Ignition Coil Service

15-06 Service ignition coils.

For an ignition system to work properly, an ignition coil(s) must step up system voltage from 14 volts to anywhere between 12,000 and 45,000 volts to ignite the air-fuel mixture under normal operating conditions. Regardless of the design—conventional oil filled, epoxy filled, waste spark coil packs, or single coil per cylinder—they all work the same and can suffer the same damage.

Like all ignition components, coils are subject to normal wear. Since most manufacturers do not supply ignition coil service intervals, they are typically due to a fault. Typical service life expectancy is between 35,000 and 80,000 miles (56,000 and 130,000 km), depending on operating conditions.

Coils, for the most part, are relatively reliable, but they can suffer damage from a variety of sources and for many reasons. The main culprit of coil failure is excessive heat caused by worn or damaged spark plugs or wires forcing the coil to work harder. When a coil overheats, the insulation surrounding the wiring breaks down, causing shorts. A short reduces the resistance, resulting in excessive current flow through the coil windings. Low supply voltage, vibration, and corrosion can also shorten the lifespan of a coil.

During routine system maintenance, verify the integrity of the complete secondary ignition system. Visually inspect the coil for signs of physical damage, and ensure that it is securely mounted to reduce vibration (FIGURE 15-25). A damaged housing can allow moisture to enter the coil, causing corrosion to form on the windings, increasing the coil’s resistance.

FIGURE 15-25 DIS coil packs are usually easy to access for testing and replacement. A Ford 3.0 L engine with the coil mounted onto the top of the front valve cover.

Check battery voltage to the coil. Remember that power is supplied to the positive side of the coil. The negative terminal of the coil connects to the module (points) or PCM. Low battery power to the coil and damaged secondary ignition components increase the coil’s charge time, resulting in overheating. Another cause of the coil overheating is a control device in the ICM or PCM sticking on, flowing excessive current through the windings.

Replacing an ignition coil is usually a straightforward procedure. Coil location varies based on system type and manufacturer. Conventional coil designs in breaker-point and electronic distributor systems are generally easily accessible. A design popular among Asian manufacturers is placing the coil under the distributor cap. If a coil is internal to the distributor, there may be a cover over it, shielding the pickup coil/signal generator from RFI developed by the coil. For example, some distributor-equipped Hondas from 1996 through 2002 will set a false P1381 CYP (Honda’s abbreviation for a CKP sensor) due to EMI if the shield is missing. Depending on the manufacturer, the coil may be serviceable separately, or it may be replaceable only with the distributor. Another location for the coil, favored by GM on its HEI system, is on top of the distributor cap (FIGURE 15-26). The coil sits in a recess of the large distributor cap, making the distributor a self-contained ignition system after supplying it with 12 volts.

FIGURE 15-26 A GM HEI distributor with an integral coil. This setup does not require a coil wire. The voltage from the coil is transferred directly through a carbon button to the rotor. The distributor cap must be large enough to mount the coil in the center, meaning the terminals are spread farther apart, which helps prevent cross fire. When current flows to the positive terminal of the coil, the distributor operates as a self-contained ignition system. Note: Not all GM HEI systems use an in-cap coil; some use an externally mounted coil.

Waste spark systems generally mount the coils on or near the engine. Typically, these coils are found near the valve cover(s) on V-type engines or at the rear of an in-line front-wheel-drive (FWD) four-cylinder engine vehicle if not attached to the coil. CNP coils mount on or near the valve covers. COP coils mount directly over the spark plug for each cylinder. Access to a COP coil may require removing the upper intake plenum since in many late-model engines, at least one bank of COP coils is usually covered by the plenum.

The following are general guidelines for COP replacement:

• Disconnect the negative battery cable.
• Locate the ignition coil(s).

  •  Some coils maybe hidden underneath a beauty cover.

• Gasoline direct-injected (GDI) coils are typically located under a sound-deadening cover.

  •  The cover helps reduce the ticking noise associated with the fuel injectors and the high-pressure fuel pump heard in the passenger compartment.

• If required, remove the air intake upper plenum.
• Using shop air, blow off any dirt and debris from around each coil to prevent it from falling into the wells when removing the coil.
• Disconnect the electrical connector(s).

