Ignition Coil Failures and Diagnosis
15-04 Dissect ignition coil failures.
The leading cause of ignition coil failure does not involve the coil itself. Often, the underlying cause of a damaged coil comes from external sources that go unaddressed during the repair. Issues include incorrect spark plug gaps, carbon tracking, moisture intrusion, and on COP applications, leaking valve covers. While damage from oil leaks can crop up on ignition systems that use spark plug wires, it is a much more common occurrence in COP systems.
Coil failures affect each system design differently. In multi-coil applications, only one or two cylinders will be affected. In a distributor (single-coil) ignition system, a failing or failed coil affects all the cylinders equally. Weak coil output in a single-coil system may cause a misfire or poor performance, especially under load. If the coil fails entirely, the vehicle will not start. In a COP system, a weak or damaged coil will affect only one cylinder. In a waste spark system, a bad coil will lead to one cylinder missing and may result in both paired cylinders missing.
When diagnosing ignition coils, remember that when the magnetic field collapses, the high-voltage energy must find a return path to ground somewhere. If a failed spark plug or plug wire prevents the normal flow of current across the plug’s electrodes to ground, the voltage will find another path—through either the coil driver or the coil. Depending on the location of the coil driver, the high voltage can damage the ICM/igniter, PCM, or the coil’s insulation as it returns to ground. Repeated ICM or PCM transistor failures are usually a result of high secondary resistance. Check the spark plug gap, spark plug wire, or COP high-voltage spring before replacing a module or PCM suffering from a driver failure. Also, when testing, do not remove a spark plug wire or COP coil and allow it to fire into the air without a ground path. To prevent damaging ignition coils and ICMs, always ground the plug wire or install a spark tester between the COP boot or spark plug boot and ground.
Poor-Quality Construction
Coil failure can occur due to poor manufacturing or excessive stress. The most common failure in an ignition coil is a breakdown of the winding’s insulation (primary or secondary) due to high secondary resistance. Faults that occur during assembly may be from using inferior materials or a shoddy manufacturing process on lower-cost coils. Undesirable resistance can result from a host of secondary ignition problems or a lean air-fuel mixture, forcing the coil to operate at maximum output for an extended time, which overheats the coil and insulation.
Occasionally, a coil will fail due to faulty construction. Failure to properly evacuate the air pockets, poor insulation on the winding, or a crack in the epoxy are all typical problems resulting from a failing coil. Construction faults typically occur due to the different expansion rates of the dissimilar materials. These materials all expand and contract as the temperature changes at different speeds. The effects of thermal expansion stress the epoxy over time, causing it to crack. Remember that a coil contains several dissimilar materials. Copper forms the windings, and the largest piece of the coil is the soft iron lam stack (core). Due to the disparity in the materials of the coil housing, epoxy, and windings, expansion rates vary. These differing expansion rates stress the epoxy, causing cracks.
The epoxy also serves as a flexible adhesive that bonds to the bobbin and wires of both the primary and secondary windings, increasing the insulating qualities of the coil. Thermal cycling can cause the epoxy to separate from the bobbin, forming a crack that allows the high voltage along the crack to ground instead of being sent to the spark plugs. Breaks also allow moisture into the coil, causing the windings to short. While there is nothing a technician can do about the construction, remember that lower-quality coils will suffer far more failures compared to a well-built coil from a reputable manufacturer. Using low-quality coils is an invitation for repeated failures.
External Coil Failures
If the construction of the coil did not cause it to fail, then look for possibilities that occurred while the coil was in use. A primary cause of coil failure is excessive stress (overload) from high secondary resistance or an open secondary circuit. Keep in mind that the coil’s output will rise only as high as required for the spark to return to ground. The actual firing voltage needed to create a spark across the plug’s electrodes depends on the electrical resistance of the spark plugs, spark plug wires or springs (COP), the size of the air gaps at the spark plug, and rotor to distributor cap if equipped (FIGURE 15-17). Non-ignition system variables include the air-fuel ratio, engine rpm, engine load and the plug’s temperature. Coil output voltage is continuously varying depending on conditions from as low as 5 kV to as high as 30 kV or more. In a properly operating high-voltage system, including the spark plugs, under most working conditions, the voltage will rise only to what it takes to jump the spark plug gap and typically will not exceed 15 to 20 kV. Under peak demand, most modern coils can produce 40 kV or more.
FIGURE 15-17 Distributor ignition and waste spark systems both use spark plug wires that can cause drivability complaints and coil failures. In addition to a misfire, high resistance in a spark plug wire causes the coil to work harder to produce the voltage needed to overcome the resistance, causing overheating and early coil failures. Refer to the service information for specifications. Use an ohm test only to verify a bad, high-resistance, or open wire. Do not rely on a resistance test to prove a wire is working correctly. Remember that a resistance test is a static test that fails to stress the component.
