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New Tech Articles from ImportCar Magazine
Understanding Ignition Coils
Edited from an article by Gary Goms, ImportCar, January 2003
The ignition coil's role is heightened with higher operating voltages in late-model import engines. Because late-model import engines may have as many as one ignition coil per cylinder, the importance of the ignition coil in both a diagnostic and a marketing sense shouldn’t be underestimated. Mathematically speaking, the more ignition coils an engine has, the greater possibility there is of failure. And, since modern ignition designs may operate at much higher operating voltages than in years past, the more likely an ignition coil will fail if critical scheduled maintenance items like spark plugs and spark plug wires are ignored.
During the past decade, the ignition coil has undergone radical changes in design. The conventional oil-filled ignition coil, for example, has been completely replaced by compact external core (E-core) configurations that are found in distributor-type and distributorless ignition designs. The new coil-on-plug (COP) designs now being incorporated in most new import vehicles are manufactured in a much more compact configuration to allow the coil to fit directly onto the spark plug itself.
Ignition coils are also being designed to simplify the modern ignition system. To illustrate, the distributor ignition system requires a distributor rotor, cap and spark plug wires to transfer voltage from a single ignition coil to an engine’s individual spark plugs. The distributorless design relies upon only a pair of high-tension spark plug wires to transfer voltage from the coil to the spark plug. COP systems, however, provide the ultimate simplicity by eliminating the spark plug wires altogether.
As for performance, distributor systems are limited in voltage output because they generally use a single coil and therefore must operate with a very limited primary circuit on/off duty cycle in order to prevent the coil windings from overheating. Distributorless systems, on the other hand, increase the duty cycle in individual primary coil circuits by using multiple ignition coils to prevent overheating. COP designs provide the ultimate efficiency by fitting an ignition coil directly onto each spark plug in the ignition system.
THE ROLE OF THE IGNITION COIL
Regardless of design configuration, the role of the ignition coil is to multiply battery voltage into ignition voltage. Following Ohm’s law for the conversion of volts to amperes, oil-filled coils generally required from three to five amperes of primary current to produce 20,000-30,000 volts of secondary current. Modern E-core and COP ignitions require as much as seven amperes of primary current in order to produce 30,000-60,000 volts of spark output. The end result is the coil producing enough voltage to arc across a spark plug gap of .035" to .060" under cylinder pressures as high as several hundred pounds per square inch.
All ignition coils, however, share the same basic operating principles. Each must have a switching device located in the coil primary circuit that times the spark with the rotation of the crankshaft. Twenty years ago, that switching device was a set of mechanical, cam-operated ignition contact points located in a distributor. Today, the switching device is a power transistor located in an ignition control module (ICM) or in the powertrain control module (PCM) itself. In older designs, the power transistor was triggered by a Hall effect, magnetic pulse or optical sensor built into the distributor. In most modern designs, the ICM is integrated with the PCM and is triggered by a crankshaft position sensor mounted at the flywheel or front harmonic balancer.
SYMPTOMS OF A BAD IGNITION COIL
Although modern ignition coils are very reliable, they can exhibit failure symptoms that are common to all coil configurations. A cranking, no-spark condition or a severe engine backfire might, for example, characterize a coil failure. The vehicle also may exhibit a temperature or humidity-related starting or stalling complaint.
While the engine computer on an OBD I distributorless or waste-spark ignition system may monitor coil operation by measuring current flow through the primary coil circuit, a modern OBD II system incorporates a misfire monitor in the PCM that measures rotational deflections in the engine’s crankshaft caused by cylinder misfires. A misfire caused by a single bad spark plug will, for example, set a generic P0300-series diagnostic trouble code (DTC) in the PCM’s diagnostic memory. A bad coil may be indicated if a misfire DTC P0301 (#1 cylinder) and a DTC P0303 (#3 cylinder) happen to be on the coil’s companion cylinders.
