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          A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

  Oscilloscope 3






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Although the oscilloscope is also used in the workshop for diagnosing the injection- and the charging system, in this case we would like to deal more precisely with the ignition system. The primary- secondary tension is applied through time and is constantly updated. This is what causes such a restless image. Of course, the image can also be frozen at any time.

If this is not done, one can see the sparking tension and afterwards, directly observe whether e.g., possible charges from the ignition coil are going to the wrong spark plug, in other words, errors are apparent. The following shows the high-voltage flow (secondary) of a transistorized ignition system.



Secondary voltage power
1Spark duration
2Post-oscillation phase
3Dwell angle

At the beginning of phase 1, the primary current is interrupted. It only takes 30 micro-seconds, for the tension in the secondary-coil of the ignition coil to increase to 15.000 volts and more. The distance between the centre- and the earth electrode (or vice versa) is ionised, i.e. conductive. The tension-needle, seen in the oscilloscope image, marks the so-called 'spark-head', followed by the 'spark-tail'. So, a considerably lower tension is necessary to maintain the spark.

Should the energy in the secondary coil no longer be sufficient, after 0,8 to 2,2 milliseconds the phase 2 begins with the oscillation resonance. At this point, the remaining energy finally peters out. The switching-on of the transistor marks the start of the third phase. From now on, as preparation for the ignition of the next cylinder in the firing order, the primary coil is charged. In the enlargement, one can see the oscillations, which, through the charging of the magnetic field and the thereby occurring countervoltage are created.



The display-mode 'Parade', can easily be noted, because in this case, the signals of the four cylinders are displayed one after the other, like soldiers on a parade-ground. This way, tension-differences between the individual cylinders, can be clearly determined. They are shown in the firing order.



The display-mode 'Stroked', is more suitable to determine the timing differences. In this case, starting at the bottom, one must designate the cylinders according to the firing order. As a rule, at least the older workshop-oscilloscopes do, offer the possibility to select either function. The four signals can also be arranged directly one above the other.

Here are a few further possible errors ...



In this case, the tension at cylinder 4, thus the third in the firing order, rises too strongly. This exaggerated rise is apparently necessary to overcome any resistances or gaps. It can of course, be too high on all the cylinders, then the cause must also affect all the cylinders. In this case, no individual ignition cable can be responsible.

Voltage needle is too high
Worn out spark plug(s).
Broken ignition cables
Injection error
Low compression

Voltage needle is too low
Dirty spark plug(s).
Electrode gap is too small
Insulator is defective
Leaky injector-valve
Low compression



Possible causes
High ignition cable resistance
Resistance in the high voltage distribution
Electrode gap is too large
Leaky injector-valve




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Translator: Don Leslie - Email: lesdon@t-online.de

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