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Air Flow Meter (Hot Wire) Waveform Notes
The voltage output from the Air Flow Meter (AFM) should be linear to airflow and this can be measured on an oscilloscope and should look similar to the example shown.
The waveform should show approximately 1.0 Volt when the engine is at idle, this voltage will rise as the engine is accelerated and air volume is increased producing an initial peak. This peak is due to the initial influx of air and drops momentarily before the voltage is seen to rise again to another peak of approximately 4.0 to 4.5 Volts. This voltage will however depend on how hard the engine is accelerated, a lower voltage is not necessarily a fault within the AFM.
On deceleration the voltage will drop sharply as the throttle butterfly closes, reducing the airflow, and the engine returns back to idle. The final voltage will drop gradually on an engine fitted with idle speed control valve as this will slowly return the engine back to base idle as an anti-stall characteristic. This function normally only affects the engine speed from around 1200 rpm back to the idle setting.
A time base of approximately 2 seconds plus is used, this enables the operator to view the AFM’s output voltage on one screen, from idle, through acceleration and back to idle again. The ‘hash’ on the waveform is due to the vacuum change from the induction pulses as the engine is running.
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