How to troubleshoot a no spark condition with a test light (Subaru)

Summary

This diagnostic video script covers the troubleshooting of a no-start issue on a 1995 Subaru Legacy. The technician follows a detailed process to check for spark, test the coils, and inspect the igniter and its control signals. By using tools like a test light, scope, and voltmeter, they determine that the igniter is faulty and not providing the correct control signals to the coils. After replacing the igniter, the car successfully starts. The video highlights the importance of accurate diagnostics, distinguishing between coil function issues and control signal problems, ensuring a precise repair.

Takeaways

• 😀 No spark from both coils is confirmed by using a test light to check for spark.
• 😀 Coil negative control wires (red and blue) show no flicker, indicating no control signal is being sent to the coils.
• 😀 The issue lies not with the coils, but with the ignition system control, particularly the igniter.
• 😀 The test light reveals a steady power feed to the coil, but no coil negative control, indicating the igniter is not functioning properly.
• 😀 The test on the igniter shows no flicker on the red and blue wires, confirming that the igniter is not receiving the proper signal from the computer.
• 😀 Ground and voltage tests on the igniter show good ground, but no voltage change when cranking, further supporting the igniter failure theory.
• 😀 The PCM (computer) is confirmed to be receiving crank signal, as indicated by the pulse seen when checking the igniter control wire with an oscilloscope.
• 😀 A voltmeter shows around 2.5 volts on the igniter control signal, indicating activity, but no actual ignition firing is happening.
• 😀 Amperage testing on the coil feed wire confirms that there is no current flow, further indicating no coil activity.
• 😀 After replacing the faulty igniter, the vehicle starts, confirming that the original issue was the igniter failure.

Q & A

What is the main issue with the 1995 Subaru Legacy in the video?

The main issue is a no-spark, no-start condition, where the car does not start due to a lack of spark from the ignition system.

How does the mechanic check for spark in the vehicle?

The mechanic uses a test light to check for spark by cranking the engine and observing the spark from each coil. Both coils fail to show spark.

What does it mean when there is no spark from either coil?

No spark from either coil indicates that the coils themselves are not the problem. This suggests a deeper issue with the ignition system, likely in the control signals or igniter.

What are the red and blue wires that the mechanic tests during the process?

The red and blue wires are the coil-negative control signals. These wires control the coils' operation by signaling the igniter to fire them.

What does the test light reveal when testing the coil-negative control signals?

The test light reveals no flicker on both the red and blue control wires, indicating there is no coil-negative control, meaning the igniter is not receiving or sending the correct signals.

Why does the mechanic check the igniter after finding no control signal to the coils?

The mechanic checks the igniter to ensure that it is functioning properly and that the wiring between the coils and the igniter is intact. The lack of control signals points to a potential issue with the igniter itself.

How does the mechanic test the integrity of the igniter circuit?

The mechanic uses a test light to check for proper coil-negative control at the igniter, verifying that the coil winding is intact and that there are no opens in the circuit.

What is the role of the computer in the ignition system?

The computer (PCM) sends signals to the igniter, controlling the timing and activation of the coils. The mechanic checks the computer's ability to send these signals to the igniter using a scope or voltmeter.

What did the mechanic find when testing the computer-to-igniter control signals?

The mechanic finds a pulse in the signals sent by the computer to the igniter, confirming that the computer is receiving the crank signal and sending the necessary control signals to the igniter.

What final test does the mechanic perform to confirm the issue is with the igniter?

The mechanic performs an amperage test using an amp probe to check for current flow through the coils. Since there is no current, it confirms the lack of coil-negative control and points to a bad igniter.