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This is a good waveform |
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Air Flow Meter (Air Vane) Waveform Notes
The voltage output from the internal track of the Air Flow Meter (AFM) should be linear to flap movement, 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 will produce an initial peak. This peak is due to the natural inertia of the air vane and drops momentarily before the voltage is seen to rise again to a peak of approximately 4.0 to 4.5 volts. This voltage will however depend on ho hard the engine is accelerated so a lower voltage is not necessarily a fault within the AFM. On deceleration the voltage will drop sharply as the wiper arm, in contact with the carbon track, returns back to the idle position. This voltage may in some cases ‘dip’ below the initial voltage before returning to idle voltage. A gradual drop will be seen on an engine fitted with an idle speed control valve as this will slowly return the engine back to base idle as an anti-stall characteristic.
A time base of approximately 2 seconds plus is used, this enables the operator to view the AFM’s movement on one screen, from idle, through acceleration and back to idle again. The waveform should be clean with no ‘drop-out’ in the voltage, as this indicates a lack of electrical continuity. A good example of this is shown on the ‘Faulty 12 volt AFM’ example waveform. This is common on an AFM with a dirty or faulty carbon track. The problem will show as a ‘flat spot’ or hesitation when the vehicle is driven, this is a typical problem on vehicles with high mileage that have spent the majority of their working life with the throttle in one predominant position. The ‘hash’ on the waveform is due to the vacuum change from the induction pulses as the engine is running.
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