A mass flow sensor responds to the amount of a fluid (usually a gas) flowing through a chamber containing the sensor. It is intended to be insensitive to the density of the fluid.
A mass airflow sensor is used to determine the mass of air entering an electronically fuel-injected engine. The air mass information is necessary for the engine control unit to calculate and deliver the correct fuel mass to the engine. Air changes its density as it expands and contracts with temperature and pressure. In automotive applications, air density varies with the vehicle's operating environment, and this is an ideal application for a mass sensor.
Mass Air Flow Sensors
The Mass Air Flow Sensors converts the amount of air drawn into the engine into a voltage signal. The ECM needs to know intake air volume to calculate engine load. This is necessary to determine how much fuel to inject, when to ignite the cylinder, and when to shift the transmission. The air flow sensor is located directly in the intake air stream, between the air cleaner and throttle body where it can measure incoming air.
There are different types of Mass Air Flow sensors. The Vane air flow meter and Karmen vortex are two older styles of air flow sensors and they can be identified by their shape. The newer, and more common is the Mass Air Flow (MAF) sensor.
Mass Air Flow Sensor: Hot Wire Type
The primary components of the MAF sensor are a thermistor, a platinum hot wire, and an electronic control circuit.
The thermistor measures the temperature of the incoming air. The hot wire is maintained at a constant temperature in relation to the thermistor by the electronic control circuit. An increase in air flow will cause the hot wire to lose heat faster and the electronic control circuitry will compensate by sending more current through the wire. The electronic control circuit simultaneously measures the current flow and puts out a voltage signal (VG) in proportion to current flow.
This type of MAE sensor also has an Intake Air Temperature (IAT) sensor as part of the housing assembly. Its operation is described in the IAT section of Temperature Sensors. When looking at the EWD, there is a ground for the MAE sensor and a ground (E2) for the IAT sensor.
Diagnosis of the MAE sensor involves visual, circuit, and component checks. The MAE sensor passage must be free of debris to operate properly. If the passage is plugged, the engine will usually start, but run poorly or stall and may not set a DTC.
Vane Air Flow Meter
The Vane Air Flow Meter provides the ECM with an accurate measure of the load placed on the engine. The ECM uses it to calculate basic injection duration and basic ignition advance angle. Vane Air Flow Meters consist of the following components:
- Measuring Plate.
- Compensation Plate.
- Return Spring.
- Bypass Air Passage.
- Idle Adjusting Screw (factory adjusted).
- Fuel Pump Switch.
- Intake Air Temperature (IAT) Sensor.
During engine operation, intake air flow reacts against the measuring plate (and return spring) and deflects the plate in proportion to the volume of air flow passing the plate. A compensation plate (which is attached to the measuring plate) is located inside a damping chamber and acts as a shock absorber to prevent rapid movement or vibration of the measuring plate.
Movement of the measuring plate is transferred through a shaft to a slider (movable arm) on the potentiometer. Movement of the slider against the potentiometer resistor causes a variable voltage signal back to the VS terminal at the ECM. Because of the relationship of the measuring plate and potentiometer, changes in the VS signal will be proportional to the air intake volume.
The r2 resistor (connected in parallel with ri) allows the meter to continue to provide a VS signal in the event that an open occurs in the main potentiometer (ri). The Vane Air Flow Meter also has a fuel pump switch built into the meter that closes to maintain fuel pump operation once the engine has started and air flow has begun.
The meter also contains a factory adjusted idle adjusting screw that is covered by a tamper - resistant plug. The repair manual does not provide procedures on resetting this screw in cases where it has been tampered with.
Types of VAF Meters
There were two major types of VAF meters. The first design, is the oldest type. It uses battery voltage for supply voltage. With this type of VAF meter, as the measuring plate opens signal voltage increases.
Karmen Vortex Air Flow Meter
This air flow meter provides the same type of information Flow Meter. It consists of the following components:
- Vortex Generator.
- Mirror (metal foil).
- Photo Coupler (LED and photo transistor).
Karman Vortex Air Flow Meter Operation
Intake air flow reacting against the vortex generator creates a swirling effect to the air downstream, very similar to the wake created in the water after a boat passes. This wake or flutter is referred to as a Karman Vortex. The frequencies of the vortices vary in proportion to the intake air velocity (engine load).
The vortices are metered into a pressure directing hole from which they act upon the metal foil mirror. The air flow against the mirror causes it to oscillate in proportion to the vortex frequency. This causes the illumination from the photo coupler's LED to be alternately applied to and diverted away from a photo transistor. As a result, the photo transistor alternately grounds or opens the 5-volt KS signal to the ECM.
This creates a 5 volt square wave signal that increases frequency in proportion to the increase in intake air flow. Because of the rapid, high frequency nature of this signal, accurate signal inspection at various engine operating ranges requires using a high quality digital multimeter (with frequency capabilities) or oscilloscope.
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