Hot Wire Air Mass Sensor

HOT WIRE AIR MASS SENSOR (M3.1, M1.2 & M1.7-M70 SYSTEMS)







The air volume input signal is produced electronically by the air mass sensor. The sensor works on the principle that as the resistance of a circuit changes with a constant voltage supply, the current flow through the circuit must change. This also causes the voltage drop across any circuit resistance to change. When the air mass sensor circuits is switched on, a voltage (and current) is applied to the sensing wire (hot wire) and temperature compensating circuits.

The sensing wire is heated to approx. 155 °C (311 F) above ambient air temperature by controlling the amount of current allowed to pass through it. This current also flows through the fixed measuring resistor. As intake air passes over the sensing wire, it creates a cooling effect, thus lowering the resistance of the wire (PTC). Current flow through the sensing wire is increased to bring the temperature of the wire back to 155 °C.

By varying the amount of current needed to maintain the 155 °C temperature (above ambient intake air temp.) of the sensing wire, the voltage drop across the measuring resistor is increased or decreased and reflects the amount (mass) of air inducted by the engine. This varying voltage is used as the input signal to the ECM for air volume.

The circuit created by the PTC Temperature Compensating Resistor is used as a comparison circuit by the micro-processor of the air mass sensor. The amount of current flow through the circuit is determined by the resistance of the PTC resistor. As ambient temperature increases, current flow will decrease. This value is monitored by the processor and represents the incoming air temperature.

It is used by the processor to determine the amount of current required to heat the sensing wire. This allows the processor to maintain the sensing wire at 155 °C above ambient temperature, which compensates for ambient air temperature and creates a basis for monitoring the cooling effect on the wire as a representation of inducted air volume (by weight).

This sensor receives a "clean burn" request signal from the engine control module to burn off contamination on the sensing wire. This function is described in the output section of this manual.