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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Reference waveform

Figure

6.5.4

Example Waveform

Figure 6.5.5

Notes:

When the positive fired plug kV's are recorded on distributor less Ignition Systems (DIS), the voltage should

be as shown and not inverted, as this would suggest that the wrong lead has been chosen.

While the engine is running, the plug voltage continuously fluctuates and the display moves up and down.

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Follow these steps:

1. Click “Vehicle->Diagnosis”, as illustrated in Figure 6.5.2

2. Click “Ignition->Secondary-> Secondary DIS (Positive-fired)”, click “OK”, as illustrated in Figure 6.5.3.

Figure 6.5.2

Figure 6.5.3

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Connect the oscilloscope
1. Plug an auto-ignition probe (HT25) into channel 1 on the oscilloscope.
2. Clip the lead's crocodile clip on suitable earth.
3. Plug the Coil-on extension lead(HT308) into the engine's positively-fired plug of an air cylinder.
4. Fit the HT25 clip on the Coil-on extension lead as illustrated in Figure 6.5.1.





Figure 6.5.1

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The modern engine with Distributorless Ignition System (DIS) has all the advantages of a constant energy

electronic ignition system, but with the added bonus of the distributor cap, king lead and rotor arm being

removed from the system. Reliability problems from dampness and tracking are now almost eliminated.

DIS has its own drawbacks by having half of the plugs firing with an acceptable negative voltage, while

the other half are fired by the less acceptable positive polarity. This has the effect of increased plug wear

on the positive fired plugs.

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The ignition probe(HT25) voltage is produced by mutual induction between the primary winding and the

secondary winding, with the central soft iron core intensifying the magnetic field between them.

The voltage measured at the spark plug is the voltage required to jump the plug gap in varying conditions.

This voltage is determined by a spark plug gap, rotor air gap etc.

The plug kilovolt (kV) requirement of older engines tends to be lower than that of the modern engine, as

the later designs run higher compression ratios, leaner air/fuel ratios, and larger spark plug gaps.

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Secondary-Distributorless (Positive Fired)

Technical information

Ignition coil is also referred to as a spark coil and used to transform lower voltages of power to the higher

voltages of power required to fire a system's spark plugs. It is similar to an electric transformer, consists

of primary and secondary winding circuits. Inside the coil's primary winding is the secondary winding.

This is coiled around a multi-laminated iron core and has approximately 20,000 to 30,000 turns.

One end is connected to the primary terminal and the other to the coil tower.

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Dwell period

Dwell is measured as an angle: with contact ignition, this is determined by the points gap. The definition

of contact ignition dwell is: 'the number of degrees of distributor rotation with the contacts closed'.

As an example, a 4 cylinder engine has a dwell of approximately 45 degrees, which is 50% of one

cylinder's complete primary cycle. The dwell period on an engine with electronic ignition is controlled

by the current-limiting circuit within the amplifier or Electronic Control Module (ECM).

The dwell angle on a constant-energy system expands as the engine speed increases, to compensate

for a shorter period of rotation and maximise the strength of the magnetic field. The term 'constant energy'

refers to the available voltage produced by the coil. This remains constant regardless of engine speed,

unlike contact ignition where an increase in engine speed means the contacts are closed for a shorter time

and gives the coil less time to saturate.

The induced voltage on a variable dwell system remains constant regardless of engine speed, while it

reduces on contact systems. This induced voltage can be seen on a primary waveform.

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Technical information

The primary ignition is so called as it forms the first part of the ignition circuit. Through the ignition coil,

it drives the secondary High Tension (HT) output. The primary circuit has evolved from the basic contact

breaker points and condenser to the distributorless and coil-per-cylinder systems in common use today.

All of these ignition systems rely on the magnetic induction principle.

Magnetic Induction

This principle starts with a magnetic field being produced, as the coil's earth circuit is completed by either

the contacts or the amplifier providing the coil negative terminal with a path to earth. When this circuit is

complete, a magnetic field is produced and builds until the coil becomes magnetically saturated. At the

predetermined point of ignition, the coil's earth is removed and the magnetic field collapses. As the field

inside the coil's 250 to 350 primary windings collapses, it induces a voltage of 150 to 350 volts.

The induced voltage is determined by:

· The number of turns in the primary winding

· The strength of the magnetic flux, which is proportional to the current in the primary circuit

· The rate of collapse, which is determined by the speed of switching of the earth path

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Reference waveform

Figure 7.1.4

Example Waveform