Today, we are going to focus on spark, and how to verify the engine is getting what it needs from the ignition system. Not only do we need the three basic elements of compression, fuel, and spark to make an engine work, but these elements also need to happen at the right time and in the right amount. For the majority of engines you’re working on, that spark is supplied via a Coil-On-Plug (COP) style ignition system, controlled and/or monitored by the Engine Control Module (ECM).
How does any ignition system work?
The heart of the system is the coil assembly. The coil has two internal windings, the primary and the secondary. The primary has fewer windings than the secondary does, making the coil a step-up transformer.
When the ECM completes the primary circuit, the current flow through the windings creates a magnetic field. When the ECM opens the circuit, the magnetic field collapses and induces a magnetic field in the secondary (Figure 1). The higher number of windings in the secondary steps up the voltage originally supplied through the primary to levels as high as 70,000V or more, depending on the coil design.
We can gain a more in-depth diagnosis using our scope. Before we do, we need to understand what to expect by understanding the different types of COP systems in use:
- 2-wire system
- 3-wire system
- 4-wire system
2-wire system
The two-wire COP connector will have a 12V power supply and a switching signal (ground path). The two-wire COP design doesn’t have an integral ignition module. Most two-wire COP designs rely on the PCM (some use a remote mounted ignition control module) to directly control dwell and timing to each COP unit by controlling the ground side of the circuit.
The two-wire COP may use internal PCM electronics to detect the primary voltage spike, created when the primary current flow is interrupted, to confirm that the coil fired. An open or unplugged coil connector will set a P035x (x indicates the COP number) coil control circuit code.
FCA (Fiat/Chrysler Automotive) PCMs monitor the primary pattern burn time and use this information to set coil ionization fault codes. If the PCM detects that the secondary ignition burn time is too short, incorrect, or not present, a code will set. Pay particular attention to the diagnostics as these “secondary circuit, insufficient ionization” codes don’t easily point to the correct COP assembly.
Testing the 2-wire system is straight forward and can easily be tested with or without a scope. Begin first by testing for spark output. This can be done using an adjustable spark tester (Figure 2). To demo testing without a scope, I've used the multimeter built in to the Autel MS919. However, any capable multimeter or DVOM will do:
1. Check for power (12V) at the two-wire COP connector with it disconnected. For some OEMs, you may have to crank the engine to see 12V present. If there isn’t 12V, refer to a wiring diagram. If 12V is present, test the switching signal using your test light.
2. Connect the test light to the B+ and, with the coil disconnected, insert the tip of the tool on the ground pin of the connector. You should see the light flash or at least change in intensity when you crank the engine. You can also use the "Frequency or Hertz" mode of the meter to confirm the ECM control of the coil.
3. With the coil connected, attach one meter lead to the battery ground and the other to the ground side of the coil connection. You should see a fluctuating frequency as the circuit is being opened and closed (Figure 3).
4. Connect one channel to the coil ground control circuit. Use a connection point that’s convenient. If you can access the connector at the coil, connect there. If the coil is hiding under the plenum, look for a common harness connector or go straight to the ECM (Figure 4). When selecting the voltage setting, many scopes (including the MS 919) allow you to select the probe type you're using and will adjust the scaling appropriately to match. By selecting the attenuator that you’re using, you won’t have to do any math to determine the actual voltage being measured.
5. If you have one, connect a low amp probe to another scope channel, select the 20A scale and "zero" the clamp before placing the probe. By monitoring and comparing current patterns, you can detect any electrical issues like open circuits or voltage drops caused by poor or corroded connections.
6. Start the vehicle and capture the pattern. If you’re not familiar with these patterns, the MS919 has an online database you can access. To make sure you have the latest information, you need to keep your tool updated.
3-wire system
The two-wire system accomplishes the same goal but does so a bit differently. The controlling electronics (which, in the two-wire design, act like the PCM) are housed internally to the coil itself. There is no way to access the primary circuitry voltage directly. Instead, you would either have to current-ramp the coil (as described above) or rely on verifying the command signal presence from the "third" wire.
The first two wires provide the voltage and ground (to allow the coil to function, just like any electrical output device). However, the third wire comes from the PCM/ECM/Igniter/Ignition Control Module (depending upon configuration). This "third" wire is the command to the coil, to function. The ECU sends the command to the coil (The internal switching device is a transistor). When the coil receives the command, the internal component of the coil allows current to flow through the primary windings. The coil itself (step-up transformer) functions the same as the two-wire device (Figure 5).
4-wire system
The four-wire COP ignition coil type isn’t as popular as the three-wire COP, but it is being used by major manufacturers in two variations. Testing these coils is like what I’ve shown you on the three-wire coil, and while you can test these with your multimeter, a scope is a much better way to check the operation of these systems.
The first variation (Toyota and Mercedes-Benz) uses three of the four wires in the connector for power (12V), ground and an ECM controlled trigger (very similar to the three-wire COP), but the addition of a fourth wire provides feedback to the ECM that the coil has actually done what the ECM asked it to do.
The Toyota version calls this fourth wire the Ignition Confirmation Signal (IGF) and uses this signal to provide fail-safe fuel cut-off if a coil has failed and isn’t firing the spark plug, protecting the catalytic convertor. The Toyota version provides a 5V bias signal on the IGF circuit that runs parallel to all the ignition coils. Internal electronics in each COP monitor the coil operation and pulse the IGF to ground when the coil fires successfully. The PCM will recognize a lack of pulse and set a trouble code for the coil that failed to fire (Figure 6).
The second variation four-wire COP design has a power (12V) and ground circuit, a trigger circuit (that GM refers to as Ignition Control or 'IC') and the fourth circuit wire is called “Reference Low.”
This reference low wire is an auxiliary ground that is provided to the COP unit, directly from the PCM. Its primary purpose is to provide a clean ground to the low current circuits and electronic in the COP, while the other body ground is managing the coils’ primary coil current. If this reference low wire or circuit loses connection to ground, it is possible for the COP to use the other ground circuit to maintain functionality.
GM’s IC trigger circuit is monitored by the PCM for opens and shorts (to ground/power) and uses a 5V signal to command COP operation. The system is quick to set trouble codes if it sees an issue on the IC line. It’s worth noting that VW/Audi also uses a four-wire COP coil and similar to GM, there is a 12V power, a 5V trigger and two grounds that both terminate on the cylinder head/cam cover.
The power output stage (terminology for 'ignition controller') uses one ground and the coil primary circuit uses a separate ground. Testing ignition systems begins with the understanding of how the systems are configured electrically, and a solid understanding of how the tools you choose to implement function.
I hope you find this helpful. Continue to be inquisitive so that you can continue to further your driveability diagnostic skills, and remember, good tools are as important as good skills.
