Air Flow Meter/Sensor: Description and Operation

Air Mass Meter:

PURPOSE
The mass air flow sensor provides the injection ECU with continuous information regarding intake "air mass [1]." The injection ECU sends a modified signal to the ignition ECU regarding engine load.

OPERATION
A "hot wire" is heated to a temperature of 212°F (100°C) above ambient temperature. This temperature is maintained regardless of outside air temperature. As air flows through the sensor, the current through the wire increases to maintain the 212° temperature.

The electronic control unit uses this current change as a signal of air mass entering the engine. The current is sensed by reading the voltage drop across the measuring resistor in the sensor. Changes in intake air temperature are corrected by the compensating resistor.

The hot wire is mounted in an inner sleeve for protection. Each end of the mass air flow sensor also has a protective screen attached to help prevent damage to the wire. To clean the heated wire of contamination, it is heated to 1830°F (1000°C) for one second when the engine is switched off.

The following conditions must exist before the burn-off circuit operates:

1. Time delay of 2.5 seconds after switching off key.
2. Coolant temperature of 140°F (60°C) or greater.
3. Prior to switching off, the engine must have been run above 2000 rpm.
4. No speed signal present.

COMPONENTS
^ Heated Wire (Platinum)
^ Measuring Resistor
^ Compensating Resistor
^ Micro Processor

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[1] Air Mass is not the same as air flow. Air flow is merely a measurement of volume. Air mass accounts for density and composition (temperature, pressure, and humidity).