Hall effect distributor pick-up waveform

4.2.6 Hall effect distributor pick-up waveform

Hall sensors are now used in a number of ways.

Figure 4.9 Temperature sensor

The ignition distributor is very common but they

Figure 4.10 Decreasing voltage from the temperature sensor

Oscilloscope diagnostics 45

are also used by ABS for monitoring wheel speed

4.2.7 Inductive distributor

and as transmission speed sensors, for example

pick-up waveform

(Figure 4.11). This form of trigger device is a simple digital

This particular type of pick-up generates its own ‘on/off switch’ which produces a square wave out-

signal and therefore does not require a voltage put that is recognised and processed by the igni-

supply to power it. The pick-up is used as a sig- tion control module or engine management ECU.

nal to trigger the ignition amplifier or an ECU. The trigger has a rotating metal disc with open-

The sensor normally has two connections. If a ings that pass between an electromagnet and the

third connection is used it is normally a screen to semiconductor (Hall chip). This action produces

reduce interference.

a square wave that is used by the ECU or ampli- As a metal rotor spins, a magnetic field is alt- fier (Figure 4.12).

ered which induces an ac voltage from the pick- The sensor will usually have three connections

up. This type of pick-up could be described as a which are: (1) a stabilised supply voltage, (2) an

small alternator because the output voltage rises earth and (3) the output signal. The square wave

as the metal rotor approaches the winding, sharply when monitored on an oscilloscope may vary in

dropping through zero volts as the two compo- amplitude; this is not usually a problem as it is

nents are aligned and producing a voltage in the the frequency that is important, not the height of

opposite direction as the rotor passes. The wave- the voltage. However, in most cases the amplitude/

form is similar to a sine wave (Figure 4.13); how- voltage will remain constant.

ever, the design of the components are such that

a more rapid switching is evident. The voltage produced by the pick-up will be determined by three main factors:

● Engine speed – the voltage produced will rise from as low as 2 to 3 V when cranking, to over

50 V, at higher engine speeds ● The proximity of the metal rotor to the pick-

up winding – an average air gap will be in the order of 0.2 to 0.6 mm (8 to 14 thou), a larger air gap will reduce the strength of the mag- netic field seen by the winding and the output voltage will be reduced

● The strength of the magnetic field offered by the magnet – the strength of this magnetic

field determines the effect it has as it ‘cuts’

Figure 4.11 Distributors usually contain a Hall effect or induc-

through the windings and the output voltage

tive pulse generator (Source: Bosch press)

will be reduced accordingly.

Figure 4.12 Hall output waveform

46 Advanced automotive fault diagnosis

Figure 4.13 Inductive pick-up output signal

an appropriate time scale must be set – in the case of the example waveform 0–500 ms and a 0–5 volt scale. The best way to test a knock sensor is to remove the knock sensor from the engine and to tap it with a small spanner; the resultant wave- form should be similar to the example shown.

Note: When refitting the sensor tighten to the correct torque setting as over tightening can damage the sensor and/or cause it to produce incorrect signals.

Figure 4.14 Knock sensor

4.2.9 Oxygen sensor (Titania) waveform

A difference between the positive and the nega- tive voltages may also be apparent as the negative

The lambda sensor, also referred to as the oxygen side of the sine wave is sometimes attenuated

sensor, plays a very important role in the control (reduced) when connected to the amplifier circuit,

of exhaust emissions on a catalyst equipped vehi- but will produce perfect ac when disconnected and

cle (Figure 4.16).

tested under cranking conditions. The main lambda sensor is fitted into the exhaust pipe before the catalytic converter. The