Worn spark plugs and, if equipped, a worn rotor and distributor cap terminals all increase the gap(s) that the spark must jump to return to ground. A spark plug’s gap is crucial to the operation and health of an ignition coil. A worn or excessive gap causes the ignition coil to work harder. As the air gap increases, more voltage is needed to bridge the gap and create the spark necessary to ionize the air-fuel mixture in the cylinder. The growing energy demands create more current flow through the coil primary. As the current flow increases, or remains higher than optimal, it can overload the primary switching transistor.
A spark plug, a plug wire, or a COP’s high-voltage spring that is open or has excessive resistance can increase the coil’s output voltage to a point where it overheats the insulation, causing a short in either set of windings. Also, be aware of spark plug wires suffering damage where the conductor pulls out of its metal sockets at either end. This is normally a result of careless servicing. Sustained operation of 30 kV or more can damage the coil’s insulation. Shorted windings decrease the coil’s output efficiency, forcing it to work harder to output the needed voltage, degrading the coil even more. Once the insulating qualities have been compromised, the coil’s output voltage drops, often causing an intermittent ignition misfire under load. If the short is severe enough, the coil may fail to produce any voltage, leading to a constant misfire. A shorted coil in a conventional ignition system may result in a no-start or an engine that will not stay running.
TECHNICIAN TIPRepeated ICM or PCM failure may be due to a failing ignition coil. Internal shorting and arcing overloads the internal circuitry and the coil driver, causing recurring failures. If a coil has failed or is failing, inspect all the cylinders for worn or damaged spark plugs, wires, or COPs. Another source of module and ignition coil failure is a grounded control wire. The short causes excessive current to flow through the primary circuit, causing overheating and subsequent failure.
A coil that is working at or near maximum voltage output stresses the coil internally as well as its high-powered driver (switching transistor) in either the PCM or ICM. Coil drivers can fail in one of two ways: intermittently or entirely. Intermittent faults of the either the coil or its driver result in an erratic spark that frequently occurs under high loads or from heat buildup after extended operation. Total driver or coil failure will lead to no spark and a dead cylinder.
Additionally, lean air-fuel mixtures, open or high-internal-resistance spark plug wires, and poor connections at the spark plug or (if used) at the distributor cap require higher-voltage output from the coil. The higher the required voltage (kV) output of the coil, the greater the stress. An open secondary circuit due to a plug wire that has fallen off at either end, an open spark plug, spring (COP), or spark plug cable will force the coil to operate at its peak output as the spark attempts to reach ground. Extended operation at or near maximum output is damaging to both the coil and the module or PCM. Coil output can reach or exceed 40 kV on newer coils during an open-circuit condition.
Corrosion or poor connections at the coil’s negative and positive terminals will reduce the available voltage to the coil, forcing it to work harder to generate the voltage needed to jump the spark plug gap and return to ground. To find high resistance in either circuit, perform a voltage drop test or check the voltage at the terminals by using a lab scope. Check pin fit and terminal tension by using the appropriate terminal end or flex probe to find loose connections.
Moisture is another cause of ignition coil damage and can be from either oil or water. One of the leading causes of moisture damage, especially on COP systems, is a leaking valve cover gasket. Oil leaks past the O-ring seal or gasket and fills the well in the cylinder head. Oil gradually erodes the boot, and the high voltage bypasses the spark plug and arcs to the cylinder head instead. This can lead to ignition coil and spark plug failure. Oil leaks can also contaminate and degrade spark plug wires in a conventional ignition system or a DIS. Whenever servicing a coil or spark plug that has been contaminated with oil, repair the cause of the oil leak at the same time as the ignition system is repaired. Failure to fix the oil leak will result in repeatedly needing to replace the ignition component.
Water intrusion is less likely and therefore often overlooked. Cracked epoxy is one of the primary causes of water intrusion in a coil. Cracked epoxy reduces insulating and sealing qualities, allowing high voltage out and moisture from rain, snow, or humidity in. Water can also enter the coil or spark plug wells from water runoff. A windshield cowl could be directing moisture under the hood to the coils, or air condition (A/C) condensation could be building up on the lines and hoses and then dripping directly on top of the coils, attacking the electronics and filling the spark plug wells. Pressure washing the engine can also force water into the spark plug holes. Over time, the moisture can short the windings, reducing the coil’s output potential and thus forcing it to work harder to deliver the spark needed. If moisture is present, always check for any relevant technical service bulletins (TSBs). Several manufacturers have issued bulletins to prevent water from contaminating the coils. TSBs may include adding or resealing cowl seals, altering the cowl drain, relocating heater hoses, or replacing faulty heater hose clamps. Others may recommend applying dielectric grease to the inside of the coil boot to help it seal to the porcelain body of the spark plug and spark plug terminal, preventing moisture from interfering with high-voltage transfer from the coil through the spring and to the spark plug.