Air Flow Meter – Hot Wire

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Electrical Connections (zirconia type only)

Single wire: this wire is the self-generated voltage output from the sensor and is generally black in color.

Two wires: this will have an output wire and an output earth return.

Three wires: this will have a single output wire and two wires for the heater element (supply and earth).

The internal heating element raises the temperature to ensure faster control when starting from cold.

Four wires: this unit has a signal and signals earth return wires. The additional two wires are for the heater

element.

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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 colored (positive) plug.

3. Place the black gator clip to the battery negative terminal and probe the lambda sensors output

connection with the acupuncture or multimeter probe as illustrated in Figure 6.6.1.

Regardless of the number of wires connecting the lambda sensor to the vehicle's ECU, the output

from the sensor will invariably be on the black wire.

Figure 6.6.1

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Operation Steps:

1. Open the software, select “Vehicle->Diagnosis Setup”, as illustrated in Figure 6.6.2.

Figure 6.6.2

2. Click “Sensors-> Lambda Sensors -> Lambda Sensor Zirconia”, and click “OK”, as illustrated in Figure 6.6.3.

Figure 6.6.3

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Reference Waveform

Example waveform

Figure 6.6.5

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Note

The sensor will have varying electrical connections and may have up to four wires; it reacts to the oxygen

content in the exhaust system and will produce a small voltage depending on the Air/Fuel mixture seen at

the time. The voltage range seen will, in most cases, vary between 0.2 and 0.8 volts: 0.2 volts indicates a

lean mixture and a voltage of 0.8v shows a richer mixture.

A vehicle equipped with a lambda sensor is said to have 'closed loop', this means that after the fuel has been

burnt during the combustion process, the sensor will analyze the emissions and re-adjust the engine's fuelling

accordingly.

Lambda sensors can have a heater element to assist the sensor reaching its optimum operating temperature.

Zirconia sensors when working correctly will switch approximately once per second (1 Hz) and will only start

to switch when at normal operating temperature. This switching can be seen on the oscilloscope, and the

waveform should look similar to the one in the example waveform. If the frequency of the switching is slower

than anticipated, remove the sensor and clean with a solvent spray and this may improve the response time.

The sensor is inoperative below 300℃.

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Primary Ignition

Connect the Oscilloscope

1. Plug the 20:1 Attenuator (HT201) into channel 1 on the oscilloscope and plug a BNC test lead

(HT30A) into the attenuator.

2. Place a large black gator clip(HT18A) on the black test plug (negative) and a small red gator

clip on the colored test plug (positive).

3. Place the black gator clip onto the battery negative terminal and probe the coil's negative terminal

with the small red crocodile clip.

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Operation Steps:

1. Open the software, select “Vehicle->Diagnosis->Setup”, as illustrated in Figure 7.1.2.

2. Click “Ignition->Primary-> Primary Ignition (Voltage)”, and click “OK”, as illustrated in Figure 7.1.3.

3. Click “OK”.

Figure 7.1.2

Figure 7.1.3

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Reference waveform

Figure 7.1.4

Example 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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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.