Diagnose ignition coil failure — How to
Most late model engines use coil-on-plug (COP) ignition coils. COP ignition coil provide higher voltage than coil packs because they operate with longer dwell time. In other words, they have more time to build a magnetic field before the power shuts off and the field collapse. So a COP produces a higher voltage spark when it collapses. Car makers need that hotter/longer spark to ignite leaner fuel mixtures in modern engines.
What a COP ignition coil schematic looks like
COP ignition coils can have 3 or more wires in addition to the secondary. There’s always a power and a ground. First generation COP coils work just like traditional ignition coils where the ECM or ignition modules toggles ground through a driver (transistor) located in the ignition module or ECM.
Any time you control the ground circuit with a transistor, you run the risk of a bad coil damaging the transistor. In those older vehicles, a bad coil could wipe out the driver, requiring the replacement of the entire ignition module or even the ECM.
In modern COP ignition coils, the driver transistor or triggering device is located in the COP coil. The ECM either grounds the transistor or sends a digital or analog signal to the triggering mechanism located in the COP coil.
In another advancement, some carmakers include a sensing device in the COP coil to confirm that the coil actually fired.
How NOT to diagnose a COP ignition coil
1) Disconnecting the electrical connector from a COP coil with the key in RUN or engine running
Warning: NEVER disconnect the electrical connector to the COP coil while the ignition is in the RUN position, especially on European vehicles. THIS IS AN OBSOLETE method of checking for coil failure. Doing this on a modern vehicle can cause a primary field collapse and spark that can damage the ECM.
2) Swapping COP coils to another cylinder
You’ll never see this technique in any carmaker’s service manual. If the primary circuit is shorted and has not yet damaged an ECM driver, swapping the coil can damage the driver on the new cylinder. (Hint: always check resistance and test continuity on the primary as one of your first checks. Doing this can avoid ECM damage).
3) Yanking the coil off the plug while the engine is running
If you think yanking the entire COP coil while the engine is running can tell you the condition of the coil, think again. A bad coil can produce a perfectly good spark in atmospheric pressure of 14-psi but fail to produce a spark in the combustion chamber at 150-psi+.
The correct COP ignition coil tests
1) Check for power and ground using a multimeter
2) Check primary resistance and check for short to ground or short to secondary using your multimeter
3) Use a capacitive probe to check for coil firing
4) Check for driver signal from the ECM
What causes COP ignition coil failures?
The most common cause of COP ignition coil failure is worn spark plugs! Yup, car owners are ignoring normal maintenance and driving too long on factory spark plugs. As the spark plug gap erodes, it take more voltage to jump the larger gap. In fact, a worn spark plug can require almost 80,000 volts to fire. A COP ignition coil can deliver that kind of voltage, but not for extended periods. Repeated 80,000 volt firings overheats the secondary windings and degrades the insulating materials inside the ignition coil. Then, the COP ignition coil begins to misfire.
Extra lean air/fuel mixtures can also cause ignition coil failures. A vacuum or air duct leak can cause a lean mixture that can’t be corrected by the PCM. Continued operation causes the ignition coil to overheat and fail.
Water or oil in the spark plug tubes can also cause ignition coil failures. A failed valve cover gasket, for example, can allow engine oil to seep into the spark plug tube and cause a short to ground. Or, faulty sealing gaskets at the top of a COP coil can allow water to enter the spark plug tube, especially during a pressure wash engine cleaning operation.
What happens when ignition coils misfire?
Once you create 80,000 volts, it has to seek ground somewhere. If it can’t jump the spark plug gap, it will find ground through degraded coil insulation. The high voltage can shoot through the plastic case or through the spark plug boot. In the most severe instances, the high voltage can seek ground through the ignition controller—either a free-standing ignition module, or the ignition “driver” inside the PCM.
Once the ignition module or the driver is destroyed, replacing the module or PCM without fixing the underlying cause of the failure will just damage the replacement components.
When does an ignition coil misfire?
Ignition coil misfire occurs when the engine is under heavy load, if the air/fuel mixture is consistently too lean, when spark plugs are worn, or when engine temperatures are too high.
Diagnose the root cause of the ignition coil failure before replacing
Always check the condition of the spark plugs before replacing an ignition coil. If the plug gap is too wide, the new coil will prematurely fail. Also, check fuel trim readings to see if the PCM is attempting to compensate for a vacuum or air duct leak.
Read this post to learn how to diagnose a misfire
©, 2017 Rick Muscopalt
Posted on by Rick Muscoplat