Air Flow Meter/Sensor: Description and Operation

Hot Wire Air Mass Sensor:

In operation, the wire is heated to a temperature of 311°F (155°C) above the 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 311° temperature. The 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. To protect the heated wire, it is mounted in an inner sleeve. Each end of the sensor also has a protective screen attached to help prevent damage to the wire. To protect the heated wire from contamination, it is heated to 1800°F (1000°C) for approximately one to five seconds when the engine is switched off. The following conditions have to exist before the burn-off circuit will operate.

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's.
4. No speed signal present.

The main components of the hot wire sensor include:

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