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Specification manual for the coil
Ignition coils supply the distributor with high voltage electricity to fire the spark plugs. The coil consists of a primary winding (coil of copper wire) and a secondary winding inside of the housing and in very close proximity to each other. The primary winding is energized with battery voltage by the ignition module or, in older engines, the points. Coils may be of the rising-field type or the collapsing-field type, meaning that the secondary winding which steps up the voltage is excited when the electrical field around the primary is expanding or contracting, respectively, in a process known as mutual induction. Testing the coil can be done with a multimeter and basic mechanical skill.
Coils can not be tested with 100 percent accuracy. A coil can test good on the bench but have an intermittent open (break in a coil) that only opens when the coil is up to operating temperature. Certain IB issues may also be exacerbated by heat and not be readily apparent on a bench test. In essence, a coil can be tested as bad with confidence but not tested as good because of these issues. The best way to insure that a coil is good is to test other components in the system such as the coil wire, battery connections and associated wiring as well to eliminate them as a factor. Another good test is to have a 'known-good' coil to temporarily install and test run.
Isolate and de-energize the coil. Remove all wires connected to the coil, then unbolt it from the engine.
Check the primary windings for continuity and resistance. Set the meter to continuity (buzz box) and touch the probes to the two posts that feed the primary winding. There should be continuity. Touch one probe to one primary post and to the case of the coil. There should be no continuity. Set the meter to ohms (resistance) and touch the probes to the two primary posts. Record the reading and refer to your owners manual or service manual to see if the reading is within operating specifications.
Check the secondary winding. Set the meter to resistance and insert one probe into the distributor output port. Touch the other probe to the mounting bracket or ground wire post (if applicable) and record the reading. Refer to the manual to verify that the resistance is within specification. Set the meter to continuity and insert one probe into the distributor port and touch one of the primary posts with the other. There should be no continuity.
Interpret your findings. Low (out of spec) resistance indicates insulation breakdown (IB) inside one of the coils. Continuity between the primary coil and the secondary indicates IB between the primary and secondary coils. Continuity between the primary coil and the case indicates IB between the primary and the case. IB allows electricity to bleed over and/or out of the coils and either diminishes the output or kills it completely.