I am often asked by car owners: the engine suddenly accelerates and tempers, the driving is weak, the idling speed is unstable, the driving is weak, and the black smoke is emitted. After doing the basic settings once, the fault is eliminated, but it repeats after a few days. Let me tell you clearly that there may be a problem with the mass air flow sensor of your car, and the computer cannot detect it.
Let me introduce you to the air flow meter
# Air flow sensor installation location
[Function]: Detect the size of the dfsk engine air intake and convert it into an electrical signal and input it into the ECU to calculate the fuel injection time and ignition time.
There are currently two types of mass flow types that are mainstream in the market: hot-wire air flow meters and hot-mode air flow meters. The other volume flow type blade type and Karman vortex type have basically been eliminated. The theoretical air-fuel ratio that we usually say is that the mass ratio of air to fuel is 14.7:1.
One, hot wire air flow sensor
1. Hotline: Nissan （Maximum）， GM （Buick Regal）， Mercedes-Benz, etc.
#NISSAN air flow meter
#NISSAN air flow meter front view
The measuring element of the hot wire air flow sensor is a platinum wire hot wire wound on the ceramic.
2, working principle
#Whiston bridge principle
The working principle of the hot wire air flow meter is shown in the figure. The hot wire resistance RH is placed in the inlet pipe. When the air flows through the heating wire, the heat of the heating wire is absorbed by the air to make it cool. The greater the air flow through the hotline, the more heat it takes away. The hot wire air flow meter uses the heat transfer phenomenon between the hot wire and the air to measure air mass flow. Its working principle is to keep the difference between the temperature of the hot wire and the temperature of the suction air at 100°C, and the temperature of the hot wire is controlled by the hybrid integrated circuit. As the mass of air entering the engine increases, the amount of heat taken away by the air also increases. In a hybrid integrated circuit, in order to maintain the temperature of the hot wire, the current through the hot wire resistance increases; conversely, the current through the hot wire resistance decreases. In this way, the current through the hot wire resistance is a single function of the air mass flow, that is, the hot wire current increases with the increase of the air mass flow, and decreases with the decrease of the air mass flow.
As the air flow increases, the temperature of the platinum wire resistance RH decreases, the resistance of RH decreases, and the heating current IH increases to maintain the temperature of the hot wire （100°C higher than the airflow temperature）。 The signal resistance RA is connected in series with RH, so the resistance RA is two. The terminal voltage V2 increases. V2 is the signal voltage that our air flow meter needs to give to the ECU.
Self-cleaning function: After the sensor is used for a period of time, the surface of the hot wire is polluted by air dust, which reduces its heat radiation ability and affects the measurement accuracy.
Self-cleaning method: ECU receives the flameout command and controls the self-cleaning circuit to be turned on. The hotline is heated to 1000°C and kept for 1S to burn the dust.
2. Thermal mode air flow sensor
It is the mainstream development direction of the current air flow meter, so it is widely used, and it is a little better than the hot-wire type.
Hot film type: Developed by General Motors, Volkswagen, BMW.
# Volkswagen hot mode air flow sensor
#Front view
1. Structure: The structure of the hot film air flow meter is shown in the figure. The heating body is not a hot wire, but a hot film, which is composed of hot metal platinum fixed on a thin layer of resin film. This structure can make the heating element not directly bear the force generated by the air flow, improve the strength of the heating element, and improve the reliability of the air flow meter. The platinum film is plated on the ceramic substrate.
#Internal view of thermal mode
2. Principle: Same as hotline
Advantages: The thermal film is not easily damaged, has high reliability, is not easy to be polluted, does not need to be self-cleaning, and has low cost.
Three, air flow sensor failure analysis
Case analysis①
Model: Jetta 5V engine
Failure: Idle speed is unstable, driving is weak and black smoke is emitted, and the basic setting troubleshooting is done once, but it repeats after a few days.
Detection: The computer showed a temporary failure of the air flowmeter. We replaced the air flowmeter and measured it again. The fault remained. After checking the entire vehicle data block, it also showed normal. However, the specific detection of the air flowmeter circuit found that the resistance value of the air flowmeter signal line was biased. The normal value is 0.5 Ω, but the actual value is 3.6 Ω.
The real reason: the line has a virtual connection, the harness plug is processed, and the fault is eliminated.
Analysis: This kind of fault is a special fault, but it is often encountered in actual maintenance and it is difficult to solve it. We can find a problem: the air flow meter signal line is located at the corner of the plug, which is prone to position failure during the production process, resulting in poor contact. In other plugs, the corresponding position is also worthy of our attention. In addition, as a vital component, the failure rate of the air flow meter is very low. When the computer prompts its failure, we must treat it with caution.
Case analysis②
# External pin in thermal mode
Model: Santana 2000GSi sedan, equipped with AJR engine, using Bosch M3.8.2 EFI system, and delivered in 2000.
Malfunction: The engine is rapidly accelerating and tempering, and driving is weak.
Inspection: Check that the oil circuit/ignition system/intake pipe is normal. Check the intake air flow sensor. Use a multimeter to check the No. 2 power supply terminal of the sensor, the voltage is 12V, normal. The No. 5 terminal is the signal voltage output terminal. The idle speed should be about 1.3V in normal mode, and 3.5-4.5V in rapid acceleration. In the detection, it was found that it was about 2.8V during rapid acceleration. Obviously the output signal voltage of the sensor was low. Immediately remove the intake air flow sensor, and check that there are some flocculent debris on the hot film metal platinum （resistor） in the sensor sampling tube. After spraying it with carburetor cleaning agent, it is installed and retested, and the engine does not return. Fire, troubleshooting.
Analysis: The intake air flow sensor is dirty, and the change in flow cannot be detected. The signal is low and the gas mixture is leaner.