DANANG UNIVERSITY OF SCIENCE AND TECHNOLOGY Essay on Mass Air Flow Sensor – Hot Wire Instructor LE MINH DUC Students VO NGOC HUY MSSV 103200086 HP Class 20 19 Class 20C4CLC1 TABLE OF CONTENTS[.]
DANANG UNIVERSITY OF SCIENCE AND TECHNOLOGY Essay on Mass Air Flow Sensor – Hot Wire Instructor : LE MINH DUC Students : VO NGOC HUY MSSV : 103200086 HP Class : 20 19 Class : 20C4CLC1 TABLE OF CONTENTS: Introduction to Mass Air Flow Sensor – Hot Wire I Structure of the intake air flow sensor II Working principle General principle Circuit explanation III Specifications of intake air flow sensor Sensor pins IV Check the intake air flow sensor Check continuity Check voltage Check sensor circuit Check the resistance and voltage of THA and E2 Check the intake air temperature sensor circuit Check the intake air temperature sensor voltage V Common failures in the thermal wire type intake air flow sensor VI Symptoms VII Pros and cons of Thermal Wire Type Intake Air Flow Sensor Advantages Defect The MAF (Mass Air Flow Sensor – Hot Wire) intake air flow sensor is responsible for determining the intake air flow rate and sending a signal to the ECU. ECU will calculate the amount of intake air and send a control signal to balance the carburetor ratio for the engine The sensor has a thermal wire that can withstand temperatures from 150 to 300 degrees Celsius When the engine runs at low or idle speed, the amount of air entering the engine is less. On the contrary, when the engine runs at high speed or under a large load, a lot of air enters I.Structure of the intake air flow sensor Airflow path through the thermal wire type intake air flow sensor inside the anemometer The thermal wire type intake air flow sensor is located on the air intake pipe just behind the air filter. The sensor contains a Platinum thermal wire and a thermistor: Thermistor (Intake Air Teamperature Measerement Resistor): It is responsible for checking the temperature of the air entering the anemometer Platinum heating wire (Heating Resistor - is actually a resistor with negative thermistor value): It is responsible for heating the surrounding air, which is regulated at a constant temperature The thermal wire type intake air flow sensor has a built-in Intake Air Temperature Sensor Thermal wire type intake air flow sensor II Working principle 1.General principle As the air flow increases, the Platinum wire loses heat and the ECU control circuit compensates for the heat by increasing the current through the wire At the same time, the ECU control circuit measures the air flow, adjusting the amperage along with the corresponding voltage signal Air flow moves through the intake air flow sensor 2.Circuit explanation In this resistor bridge circuit, the voltages at points A and B are equal: [Ra+R3]*R1=Rh*R2 As the resistance increases, there is a voltage difference between points A and B The amplifier senses this voltage difference. When the voltage at Rh is lower than the standard voltage, the transistor opens to allow current to flow down Rh. Thus, it increases the current flowing through the fusing wire (Rh). The more airflow moves through the wire, the more amperage is needed to keep it hot. This temperature regulation at the resistor Rh results in the potentials A and B gradually becoming equal. The heat wire will be kept constant Drying wire temperature (Rh) and intake air temperature The temperature of the thermocouple Rh is maintained but will be higher than the temperature of the intake air due to the thermistor Ra in the circuit Due to the nature of a negative thermistor, when the temperature increases, the resistance decreases, when the temperature decreases, the resistance increases. In some new vehicles, this thermistor is a positive resistor The voltage VG is proportional to the volume of air being fed (approximately 20 g/s at 575 rpm – 50 g/s at 1400 rpm) III Specifications of intake air flow sensor Sensor pins A thermal wire type intake air flow sensor typically has pins in order from 1-5: B+,EVG, VG, THA, E2 Thermal wire type intake air flow sensor pins In which, the meaning of the pins: Pin B+ : Positive after the power switch for the MAF sensor EVG foot : Mass anemometer VG pin : Signal to determine the intake air flow THA leg: Air temperature sensor signal E2 pin : Mass air temperature sensor IV Check the intake air flow sensor Check continuity Use VOM to check whether the connections, jacks, contacts are in good contact or not. If not, check and repair the intake air flow sensor Check voltage Turn the ignition switch in the ON position (engine is still off) Use the diagnostic device to read the voltage value of the MAF sensor: Display voltage: ~1.0 V If the displayed voltage is different from the reference voltage, go to the sensor circuit test step. If this circuit is not damaged, the sensor must be replaced If the same as the reference voltage then: Start and warm up the engine to normal operating temperature Let the engine run in idle mode Use the diagnostic device to read the voltage value of the MAF sensor: Display voltage: ~1.6 – 2.3V If the displayed voltage is different from the reference voltage, go to the sensor circuit test step Check sensor circuit Unplug the connector of the MAF sensor and the jack of the ECM Use an Ohm meter to measure the continuity between the legs: MAF-1 (B+) with positive wire after Main Relay: R ~ 0: good; R ~ ∞: break the positive wire MAF-2 (Mass anemometer) with ECU intake air flow sensor mass: R ~ 0: good; R ~ ∞: broken ground wire MAF-3 (VG) with ECU intake air flow sensor signal pin: R ~ 0: good; R ~ ∞: broken signal wire Repair or replace the conductors if the test results are not good Check the resistance and voltage of THA and E2 Remove the anemometer jack Use VOM to measure the resistance between THA and E2 pins Compare with the standard value of THA signal pin in the table below: Check voltage : Insert the jack into the anemometer => Start the engine, check the voltage between the terminals THA and E2. The permissible voltage is 0.5÷2.4V for the motor running at a temperature of 20 - 80 degrees Celsius Check the intake air temperature sensor circuit Unplug the IAT sensor and the ECM connector Use an Ohm meter to measure the continuity between the legs: THA MAF sensor with Intake air temperature sensor signal pin in ECU: R ~ 0: good; R ~ ∞: broken signal wire E2 MAF sensor with ECU intake air temperature sensor mass: R ~ 0: good; R ~ ∞: broken ground wire Repair or replace the conductors if the test results are not good Check the intake air temperature sensor voltage Unplug the IAT sensor connector Use a voltmeter to measure the voltage between the pins: THA MAF sensor with cooler: U ~ 0V: good E2 MAF sensor with cooler: U~5V: good V Common failures in the thermal wire type intake air flow sensor It is very easy to break the Platinum thermal wire because it is very thin The connector pins are often dirty resulting in poor contact Wire break VI Symptoms Poor starting: When starting, the fuel combustion is not clean Unstable operation: The idle speed is high, low or unstable, the machine is shaken and idle Engine power is reduced Poor driving Check Engine light is on Cars often stall when stopping at red lights VII Pros and cons of Thermal Wire Type Intake Air Flow Sensor Advantages Responds faster to changes in air flow rate Durable because there are no moving parts Low cost of replacement and repair Avoid using separate pressure sensors and temperature sensors to determine air mass Smaller size than other types of intake air flow sensor Defect Oil and dirt contaminate the heating wire, thereby reducing the accuracy of the sensor Thin platinum wire can be damaged if not handled properly