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Oscilloscope testing
Interpreting wave forms
Typical wave forms - --
FI~. 22 81 FI~. 23
Oscilloscope wave form patterns can vary greatly and are dependent on many factors. Therefore prior to making a
diagnosis or changing components the following points should be considered when the wave form obtained does not
appear to be correct when compared with the 'typical' wave form.
Fig. 22
Voltage
Mgital wave tom
Fig. 23
Typical wave forms indicate the approximate position of the wave form in relationship to the 'zero grid', but this may vary
Fig. 22 [I], dependent on the system being tested and may be positioned anywhere within the approximate 'zero range'
Fig. 22 [2] & Fig. 23 [I].
Themplitu~verall height of the pattern (the voltage) Fig. 22 [3] & Fig. 23 [2], will depend on the circuit's operating
voltage.
For direct current (DC) circuits this will depend on the voltage being switched, for example, idle speed control device
voltage will be constant and will not vary with engine speed.
For alternating current (AC) circuits this will depend on the speed of the signal generator, for example, an inductive
crankshaft position (CKP) sensor's output voltage will increase with engine speed.
Therefore if the oscilloscope pattern is too high (or the upper part is missing) increase the voltage scale to obtain the view
required. If the pattern is too low decrease the voltage scale.
Some circuits operating solenoid components, for example idle speed control devices, may display voltage spikes Fig. 22
[4], when the circuit is switched off. This voltage is generated by the component and can normally be ignored. -
Some circuits that have a square wave type of typical wave form may display the voltage decaying at the end of the
switching period Fig. 22 [5]. This is a characteristic of some systems and can normally be ignored, as it does not by itself
indicate a fault. -
Frequency
The overall width of the pattern (the frequency), will depend on the circuit's operating speed.
The typical wave forms illustrated show the wave form viewed with the oscilloscope's time scale set to enable detailed
observation.
In direct current (DC) circuits the time scale will be dependant on the speed at which the circuit is switched Fig. 22 [6], for
example, the frequency of an idle speed control device will vary with engine load.
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