The function of an air flow meter is to accurately measure the intake gas under all engine operating conditions, thereby enabling better engine management and reducing exhaust emissions.
I. Basic Functions and Installation Location
Both hot-film and hot-wire air flow meters are thermal flow meters. They measure a portion of the airflow passing through the sensor unit in the intake manifold via a measuring channel within the air flow meter housing. The measured temperature value is analyzed in an electronic analysis unit, and the air flow meter transmits a voltage corresponding to the air mass to the engine electronic control unit. This voltage signal is used to calculate the fuel injection time and the engine's actual torque.
When the air flow sensor malfunctions, the engine will calculate the air quality using a characteristic curve (throttle angle and engine speed), which will result in abnormal engine power.
Air flow meters are typically installed after the air filter and before the throttle body; they can also be installed on the air filter or the throttle body.
II. Structural Principles and Testing
1. Hot-wire air flow meter
(1) Basic structure
The heating element of a hot-wire air flow sensor is a platinum wire, and it comes in two structural forms: hot-wire main flow and hot-wire bypass.
The structure of a hot-wire mainstream airflow sensor is shown in the figure below. Inside the airflow meter is a heated platinum wire exposed to the intake airflow. By applying a specified current to the platinum wire, the ECM heats it to a designated temperature. The intake airflow cools the platinum wire and the internal thermistor, thus changing their resistance. To maintain a stable current value, the ECM changes the voltage applied to the platinum wire and the internal thermistor. The voltage value is proportional to the airflow through the sensor, and the ECM uses this value to calculate the intake air volume.
The platinum heating wire and the temperature sensor form a bridge circuit, and the predetermined temperature is maintained by controlling the transistor to keep the pressure difference between A and B equal.
(2) Typical circuit connection
The Toyota Camry 1ZR-FE, 2ZR-FE, and 5AR-FE engines use hot-wire air flow meters. The circuit connection of the air flow meter for the 5AR-FE engine is shown in the figure below.
Sensor 3# provides 12V power to relay 1. Sensors 4# and 5# are signal output terminals, which are connected to terminals 92# and 91# of the E26 terminal of the engine control unit, respectively.
(3) Individual detection of air flow sensor
Remove the air flow sensor assembly and visually inspect the platinum heating wire (heater) of the mass air flow meter assembly for foreign objects. If foreign objects are found, replace the sensor assembly.
Check the resistance of the intake air temperature sensor built into the air flow sensor. For connector E14, pin 1 is the intake air temperature sensor signal output terminal, and pin 2 is the ground terminal.
2. Hot-film air flow meter
(1) Basic principles
The working principle of a hot-film air flow sensor is similar to that of a hot-wire air flow sensor. The difference is that the hot-film type does not use platinum wire as the heating wire. Instead, the hot-film resistor, compensation resistor, and bridge resistor are fabricated on the same ceramic substrate using a thick-film process. This structure prevents the heating element from directly bearing the force generated by airflow, increasing the strength of the heating element, improving the reliability of the air flow meter, resulting in smaller errors, lower cost, higher resistance, and lower current consumption, allowing for smaller and lighter designs. The signal output voltage changes linearly between 0 and 5V. However, the responsiveness of the hot-film heating element is slightly inferior.
The 2016 Toyota Camry Hybrid's 6AR-FSE engine uses a new silicon-based thermal mode air flow sensor (HMF6), which integrates digital processing circuitry for more accurate output. The sensor structure is shown in the diagram below.
The airflow sensor has a built-in intake air temperature sensor. The intake airflow passes through a silicon wafer sensor (before the heater), the heater, and then another temperature sensor (after the heater) in a bypass duct. Because the intake air temperature rises upon contact with the heater, the temperature of the intake airflow passing through the temperature sensor (after the heater) is higher than its temperature passing through the temperature sensor (before the heater). The intake air temperature difference varies at each temperature sensor depending on the intake airflow rate through the silicon wafer sensor. The temperature sensor bridging circuit detects the temperature difference, and the control circuit converts it into a pulse signal and outputs it to the ECM. When the temperature detected by the temperature sensor (before the heater) is higher than the temperature detected by the temperature sensor (after the heater), intake air recirculation is determined.
Sensor internal circuit
Sensor and ECU connection circuit diagram
(2) Sensor unit detection
Disconnect the ignition switch and remove the sensor. Visually inspect the air flow sensor's temperature sensor (thermometer) for foreign objects. If foreign objects are found, replace the sensor. Check the resistance between pins 2 and 3 of sensor E78 (intake air temperature sensor) with a multimeter. The test result should conform to the diagram below. If it does not conform, replace the sensor.