HOW TO TEST IGNITION COILS
All ignition coils may be tested by measuring, a) open-circuit spark output, b) passive resistance in the primary and secondary coil circuits, and c) the current rise or "ramp" through the primary windings. The open-circuit spark test indicates a good coil when it produces a bright blue spark that jumps a 1/4-inch gap on a spark tester. This test simultaneously tests the integrity of the triggering system and ignition coil. If spark isn’t present, measuring for a switching action at the negative primary coil terminal is the next step. Because a conventional test light may not detect triggering durations or "dwell times" of as little as seven to 10 degrees of crankshaft rotation on modern ignition systems, the triggering or switching action should be measured with a lab scope or digital multimeter.
Although the resistance test is not a definitive measure of a coil’s electrical integrity, a coil should be replaced if the resistance values don’t fall within specifications. Along those same lines, the diversity of modern ignition coil designs has made ignition oscilloscope analysis more difficult because many ignition coil waveforms deviate from the conventional norm. Consequently, oscilloscope diagnosis shouldn’t be considered a definitive test of ignition coil condition unless it can be compared with a known-good waveform.
On the other hand, using a low-amperage current probe to measure the current "ramp" through the primary ignition circuit is perhaps the most definitive method of determining the electrical integrity of the coil and the quality of the triggering action. Many defective ignition coils, for example, will pass a resistance test, but fail a current ramp test. When testing multiple coil systems, the current ramp gives an excellent comparison of current flow through each coil in the ignition system and usually helps the technician arrive at a more accurate diagnostic conclusion. See Photo 1.
WHY COILS FAIL
Because worn spark plugs and open-circuit spark plug wires force ignition coils to operate at maximum output, neglected ignition system maintenance is the most common cause of modern ignition coil failure. As the spark plug air gap widens due to normal erosion, increased voltage is required to create a spark in the combustion chamber. This increased voltage, in turn, demands more current flow through the coil’s primary circuit. And, this increased primary current flow can overload the ICM’s primary transistor. The increased secondary current created by the ignition coil also can increase to the point that it perforates the secondary circuit in the coil itself. See Photo 2.
SELLING IGNITION COILS
Although ignition coils are not a routine replacement item like spark plugs, a coil replacement is a highly recommended preventive maintenance item in many cases. To better illustrate, excessive current draw caused by shorted ignition coil windings can damage the ICM. Some Honda models with the coil integrated into the distributor are particularly sensitive to this issue. In addition, a coil should be replaced if it shows traces of spark perforation, carbon tracking or overheating.
A coil should also be replaced if it has been saturated with battery acid or other corrosive agents that decrease the dielectric strength of the insulating material. Last, in many cases where it’s apparent that diagnostic time for locating an intermittent no-spark condition will exceed the cost of the coil itself, the coil should be replaced as a faster, more economical method of eliminating an intermittent ignition system failure.
IGNITION COIL REPLACEMENT
1. When the ICM fails: Shorted primary windings in an ignition coil can overheat the ignition control module (ICM) and cause a repeat failure.
2. When the spark plug wires fail: Open-circuit spark plug wires can create up to 60,000 volts of potential electrical energy, which can cause an invisible "tunneling" failure inside the coil windings or a more visible carbon burn or tracking on the coil tower.
3. When the spark plugs are neglected: Neglecting to replace worn spark plugs can cause an ignition coil failure similar to open-circuit spark plug wires. If an ignition misfire continues after spark plug replacement, the coil is a good place to look.
4. When an ignition coil is cracked, burned, severely discolored or corroded from acids or other chemicals.
5. When a temperature-related ignition system performance complaint occurs: Defective ignition coils are notoriously sensitive to extreme changes in ambient temperature and humidity. An ignition coil test or replacement may be called for when a "weather-related" performance complaint is being investigated.
Gary Goms owns the Buena Vista Auto Clinic in Buena Vista, CO. He is an ASE-certified Master Auto Technician, ASE L-1 Advanced Engine Performance Technician, and a MACS- and IMACA-certified Air Conditioning Technician.