal shifts but overdrive will be disabled

37. CHAPTER QUIZ 10. A P0118 DTC is being discussed. Technician A says that the ECT sensor could be shorted internally. Technician B says that the signal wire could be open. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

38. CHAPTER QUIZ 10. A P0118 DTC is being discussed. Technician A says that the ECT sensor could be shorted internally. Technician B says that the signal wire could be open. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B