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Difficulty: MediumIgnition System

A technician is diagnosing an engine no-start condition on a vehicle equipped with an electronic ignition system. Testing reveals that current is continuously flowing through the primary coil winding because the ignition control module fails to open or switch the circuit. What is the direct consequence of this failure on the ignition system?

  1. A
    The alternator cannot generate alternating current to recharge the vehicle battery during cranking.
  2. B
    The air-fuel mixture delivered to the intake manifold becomes excessively rich.
  3. The primary magnetic field cannot rapidly collapse, preventing the induction of high secondary voltage needed to fire the spark plugs.Answer
  4. D
    The engine shifts its combustion timing to ignite during the exhaust stroke rather than the power stroke.

Answer

The primary magnetic field cannot rapidly collapse, preventing the induction of high secondary voltage needed to fire the spark plugs.
In an automotive ignition system, high voltage (typically 20,000 to 50,000 volts) is produced through mutual induction. Low-voltage battery current flows through the primary winding to build a strong magnetic field. When the ignition control module interrupts this primary current, the magnetic field collapses abruptly across the secondary winding, inducing the high voltage required to jump the spark plug gap. If the primary circuit is never opened, the magnetic field remains stationary, and no spark can be generated.

Step-by-Step Solution

1
Identify the operating principle of an automotive ignition coil.
An ignition coil acts as a step-up transformer that stores energy in a magnetic field created by low-voltage current in the primary winding.
Understanding transformer action is necessary to determine what triggers high-voltage generation.
2
Analyze the effect of interrupting primary circuit current.
When the ignition control module or switching transistor turns off (opens the circuit), the primary current drops to zero instantly, causing the magnetic field to collapse rapidly across the secondary winding.
Electromagnetic induction requires a rapid change in magnetic flux to induce high voltage (thousands of volts) across the thousands of secondary wire turns.
3
Determine the outcome if the primary circuit fails to open.
If current remains constant and does not collapse, no voltage is induced in the secondary winding, resulting in no spark at the spark plug.
Constant direct current creates a stationary magnetic field, which induces zero voltage.

Key Concept

Electromagnetic induction in ignition coils via rapid primary magnetic field collapse
Estimated Time:1m 0s
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Topics

Ignition System

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