ENGINE 1.3 QUADRAJET (75PS) 95 Fig. 185 1. Electronic circuit cover and measuring duct cover 2. Electronic control circuit 3. Sensitive element with int ake air temperature sensor incorporated 4. Electronic circuit metal support plate and sensitive element 5. Plastic support casing NOTE : The air flow meter cannot be dismantled PIN OUT 1. 12 V power supply 2. Earth 3. Air temperature signal 4. Air mass signal Fig. 186 OPERATION The operating principle is based on the heating of a diaphragm (sensitive element cont aining two resistances upstream and downstream of the support) in a by-pass duct through which the engine intake air flows. The hot film membrane is maint ained at const ant temperature (about 120 °C higher than the incoming air temperature) by the specific heating resist ance controlled by the management electronics. The mass of air passing through the by-pass duct, giving off heat to the membrane, thermal ly unbalances the two resistances to an extent directly proportional to the mass of the incoming fluid, an imbalance that is translated through the control circuit into a resistance variation, electronically processed and translated into a frequency value to be sent to the engine management control unit. The imbalance in the resistance value also makes it possible to determine the direction of the column of air. As far as the air temperature informat ion is concerned, it is sent to the engine management control unit via a dedicated pin in the form of a duty cycle signal (rectangular wave). The value of the duty is proportional to the value of the temperature. NOTE : The flow meter measures the air mass directly (not the volume) to eliminate problems of compensation correlated to the temperature, altitude, pressure values etc. 1 4
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