Air Flow Meter – Hot Wire

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Air Flow Meter – Hot Wire

An air flow meter is used in some automobiles to measure the quantity of air going into the internal combustion engine.

All modern electronically controlled diesel engines use air flow meter, as it is the only possible means of determining the

air intake for them. In the case of a petrol engine, the electronic control unit (ECU) then calculates how much fuel is

needed to inject into the cylinder ports. In the diesel engine, the ECU meters the fuel through the injectors into the

engines cylinders during the compression stroke.

The AFM is once again located between the air filter and the throttle butterfly. Inside the component are two wires, one

of which is used to convey the temperature of the incoming air and the other heated to a high temperature (approximately

120℃) by passing a small current through it. As the air flows across the heated wire, it has a cooling effect on it causing

a temperature change; a small circuit inside the component increases the current passing through the wire to maintain

the temperature, and it is the recognition of this current that signals to the ECM the mass air flow.

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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

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Example Waveform

Figure 6.4.5

Notes:

Neither wire of the injector should be connected to the negative (ground) input of the oscilloscope as this could

cause a short circuit.

The 20:1 Attenuator is used to monitor the induced voltage that is created when the earth path to the injector is

removed. This voltage will be in the region of 60 to 80 volts.

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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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Figure 6.1.3

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Reference waveform

Figure 6.1.4

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Example waveform

Figure 6.1.5

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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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Camshaft Sensor

Figure 6.2.5 Idling 1500

Notes:

The camshaft sensor is sometimes referred to as the Cylinder Identification Sensor(CIS) and is used as a reference to

time the sequential fuel injection. The signal waveform can be either a permanent magnetic sine wave or in this case a

digital square wave.

The Electronic Control Unit (ECU) needs to see the signal when the engine is started for its reference; if this is absent,

it can put the ECU into 'limp-home'.

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Throttle Position - Switch

The throttle position sensor is sometimes referred to as the throttle switch. It measures and reports the

amount of throttle opening to the Engine Control Module (ECM). The ECM uses this information to make

adjustments to the mixture of fuel and air used for combustion. As the throttle is opened, the engine requires

more fuel in the mixture to provide needed power.

The throttle switch is a 3 wire twin contact device that is operated by the throttle butterfly spindle. This

will provide information to the Electronic Control Module (ECM) for throttle closed, throttle open and

wide open throttle position (90% open). When the engine is at tick over the idle contacts are closed,

as the throttle is opened these contacts open, sending a signal to the ECM to indicate cruise/part load

condition. At the 90% wide open throttle position the other contacts are closed and the signal is sent to

the ECM to provide extra fuel enrichment for acceleration.

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Connect the oscilloscope

1. Plug a BNC test lead(HT30A) into channel 1 on the DSO3064(A), place a large black gator clip (HT18A)

on the test lead with the black moulding (negative) and an acupuncture(HT307) or multimeter probe onto the

test lead with the red moulding (positive). Place the black gator clip onto the battery negative terminal and probe

the throttle switch with the acupuncture or multimeter probe.

2. Plug a BNC test lead into channel 2 on the DSO3064(A) and an acupuncture or multimeter probe onto the test

lead with the red moulding (positive). Probe the throttle pedal switch with the other acupuncture or multimeter as

illustrated in Figure 6.3.1.