Automotive Fuel and Emissions Control Systems, 2/e By James D. Halderman and Jim Linder © 2009 Pearson Higher Education, Inc. Pearson Prentice Hall -

Automotive Fuel and Emissions Control Systems, 2/eBy James D. Halderman and Jim Linder

© 2009 Pearson Higher Education, Inc.Pearson Prentice Hall - Upper Saddle River, NJ 07458

OBJECTIVES

After studying Chapter 17, the reader will be able to:1. Prepare for ASE Engine Performance (A8)

certification test content area “E” (Computerized Engine Controls Diagnosis and Repair).

2. Discuss how MAF sensors work.3. List the methods that can be used to test MAF

sensors.4. Describe the symptoms of a failed MAF sensor.5. List how the operation of the MAF sensor affects

vehicle operation.6. Discuss MAF sensor rationality tests.



AIR FLOW SENSORS

• Port electronic fuel-injection systems that use air flow volume for fuel calculation usually have a movable vane in the intake stream.

• The vane is part of the vane air flow (VAF) sensor.

• The vane is deflected by intake air flow.

FIGURE 17-1 A vane air flow (VAF) sensor.



AIR FLOW SENSORS

FIGURE 17-2 A typical air vane sensor with the cover removed. The movable arm contacts a carbon resistance path as the vane opens. Many air vane sensors also have contacts that close to supply voltage to the electric fuel pump as the air vane starts to open when the engine is being cranked and air is being drawn into the engine.



MASS AIR FLOW SENSOR TYPES

• Hot Film Sensor– The hot film sensor uses a temperature-sensing resistor

(thermistor) to measure the temperature of the incoming air.

• Hot Wire Sensor– The hot wire sensor is similar to the hot film type, but

uses a hot wire to sense the mass air flow instead of the hot film.




FIGURE 17-3 This five-wire mass air flow sensor consists of a metal foil sensing unit, an intake air temperature (IAT) sensor, and the electronic module.




FIGURE 17-4 The sensing wire in a typical hot wire mass air flow sensor.



KARMAN VORTEX SENSORS

• In 1912, a Hungarian scientist named Theodore Van Karman observed that vortexes were created when air passed over a pointed surface.

• There are two basic designs of Karman Vortex air flow sensors. The two types include:– Ultrasonic. – Pressure-type.



KARMAN VORTEX SENSORS

FIGURE 17-5 A Karman Vortex air flow sensor uses a triangle-shaped rod to create vortexes as the air flows through the sensor. The electronics in the sensor itself converts these vortexes to a digital square wave signal.



PCM USES FOR AIR FLOW SENSORS

• The PCM uses the information from the air flow sensor for the following purposes:– Air flow sensors are used mostly to determine the amount

of fuel needed and base pulse-width numbers. • The greater the mass of the incoming air, the longer the injectors are

pulsed on.

– Air flow sensors back up the TP sensor in the event of a loss of signal or an inaccurate throttle position sensor signal.

• If the MAF sensor fails, then the PCM will calculate the fuel delivery needs of the engine based on throttle position and engine speed (RPM).



TESTING MASS AIR FLOW SENSORS

• Start the testing of a MAF sensor by performing a thorough visual inspection.

• Look at all the hoses that direct and send air, especially between the MAF sensor and the throttle body.

• Also check the electrical connector for:– Corrosion– Terminals that are bent or pushed out of the plastic

connector– Frayed wiring



TESTING MASS AIR FLOW SENSORS

• MAF Sensor Output Test• Tap Test• Digital Meter Test of a MAF Sensor



WHAT IS FALSE AIR?

FIGURE 17-6 Carefully check the hose between the MAF sensor and the throttle plate for cracks or splits that could create extra (false) air into the engine that is not measured by the MAF sensor.



MAF SENSOR CONTAMINATION

• Dirt, oil, silicon, or even spider webs can coat the sensing wire.– Because it tends to insulate the sensing wire at low air flow rates, a

contaminated sensor often overestimates the amount of air entering the engine at idle, and therefore causes the fuel system to go rich.

• At higher engine speeds near wideopen throttle (WOT), the contamination can cause the sensor to underestimate the amount of air entering the engine. – As a result, the fuel system will go lean, causing spark knock and

lack of power concerns.

• To check for contamination, check the fuel trim numbers.



THE RICH RUNNING TOYOTA

FIGURE 17-7 (a) Air flow sensor with the protective cover removed.(b) Broken air flow vane return spring.



MAF-RELATED DIAGNOSTIC TROUBLE CODES

• The diagnostic trouble codes (DTCs) associated with the mass air flow and air vane sensors include:



SUMMARY

1. A mass air flow sensor actually measures the density and amount of air flowing into the engine, which results in accurate engine control.

2. An air vane sensor measures the volume of the air, and the intake air temperature sensor is used by the PCM to calculate the mass of the air entering the engine.

3. A hot wire MAF sensor uses the electronics in the sensor itself to heat a wire 70°C above the temperature of the air entering the engine.



REVIEW QUESTIONS

1. How does a hot film MAF sensor work?2. What type of voltage signal is produced by a MAF?3. What change in the signal will occur if engine

speed is increased?4. How is a MAF sensor tested?5. What is the purpose of a MAF sensor?6. What are the types of air flow sensors?



