Air Flow Meter – Hot Wire

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Notes:

The voltage output from the Air Flow Meter (AFM) should be proportional to airflow.

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, but 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 rises again to another peak of about 4.0 to 4.5 volts. This voltage

will, however, depend on how hard the engine is accelerated, and a lower voltage is not

necessarily a fault within the AFM.

On deceleration the voltage drops sharply as the throttle butterfly closes, reducing the

airflow, and the engine returns to idle. The final voltage drops gradually on an engine

fitted with an idle speed control valve, as this slowly returns the engine to base idle as

an anti-stall characteristic.

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Example waveform

Figure 6.1.5

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Reference waveform

Figure 6.1.4

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Figure 6.1.3

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Operation Steps:

1. Open the software, select “Vehicle->Diagnosis->Setup”, as illustrated in Figure 6.1.2.

Figure 6.1.2

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Connect the oscilloscope

1. Plug a BNC test lead(HT30A) into channel 1 on the DSO3064(A).

2. Fit a large black gator clip on the black (negative) plug on the test lead and an acupuncture probe(HT307)

or multimeter probe on the coloured (positive) plug.

3. Place the black gator clip to the battery negative terminal and probe the air flow sensor's output terminal with

the acupuncture or multimeter probe as illustrated in Figure 6.1.1. If you cannot reach the terminal or plug with a

probe, then you may be able to use a breakout box or lead if you have one available.

Figure 6.1.1