  •  If the primary wires are not part of a connector, label them to prevent crossing the battery feed and ground circuits.

• Remove the hold-down hardware. This can include bolts, clips, screws, and brackets.
• For most conventional, waste spark, and CNP systems, the coil(s) can now be removed and replaced.
• To remove a COP coil, slowly twist the coil back and forth to break the seal between the boot and spark plug.
• While twisting, pull straight up on the coil body.
• On some systems, the coil will have to be maneuvered past some fuel injectors or other engine obstructions.
• To prevent damage, some COP coils require a special tool to remove the coil from the cylinder head (FIGURE 15-27).
• Clean the spark plug threads in the cylinder head by using a blast of compressed air.
• Before installing the coil, apply a thin layer of dielectric grease on the inside of the rubber boot.
• Correctly torque the coil hold-down bolt.
• Connect the coil wiring and secure any locks or assurance position connectors.
• Connect the battery, start the vehicle, and check for misfires.

FIGURE 15-27 A specially designed ignition coil removal tool to safely remove push-down coil packs for Volkswagen (VW)/Audi engines on the left and GM’s version for Ecotec engines on the right. The VW tool is intended to grasp the coil securely and safely, preventing damage during removal. The GM tools thread into the top of the coil/module housing so that they can be pulled up equally. Using other makeshift methods, including pry bars or screwdrivers, may damage the coil.

Coil-on-Plug Boot Service

A common failure to be aware of on some COP systems occurs at the coil boot. The boot deteriorates over time due to high voltage, extreme thermal cycling due to its location, and vibration (FIGURE 15-28). As the boot ages, the firing voltage can leak out and fire to the cylinder, causing a misfire. If only the boot has failed and the mileage on the coil is not excessive, replacing only the boot is an industry-accepted repair. Having the option just to replace the boot, instead of the entire coil assembly, is a much less costly option for the customer. Due to the relatively low cost of coil boots and springs, some technicians also replace these components when they service the spark plugs, as a preventive measure (FIGURE 15-29).

FIGURE 15-28 COP coil boots are subject to extreme operating conditions that intensify when a spark plug begins to misfire. The high voltage attempts to find an “easier path to ground,” jumping from the COP boot to the cylinder head. When removing the coil for routine service, or when troubleshooting a misfire, inspect the COP boots carefully. Look for discolored spots that appear as a white/grayish spot or circle on the boot where the energy was firing through the boot to ground. At first, the spot may be difficult to see. As the condition worsens, the spot will become larger. Another option is to lightly mist the boot with water or wrap it in aluminum foil. Reinstall the COP and retest. If the misfire condition worsens or becomes more apparent, replace the boot and spring or COP assembly. Note: If the COP is in a hard-to-access location, swap the COP and spark plug with a known-good cylinder that is easier to get to in case it needs to be replaced after testing.

FIGURE 15-29 When servicing spark plugs on a COP vehicle, replacing the boots and high-voltage stainless steel springs, if available, is recommended. Over time, heat and high voltage begin to break down the rubber and can cause a misfire. When replacing the boots, apply a light coating of dielectric grease to the inside of the boot.

High-Performance Coils

For performance applications, especially on forced induction or highly tuned engines, the increase in cylinder pressures escalates the demand on the ignition system. While original equipment coils may deliver up to 50,000 volts, they are only manufactured to do so occasionally and for only a very brief time. Consistently working at maximum output to provide the high-energy spark needed to overcome elevated cylinder pressures will lead to coil failure and possibly module/igniter failure. Performance aftermarket and racing coils are designed to provide sustained voltage outputs exceeding 40,000 to 50,000 volts without damage. For racing applications, some of these coils can exceed 100,000 volts.

In the world of high performance, where more of anything is always considered better, merely installing a coil that outputs more voltage does not guarantee better performance. It is true that upgrading the coil can result in an increase in power, but the coil must also be able to maintain the additional voltage under load with virtually no drop-off. In addition to peak kV, the coil must have enough reserve energy to sustain the spark event. Spark duration is a critical component of the coil’s ability to maximize complete combustion. The longer the spark burn time, the better the chance of fully igniting and burning the air-fuel mixture, generating maximum power output. If the coil fails to provide the necessary energy to the spark plugs through the entire operating range of the engine, there will be little to no increase in performance.