Ignition systems - testing

Ignition systems / Ignition systems - testing

Warning: Extreme care must be taken when working on the system with the ignition switched on; it is possible to get a substantial electric shock from a vehicle’s ignition system. Persons with cardiac pacemaker devices should keep well clear of the ignition circuits, components and test equipment. Always switch off the ignition before disconnecting or connecting any component and when using a multi-meter to check resistances.

General information
1 Comprehensive fault diagnosis of all the engine management systems described in this Chapter is only possible with dedicated electronic test equipment. Problems with the systems operation that cannot be pinpointed by following the basic guidelines described in this Section should therefore be referred to a VW dealer for assessment. Once the fault has been identified, the removal/refitting sequences detailed in the following Sections will then allow the appropriate component(s) to renewed as required.

2 Most ignition system faults are likely to be due to loose or dirty connections or to ‘tracking’ (unintentional earthing) of HT voltage due to dirt, dampness or damaged insulation, rather than by the failure of any of the system’s components. Always check all wiring thoroughly before condemning an electrical component and work methodically to eliminate all other possibilities before deciding that a particular component is faulty.

3 The old practice of checking for a spark by holding the live end of an HT lead a short distance away from the engine is not recommended. Similarly, no attempt should be made to ‘diagnose’ misfires by pulling off one HT lead at a time. Not only is there a high risk of an electric shock, but the HT coil and/or exhaust catalyst could easily be damaged.

4 Problems with the systems operation that cannot be pinpointed by using the guidelines in the following paragraphs should be referred to a VW dealer for assessment.

Engine will not start
5 If the engine either will not turn over at all, or only turns very slowly, check the battery and starter motor . Connect a voltmeter across the battery terminals (meter positive probe to battery positive terminal), disconnect Hall sender multiway connector from the distributor, then note the voltage reading obtained while turning over the engine on the starter for (no more than) ten seconds. If the reading obtained is less than approximately 9.5 volts, first check the battery, starter motor and charging systems (see Chapter 5A).

6 If the engine turns over at normal speed but will not start, check the HT circuit by connecting a timing light (following the manufacturer’s instructions) and turning the engine over on the starter motor: if the light flashes, voltage is reaching the spark plugs, so these should be checked first. If the light does not flash, check the HT leads themselves followed by the distributor cap, carbon brush and rotor arm using the information given in Chapter 1.

7 If there is a spark, check the fuel system for faults referring to the relevant part of Chapter 4 for further information.

8 If there is still no spark, then the problem must lie within the engine management system. In these cases, the vehicle should be referred to a VW dealer for assessment.

Engine misfires
9 An irregular misfire suggests either a loose connection or intermittent fault on the primary circuit, or an HT fault on the coil side of the rotor arm.

10 With the ignition switched off, check carefully through the system ensuring that all connections are clean and securely fastened.

11 Check that the HT coil, the distributor cap and the HT leads are clean and dry. Check the leads themselves and the spark plugs (by substitution, if necessary), then check the distributor cap, carbon brush and rotor arm as described in Chapter 1.

12 Regular misfiring is almost certainly due to a fault in the distributor cap, HT leads or spark plugs. Use a timing light (paragraph 4 above) to check whether HT voltage is present at all leads.

13 If HT voltage is not present on one particular lead, the fault will be in that lead or in the distributor cap. If HT is present on all leads, the fault will be in the spark plugs; check and renew them if there is any doubt about their condition.

14 If no HT voltage is present, check the ignition coil; its secondary windings may be breaking down under load.