Yesterday, a question arose about the best methods for testing Coil on Plug (COP) ignition systems. While a definitive, one-size-fits-all strategy can be elusive due to varying symptoms (like no-code misfires) and vehicle accessibility, a consistent approach is crucial for efficient diagnostics. Over time, I’ve developed a reliable method, and I’m eager to share it and explore if others in the field employ similar or more effective techniques. Here’s my go-to approach for testing COP ignition, aiming to provide you with a clear “How To Test Coil On Plug Ignition Coil” methodology.
1. Initial Assessment: Secondary Signal Inspection
Whenever possible, my first step in testing coil on plug ignition coil systems is to examine the secondary ignition waveform using a COP probe. This initial check offers a comprehensive overview. If the secondary waveform appears normal, it generally indicates that the coil itself, its power supply, the switching mechanism, and related components are functioning correctly. Furthermore, experienced technicians can glean valuable insights from the secondary waveform, potentially identifying issues like fouled spark plugs or lean cylinders. In straightforward cases such as idle misfires, analyzing all cylinders can often be completed swiftly, sometimes within minutes.
Tip: To streamline comparison, I typically retain the waveform from the first cylinder as a reference point. If a subsequent cylinder’s waveform deviates significantly from this baseline, it immediately flags that cylinder for closer inspection and further testing to pinpoint the root cause of the discrepancy in the “how to test coil on plug ignition coil” context.
2. Primary Current Testing: A Practical Alternative
In situations where access to the coils is restricted, testing the primary current becomes a valuable initial step in how to test coil on plug ignition coil systems. I prefer to assess all ignition coils simultaneously by positioning a current clamp around the common 12V power supply feed. Often, the fuse box provides a convenient access point for this measurement. However, caution is necessary to ensure the fuse circuit is not shared with other critical components, which could skew the readings and complicate the diagnostic process of how to test coil on plug ignition coil effectively.
At this stage, I often employ a second current clamp to simultaneously monitor the injector current. This dual clamp approach allows for a broader system overview. (I’m known for leveraging current clamp diagnostics directly from the fuse box to gain a holistic view of electrical system behavior.)
Tip: When analyzing both injector and ignition current patterns, observe if both are absent for a specific cylinder. This could indicate that an ignition issue has triggered the Engine Control Unit (ECU) to disable the injector for that cylinder as a protective measure against unburnt fuel damaging the catalytic converter. To investigate this, cycle the ignition key off and then on. Then, observe the current patterns in the initial seconds after engine start-up to see if the injector and ignition activity resumes, helping to understand how to test coil on plug ignition coil systems in complex scenarios.
A significant advantage of both the secondary signal and primary current tests is their versatility. They are applicable regardless of whether you are working with 2, 3, or 4-wire COP systems, simplifying the initial diagnostic approach for how to test coil on plug ignition coil across different vehicle configurations. If these initial tests pinpoint a problem within a specific cylinder, the subsequent steps become component-specific, invariably involving back-probing the coil wiring to isolate the fault.
3. Deep Dive: Testing 2-Wire COP Systems
In 2-wire COP systems, the ignition switching is managed by an external ignition amplifier, which can be a standalone module or integrated within the ECU. Some sophisticated ECUs utilize the inductive primary voltage kickback as a feedback mechanism to verify ignition events. Consequently, a malfunctioning coil in these systems may trigger diagnostic trouble codes (DTCs), aiding in the process of how to test coil on plug ignition coil effectively.
To test a 2-wire COP:
- Channel A – Switching Signal (Primary Voltage): Connect channel A of your oscilloscope to measure the primary voltage, which represents the switching signal controlling the coil.
- Channel B – 12V Feed: Use channel B to verify the presence and stability of the 12V power supply to the coil, ensuring there are no voltage drops that could affect coil performance in the context of how to test coil on plug ignition coil.
Tip: If direct access to the coil terminals is challenging, the switching signal can often be measured at the ignition amplifier or ECU connector. This centralized access point allows for efficient signal acquisition from all cylinders from a single location, streamlining the comparative analysis when learning how to test coil on plug ignition coil systems.
