Tech feature — COP ignition

How to test COP ignitions

THE RECIPE NEVER CHANGES: PETROL ENGINES NEED THE RIGHT AIR AND FUEL MIXTURE, COMPRESSION, AND A GOOD SPARK

Acoil-on-plug (COP) ignition system monitored by the engine control module supplies the spark in most modern petrol engines.

THE BASICS

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 ‘stepup 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 coil (Figure 1). The higher the 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.

There are three different types of COP systems in use: two, three, and four-wire systems.

TWO-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 PO35x (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 ionisation fault codes. If the PCM detects that the secondary ignition burn time is too short, incorrect, or not present, a code will be set. Pay particular attention to the diagnostics as these ‘secondary circuit, insufficient ionisation’ codes don’t easily point to the correct COP assembly. Figure 2. This device is used to load the ignition system. As the gap increases, so does the load. A faulty ignition coil will fail to create a strong enough spark to jump the gap

Testing the two-wire system is straightforward and can be achieved with or without a scope. Begin first by testing for spark output. This can be done by 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 it 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 MS919) 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 maths 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.

FOUR-WIRE SYSTEM

Figure 3. A DVOM can be used to evaluate the operation of the trigger circuit. When set to Hertz, an increase in frequency should be expected if the circuit is functioning properly

Figure 4. Using a scope lead and piercing probe to test for available voltage. Note: Be sure to seal the wire insulation after testing if piercing probes are utilised

THREE-WIRE SYSTEM

The three-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 on 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 twowire device (Figure 5).

Figure 5. Using a lab scope for coil testing provides a visual representation of the circuit functioning. As the trigger circuit is energised, the coil begins to dwell. When the trigger circuit is de-energised, the coil discharges a spark

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 as per the three-wire coil. You can test these with your multimeter but 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 converter. 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 recognise 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 electronics 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 understanding how the systems are configured electrically, and a solid understanding of how the tools you choose implement function.

Figure 6. The purple wire at the ECM represents ‘IGF’. This is the feedback circuit to provide the ECM with a confirmation that the Ignition event occurred. This circuit is parallel by design and therefore only one input is available for all coils commonly