Deciphering the "good" and the "bad" related to modern ignition coil technology
Every technician knows it’s “AIR – SPARK – FUEL” to make a combustion event happen. If any of those three are missing, you’re unlikely to have combustion on a SI (Spark Ignition) engine. But what if you have all three and still have a misfire or a P0350 series DTC related to a specific cylinder? In this article, we'll describe the "SPARK" side of combustion along with the "good" and the "bad" related to modern ignition coil technology — the C.O.P. (Coil-On-Plug). Just as the men and women of law enforcement vary in their shapes, sizes and backgrounds, so do ignition coils of the C.O.P. design. There are 2-wire, 3-wire and 4-wire C.O.Ps in addition to coil-near-plug (which we’ll lump into the C.O.P. category).
Simple diagnostics - C.O.P. swapping
The retrieval for a P0301-P0308 DTC is almost always followed by the classic swapping of the coils, to see if the misfire DTC moves. Why? For two obvious reasons:
It’s usually a quick and easy diagnostic step.
A faulty C.O.P. is a common cause of a misfire.
Is there a downside to this common test method? Yes! Moving C.O.P.s can sometimes make an intermittent condition go into remission, which of course makes diagnostics next to impossible! Diagnostics are not always as simple as moving coils from an affected cylinder to an unaffected cylinder. Let's look at ignition coil design differences and then move on to diagnostic procedures that go beyond a simple C.O.P. swap.
2-wire C.O.P.s
Two-wire coils are the simplest design out there. There are three terminals if you include the coil’s secondary output that connects directly to the top of the spark plug. If you're an older tech who remembers oil-filled tower coils that provided HV (High Voltage) spark to a distributor, these work the same except for the trigger to the ground side of the coil’s primary winding. Although “good C.O.P.s” are Ohmic loads (they follow Ohm’s law to the letter), the average current (amps) in the coil’s primary winding (Figure 1) can vary due to:
1. Temperature
2. Saturation Time
3. Current Limiting Characteristics of the IT / IC (Ignition Timing / Ignition Control) Electronics
The latter of those three variables is always in the remote-control circuit for the 2-wire C.O.P. designs. The ECM/PCM typically is the remote driver device for 2-wire C.O.P.s providing the ground trigger (just like points did in some of our readers’ yesterdays), while the 12-volt positive power source comes from an ignition relay (or ASD relay in FCA models). Although most 2-wire C.O.P. applications have evolved to the 3-wire variations, there are many vehicles on the road still using this design.
2-wire C.O.P. diagnostics
How do you diagnose them? They’re quite simple. First, make sure you’re not chasing a wiring harness problem such as a lack of power to the C.O.P.s. (Figure 2). This applies to any design of C.O.P. You can measure the resistance across the two terminals in the harness connector (primary coil winding). This should be around 1 ohm (or slightly less) but not a dead short (0.1 ohms) or open (OL). Some 2-wire C.O.P.s feature a diode between the two primary terminals. Depending on how the meter leads were placed, there may be an open, but your meter could still test that diode. Dead shorts blow C.O.P. 12-volt supply fuses, and opens cause no-spark conditions.
Secondary winding? Connect between either of the 2 harness terminals and the secondary output (where the spark plug goes). You should measure a few thousand ohms. A few ohms? That'd be a short, and there would be no spark. OL? That might be a case where there's a short break internal to the coil secondary winding. What does high voltage secondary do? It jumps gaps! So, it might fire but might again misfire, depending on factors such as air-fuel ratio, engine load, RPMs, etc.
How does the ECM / PCM diagnose 2-wire C.O.P.s? Don’t forget other obvious lack of proper connections (i.e. C.O.P. spring or power supply relay), which can also apply to any coil design (Figures 3 and 4). Of course, it looks for misfires via the CKP sensor (P0300 series DTCs) but it also may look at C.O.P. performance. (P0350 series DTCs). Typically, a Zener diode is internal to the PCM in the C.O.P. control circuit. A Zener diode blocks reverse polarity UNTIL a certain voltage is met. When the C.O.P. fires, that voltage is met and the PCM says, “that coil fired.” The PCM also monitors the current draw of the 2-wire C.O.P. to ensure it doesn’t have a short or open circuit condition.
3-wire C.O.P.s
These are becoming the most common of all C.O.P.s. Many vehicles using the 2-wire C.O.P.s in the past now use 3-wire C.O.P.s. Many newer Toyotas that used to use 4-wire C.O.P.s now use the 3-wire design as well. The three wires (not including the secondary high voltage output) are:
1. 12-volt power
2. Ground
3. IC (Ignition Control) / IT (Ignition Timing)
You CANNOT scope the primary voltage waveform or resistance-test the primary coil winding. However, you can current-ramp (scope waveform) the primary circuit. Beyond misfires, the PCM can monitor the shape of the IC circuit it sends the C.O.P. to determine the internal condition of the assembly.
4-wire C.O.P.s
As previously stated, these are a dying breed but are still around. Toyota / Lexus C.O.P.s have the following circuits (Figure 5):
1. 12-volt power
2. Ground
3. IC
4. Diagnostic feedback circuits
GM 4-wire C.O.P.s are entirely different. They use the same circuits as their 3-wire cousins but add a “reference ground,” which aids the PCM in determining the C.O.P.’s performance.
Additional advanced diagnostics
In addition to basic visual inspection and the use of the proper air-gap tester to check for spark (no, laying it next to the manifold is NOT an accurate test for spark) there are advanced current ramping methods using a scope and an inductive amp clamp (Figure 6). Variations between brands and quality levels of C.O.P.s can be a big factor in using current ramping to sort the good C.O.P.s from the bad C.O.P.s. (Figures 7 and 8). The current ramp of the coil does NOT allow you to see nearly as much information regarding the spark event and combustion conditions as a secondary waveform analysis. That didn’t go away with those Sun and Allen scopes. They are alive and well in the form of secondary waveform adapters and time-honored pattern analysis (Figure 9). Don’t let this type of testing intimidate you. Your dad scoped ignition systems — now you go do it. Sometimes a scope pattern is the only way to sort out “the Good C.O.P.s and the Bad C.O.P.s (Figure 10)!"