Bonus Tip: Be aware that many Ford and some other manufacturers’ 2-wire COP systems employ multi-strike ignition at low RPMs to reduce emissions. This characteristic waveform is normal and should not be misinterpreted as a fault during your “how to test coil on plug ignition coil” procedures.
4. Advanced Diagnostics: Testing 3-Wire COP Systems
Three and four-wire COP systems integrate the ignition amplifier directly into the coil assembly, eliminating external ignition modules and changing the diagnostic approach for how to test coil on plug ignition coil. In these systems, the ECU triggers ignition by sending a (typically) 5V pulse to the coil. This integrated design enhances system reliability and can often reduce repair costs, particularly in ECUs with built-in ignition amplification capabilities.
For testing 3-wire COP systems:
- Channel A – 5V Trigger Signal: Connect channel A to measure the 5V trigger signal from the ECU that initiates coil firing.
- Channel B – 12V Feed: Use channel B to confirm the presence and stability of the 12V power supply to the coil.
- Channel C – Ground (GND): Channel C should be connected to the ground to ensure a good ground connection is present, which is crucial for proper coil function and accurate testing when learning how to test coil on plug ignition coil.
Tip: A diagnostic drawback of many 3-wire systems is that the ECU often cannot directly verify if ignition has actually occurred. In fact, in many cases, you can disconnect a 3-wire coil entirely without the ECU registering a fault code. This represents a step backward diagnostically compared to 2-wire systems where ignition feedback was more common. If a 3-wire system ECU can detect ignition problems, it typically does so by monitoring the amplitude of the trigger pulse to detect open or short circuit conditions, which is important to consider when you how to test coil on plug ignition coil effectively.
The example below illustrates a 3-wire COP system, including the primary current waveform for a comprehensive view. I often split my oscilloscope screen (using “Add View” functionality) to display trigger and feedback signals at the top and power/ground signals at the bottom, optimizing the display for clarity during “how to test coil on plug ignition coil” procedures.
5. Comprehensive Analysis: Testing 4-Wire COP Systems (with Feedback)
When dealing with 4-wire COP systems, the initial task is to determine if it’s a true 4-wire system incorporating a feedback pulse to the ECU. Some 4-wire configurations, particularly in many General Motors (GM) vehicles, utilize an extra ground wire from the ECU instead of a feedback signal. These can be considered “3-wire COPs with an extra ground wire.” Many of these systems, however, can still set fault codes by analyzing the trigger signal amplitude, which helps in diagnosing how to test coil on plug ignition coil problems.
True 4-wire COPs may have:
- Two Ground Wires: For enhanced grounding.
- Feedback Signal: A dedicated signal wire that sends a pulse back to the ECU each time the ignition coil fires, providing direct confirmation of ignition events and crucial data for “how to test coil on plug ignition coil” diagnostics.
For testing 4-wire COP systems:
- Channel A – 5V Trigger Signal: Connect channel A to measure the ECU’s 5V trigger signal.
- Channel B – Feedback Signal or 2nd Ground: Use channel B to identify if there is a feedback signal present or if it is a second ground wire, depending on the specific system design.
- Channel C – 12V Feed: Verify the 12V power supply stability using channel C.
- Channel D – Ground (GND): Ensure a solid ground connection with channel D.
Tip: Feedback signals in 4-wire systems are typically wired in parallel from each cylinder to minimize wiring complexity, while trigger signals are individually routed to each coil. Expect to observe more feedback pulses per engine cycle than trigger signals due to the parallel configuration. If you see an equal number of trigger and feedback signals, it might indicate an open circuit in the feedback wiring for that specific coil, a key diagnostic insight when learning how to test coil on plug ignition coil systems.
Bonus Tip: The specific channel order in your oscilloscope setup is not critical for how to test coil on plug ignition coil. While a standardized setup can be helpful, you can often deduce the signal type based on the waveform characteristics. Starting with a timebase of 20ms/division is a good practice as it allows you to capture sufficient data without immediately needing to configure triggering. You can always add a trigger later once you have identified each channel’s signal type.
In conclusion, this outlines my standard procedure for how to test coil on plug ignition coil systems. However, the field of automotive diagnostics is constantly evolving, and I am always keen to learn from the experiences of others. If you have alternative or more effective methods, I encourage you to share them, as collaborative knowledge sharing benefits the entire automotive repair community.
