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INTERNAL SHORT CIRCUITS
Overheating of the coil caused by the aging process, a faulty ignition module, or a faulty output stage in the electronic control unit.
FAULT IN THE VOLTAGE SUPPLY
The coil charging time increases on account of the voltage supply being too low, this can lead to premature wear or overload on the ignition control unit or the output stages in the electronic control unit. This can be caused by faulty wiring or a weak battery.
MECHANICAL DAMAGE
Damage to the ignition cables caused by marten bites. A faulty valve cover gasket and resulting engine oil leaks can damage the insulation of plug slot coils. Both of these causes lead to sparkover, and thus premature wear.
CONTACT FAULT
Contact resistance in the wiring due to humidity penetrating in the primary and secondary area, also frequently caused by engine washing or the use of grit in winter.
3 SYMPTOMS OF A FAULTY IGNITION COIL: SYMPTOMS
•Engine does not start
•Vehicle misfires
•Poor acceleration or loss of power
•Engine control unit switches to limp-home mode
•Engine warning lamp lights up
•Fault code is stored
4 MEASURING THE IGNITION COIL: BASIC PRINCIPLES
Dismantled state
There are different ways of checking the ignition coil:
Testing the resistance values of the coils using the ohmmeter.
Depending on the ignition system and ignition coil design, the following reference values apply: (observe the manufacturer's specifications)
Cylinder ignition coil (transistor ignition system)
Primary: 0.5 Ω–2.0 Ω/Secondary: 8.0 kΩ–19.0 kΩ
Cylinder ignition coil (electronic ignition system with map-controlled ignition)
Primary: 0.5 Ω–2.0 Ω/Secondary: 8.0 kΩ–19.0 kΩ
Single-spark or dual-spark ignition coil (fully electronic ignition system)
Primary: 0.3 Ω–1.0 Ω/Secondary: 8.0 kΩ–15.0 kΩ
PRACTICAL TIP
Note:
If a high-voltage diode is built into an ignition coil to suppress sparks, it is not possible to measure the resistance of the secondary coil.
In this case, the following method is helpful:
Connect a voltmeter in series between the secondary winding of the ignition coil and a battery. If the battery is connected in the diode's conducting direction, the voltmeter must display a voltage. After reversing the polarity of the connections in the blocking direction of the diode, no voltage must be displayed. If no voltage is indicated in either direction, it can be assumed that there is an interruption in the secondary circuit. If a voltage is indicated in both directions, the high-voltage diode is faulty
Installed state
The following checks can be used:
Visual inspection
•Check the ignition coil for mechanical damage
•Check the housing for hairline cracks and sealant leaks.
•Check the electrical wiring and plug connections for damage and oxidation.
Check the electrics using a multimeter or oscilloscope
•Check the voltage supply to the ignition coil
•Check the triggering signal from the ignition distributor, ignition control unit, or engine control unit
•Illustration of the high-voltage curve using an oscilloscope or ignition oscilloscope
Testing with the diagnostic unit
•Read out the fault memory of the ignition system or engine control
•Read out parameters
During all testing work on the ignition system, please note that faults established during tests with the oscilloscope are not necessarily faults caused by the electronic system; they can also be caused by a mechanical problem in the engine. This may be the case, for example, if compression is too low in one cylinder, which means the oscilloscope shows the ignition voltage for this cylinder to be lower than that of the other cylinders.
NOTE
Although "diagnosable engine management systems" are installed in today's vehicles, a multimeter or oscilloscope must be used when checking ignition systems. In order to interpret the displayed measuring results and figures correctly, additional employee training is usually required. One important pre-requisite for successful diagnostics is a careful visual inspection at the beginning of the troubleshooting process.
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