Maintenance instruction for sunray hfc4da1-2c china-IV diesel engines
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2) Working Principle
This sensor is composed of two sensors, namely airflow sensor and intake temperature sensor, and is installed on the
intake hose after the air cleaner. This type of sensor requires that there shall be free of air leakage from the sensor to
the engine combustion chamber, or it will lead to unstable idling of engine or even engine flameout.
The airflow sensor integrates some micro-electronic units on a ceramic substrate. When the engine is working, the
diaphragm will be heated. The intake airflow passing through the diaphragm will take away the heat and the
Wheatstone bridge integrated on the diaphragm will increase the current to supplement the heat loss, which leads to
the variation of electric signal. This signal is transmitted to the engine control unit ECU, based on which the ECU can
obtain the load variation of the engine and thus control the fuel injection pulse width.
The intake temperature sensing unit is one negative temperature coefficient (NTC) thermistor. The sensor installs two
same sensing units on two sides of the thermal diaphragm. When the intake airflow flows through the diaphragm, the
temperature of the temperature sensing unit on the front of diaphragm is slightly lower than that of temperature
sensing unit on the rear. Based on this characteristic, the ECU can judge the direction of airflow.
3) Measurement analysis
Wiring terminals: 1. +12V power supply; 2. Grounding; 3. Air temperature signal; 4. Airflow rate signal.
4) Malfunction Mode
The subsequent electronic device of the air flowmeter in the ECU can judge the malfunctions such as open-circuit or
short-circuit of internal circuit and connecting circuit of air flowmeter and the damage of sensor. Upon the detection
that the output signal of the sensor is out of its output characteristic curve, the ECU will judge as malfunction of
sensor.
5) Troubleshooting
When the airflow sensor is failed, the engine cooling fan will run at high speed for one period after the ignition key is
turned to Off. For instance: When the airflow rate is higher than the upper limit of airflow or less than the lower limit
of airflow, the ECU will judge as malfunction of sensor (Though the intake airflow rate is less than lower limit
(closing to zero) at the start of the vehicle, the ECU can judge the start condition) and at the same time light up the
engine malfunction lamp and work under malfunction mode.
Air flowmeter
Voltage output V
Common rail
pressure