771 Tests On Distributorless Ignitiondis

Firstly, it is important to observe 'Key skill 7: work in a safe manner'. Electric shocks from ignition systems must be avoided - not only is the shock dangerous in itself but it can also cause involuntary muscular actions which can cause limbs to be thrown into contact with moving or hot parts.

The secondary voltage trace is often used in the analysis of ignition system performance. Figure 7.32 shows the portable oscilloscope set-up for this test. The inductive pick-up is clamped to the HT lead, as close as possible to the spark plug.

Note that the screen display shows data about engine speed and burn time. These and other details may more easily be seen by taking an enlargement of the trace for a single cylinder, as shown in Fig. 7.33. With reference to this figure the details are as follows.

1. Firing line. This represents the high voltage needed to cause the spark to bridge the plug gap.

Fig. 7.32 The oscilloscope set-up for obtaining a secondary voltage trace from a DIS

Fig. 7.33 Details of the HT voltage trace for a single cylinder r*—*r

Fig. 7.33 Details of the HT voltage trace for a single cylinder

2. The spark line.

3. Spark ceases.

4. Coil oscillations.

5. Intermediate section (any remaining energy is dissipated prior to the next spark).

6. Firing section (represents burn time).

7. Dwell section.

8. Primary winding current is interrupted by transistor controlled by the ECM.

9. Primary winding current is switched on to energize the primary. The dwell period is important because of the time required for the current to reach its maximum value.

A comparison of the secondary voltage traces for each of the cylinders should show them to be broadly similar. If there are major differences between the patterns then it is an indication of a defect. For example, a low firing voltage (1) indicates low resistance in the HT cable or at the spark plug. The low resistance could be attributed to several factors, including oil- or carbon-fouled spark plug, incorrect plug gap, low cylinder pressure or defective HT cable. A high voltage at (1) indicates high resistance in the HT cable, or at the spark plug. Factors to consider here include a loose HT lead, wide plug gap, or an excessive amount of resistance which has developed in the HT cable. Table 7.1 summarizes the major points.

Table 7.1 Factors affecting firing voltage for high and low firing voltages


High firing voltage

Low firing voltage

Spark plug gap



Compression pressure



Air-fuel ratio



Ignition point



Sparking plugs can be removed and examined, HT leads can also be examined for tightness in their fittings and their resistance can be checked with an ohm-meter. Resistive HT leads are used for electrical interference purposes and they should have a resistance of approximately 15 000-25 000 ohms/meter length.

Here, as in all cases, it is important to have to hand the information and data that relates to the system being worked on.

Do It Yourself Car Diagnosis

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