Warning: The voltage generated by the electronic ignition system is significantly higher than that of a conventional system. Special care should be taken when the ignition system is on. If you have a surgically installed pacemaker, stay away from ignition circuits, components, and test equipment. Always turn off the ignition before disconnecting or connecting components and when using a multimeter to check resistance.
General information
1. Ignition system components are usually very reliable: most faults are due to loose or dirty connections, high voltage leaks due to dirt, dampness or damaged insulation. Before suspecting any component of the system, first carefully check all the wiring and methodically make sure that the fault is caused by this particular component.
2. The old practice of checking for spark by holding the spark plug end of the high-tension lead to the engine is not recommended, not only because you risk getting a severe shock, but also because you can damage the electronic control unit (ECU) high-tension coil, or commutator. Likewise, never try to "diagnose" ignition failures by removing the high-tension lead caps from the spark plugs one by one, since in addition to the risk of shock, the test may be useless, since later models have an ignition system that has the ability to detect and temporarily disable high-tension circuits that are open.
3. If problems occur that are not described in the following subsections, please contact a service station.
If the engine does not start
4. If the engine either does not turn over at all or turns over very slowly, check the battery and starter. Connect a voltmeter to the battery terminals (positive probe of the voltmeter to the positive terminal of the battery), disconnect the ignition coil high-voltage wire from the distributor cap and measure the voltage with the ignition on (no more than ten seconds) starter. If the voltmeter reads less than 9.5 V, check the battery, starter, and charging system as described in Part A of this chapter.
5. If the engine turns over at normal speed but does not start, check the high voltage circuit by connecting an ignition test light (in accordance with the manufacturer's instructions) and turning the engine with the starter: if the indicator flashes, the voltage is reaching the spark plugs, so you should check them first. If the indicator does not flash, check first the high-voltage wires, then the distributor cap, the carbon of the central electrode and the "runner", as described in chapter 1.
6. If there is a spark, check the serviceability of the power supply system as described in chapter 4.
7. If there is no spark, check the voltage at the "+" terminal of the ignition coil. The cause of the malfunction may be the ignition / engine management system. In this case, contact a service station.
Ignition failures
8. Irregular ignition failures indicate that either the contact has weakened or some intermittent fault has occurred in the primary winding circuit of the ignition coil, or a fault in the high-voltage circuit from the ignition coil to the distributor rotor.
[Original source of information: MERCEDESMAN]
9. With the ignition off, carefully check the entire system, making sure all connections are clean and secure. If you have the appropriate equipment, check the low voltage circuit as described above.
10. Make sure the ignition coil, distributor cap and high-voltage wires are clean and dry. Check the high-voltage wires, spark plugs yourself (replacement if necessary), then check the distributor cap, the center electrode carbon and the distributor rotor ("runner") as described in chapter 1A.
11. Regular ignition failures most often occur due to faulty distributor cap, high-voltage wires and spark plugs. To check for high voltage on all wires, use the ignition indicator mentioned above (see item 5).
12. If there is no high voltage at the tip of one of the wires, then this wire or its tip or distributor cap is faulty. If there is high voltage at all wires, then the spark plugs are faulty - check them and replace them if necessary.
13. If there is no high voltage on any of the wires, check the ignition coil - under load, a current leak may occur in the secondary winding.