CHAPTER QUIZ

1. A fuel-injection system that does not use a sensor to measure the amount (or mass) of air entering the engine is usually called a(n) _____ type of system.a) Air vane-controlledb) Speed densityc) Mass air flowd) Hot wire



CHAPTER QUIZ

1. A fuel-injection system that does not use a sensor to measure the amount (or mass) of air entering the engine is usually called a(n) _____ type of system.a) Air vane-controlledb) Speed densityc) Mass air flowd) Hot wire



CHAPTER QUIZ

2. Which type of sensor uses a burn-off circuit?a) Hot wire MAF sensorb) Hot film MAF sensorc) Vane-type air flow sensord) Both a and b



CHAPTER QUIZ

2. Which type of sensor uses a burn-off circuit?a) Hot wire MAF sensorb) Hot film MAF sensorc) Vane-type air flow sensord) Both a and b



CHAPTER QUIZ

3. Which sensor has a switch that controls the electric fuel pump?a) VAFb) Hot wire MAFc) Hot filter MAFd) Karman Vortex sensor



CHAPTER QUIZ

3. Which sensor has a switch that controls the electric fuel pump?a) VAFb) Hot wire MAFc) Hot filter MAFd) Karman Vortex sensor



CHAPTER QUIZ

4. Two technicians are discussing Karman Vortex sensors. Technician A says that they contain a burn-off circuit to keep them clean. Technician B says that they contain a movable vane. Which technician is correct?a) Technician A onlyb) Technician B onlyc) Both Technicians A and Bd) Neither Technician A nor B



CHAPTER QUIZ

4. Two technicians are discussing Karman Vortex sensors. Technician A says that they contain a burn-off circuit to keep them clean. Technician B says that they contain a movable vane. Which technician is correct?a) Technician A onlyb) Technician B onlyc) Both Technicians A and Bd) Neither Technician A nor B



CHAPTER QUIZ

5. The typical MAF reading on a scan tool with the engine at idle speed and normal operating temperature is _____.a) 1 to 3 grams per secondb) 3 to 7 grams per secondc) 8 to 12 grams per secondd) 14 to 24 grams per second



CHAPTER QUIZ

5. The typical MAF reading on a scan tool with the engine at idle speed and normal operating temperature is _____.a) 1 to 3 grams per secondb) 3 to 7 grams per secondc) 8 to 12 grams per secondd) 14 to 24 grams per second



CHAPTER QUIZ

6. Two technicians are diagnosing a poorly running engine. There are no diagnostic trouble codes. When the MAF sensor is unplugged, the engine runs better. Technician A says that this means that the MAF is supplying incorrect air flow information to the PCM. Technician B says that this indicates that the PCM is defective. Which technician is correct?a) Technician A onlyb) Technician B onlyc) Both Technicians A and Bd) Neither Technician A nor B



CHAPTER QUIZ

6. Two technicians are diagnosing a poorly running engine. There are no diagnostic trouble codes. When the MAF sensor is unplugged, the engine runs better. Technician A says that this means that the MAF is supplying incorrect air flow information to the PCM. Technician B says that this indicates that the PCM is defective. Which technician is correct?a) Technician A onlyb) Technician B onlyc) Both Technicians A and Bd) Neither Technician A nor B



CHAPTER QUIZ

7. A MAF sensor on a General Motors 3800 V-6 is being tested for contamination. Technician A says that the sensor should show over 100 grams per second on a scan tool display when the accelerator is depressed to WOT on a running engine. Technician B says that the output frequency should exceed 7,000 Hz when the accelerator pedal is depressed to WOT on a running engine. Which technician is correct?

a) Technician A onlyb) Technician B onlyc) Both Technicians A and Bd) Neither Technician A nor B



CHAPTER QUIZ

7. A MAF sensor on a General Motors 3800 V-6 is being tested for contamination. Technician A says that the sensor should show over 100 grams per second on a scan tool display when the accelerator is depressed to WOT on a running engine. Technician B says that the output frequency should exceed 7,000 Hz when the accelerator pedal is depressed to WOT on a running engine. Which technician is correct?

a) Technician A onlyb) Technician B onlyc) Both Technicians A and Bd) Neither Technician A nor B



CHAPTER QUIZ

8. Which air flow sensor has a dampening chamber?a) A vane airflowb) A hot film MAFc) A hot wire MAFd) A Karman Vortex



CHAPTER QUIZ

8. Which air flow sensor has a dampening chamber?a) A vane airflowb) A hot film MAFc) A hot wire MAFd) A Karman Vortex



CHAPTER QUIZ

9. Air that enters the engine without passing through the air flow sensor is called _____.a) Bypass airb) Dirty airc) False aird) Measured air



CHAPTER QUIZ

9. Air that enters the engine without passing through the air flow sensor is called _____.a) Bypass airb) Dirty airc) False aird) Measured air



CHAPTER QUIZ

10.A P0102 DTC is being discussed. Technician A says that a sensor circuit shorted-to-ground can be the cause. Technician B says that an open sensor voltage supply circuit could be the cause. Which technician is correct?a) Technician A onlyb) Technician B onlyc) Both Technicians A and Bd) Neither Technician A nor B



CHAPTER QUIZ

10.A P0102 DTC is being discussed. Technician A says that a sensor circuit shorted-to-ground can be the cause. Technician B says that an open sensor voltage supply circuit could be the cause. Which technician is correct?a) Technician A onlyb) Technician B onlyc) Both Technicians A and Bd) Neither Technician A nor B

Documents

Automotive Fuel and Emissions Control Systems, 2/e By James D. Halderman and Jim Linder © 2009 Pearson Higher Education, Inc. Pearson Prentice Hall -