The purpose of this test is to evaluate the voltage output and response time of the AFM during engine idle, WOT and over-run conditions.
The Example Waveform shows the initial idle voltage rise rapidly to almost 4.0 volts, then fall to around 2.5 volts. This initial peak and drop is due to the rapid influx of air at the instant the throttle opens. The voltage then climbs to around 4 volts with the throttle fully open.
You can then see a rapid drop in voltage as the throttle closes. This is followed by a gradual reduction as the engine returns to idle.
There are a number of control units that will maintain or even increase air flow in this phase as an anti-stall protocol.
The hash on the waveform is due to fluctuations caused by induction pulses from the engine.
The Hot Wire Air Flow Meter has a few major advantages over the earlier Vane type meters in that it has very little restriction to air flow. The mass air flow is measured by the cooling effect of the incoming air on an electrically heated wire suspended in the flow. The Hot Wire sensor will also respond to air density, increased density will increase this cooling effect. Vane type meters cannot do this and require a separate pressure sensor to determine mass.
The Hot wire meter is able to react very quickly to temperature changes in the wire by increasing current to heat up or reducing current to cool down. The system attempts to maintain a constant wire temperature of around 120°C. These current changes are therefore directly proportional to air mass flow.
The current flow changes are converted by the meter into a calibrated voltage signal which is sent to the Engine Control Module (ECM), which in turn changes fuel delivery to maintain efficiency.
The heated wire will suffer from a build-up of oxides on the surface. This is negated by the fact that the ECM will clean the wire after each engine run by increasing current to get the temperature up to around 1000°C.
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.
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