Signs of coil failure

Coils are in the rather small handful of parts which, by virtue of being relatively affordable and easy to change, are commonly used in “replacement therapy” (replacing parts simply because it’s easier than figuring out what’s really causing the problem). The goal of this newsletter is to present a list of suggestions that can assist you in determining if your coil is at risk of failure before actually having a problem, and a couple suggestions on how coils behave when failing.

Some signs that your coil may be having a problem which you can inspect for and replace before a problem emerges:

1) Any noticeable dent or other mechanical damage in the outer surface of the coil would be reason for replacement. The windings and connecting circuitry within coils are rather delicate and don’t handle physical abuse very easily. Solid epoxy coils are somewhat more resistant to physical damage but not much – even a small hairline crack in an epoxy coil would be reason for replacement.

2) Any oil leakage would be reason for replacement. Metal “canister” type of coils are oil filled to aid in dissipating internal heat. Any sign of oil leakage through rust spots in the metal canister, and/or appearing at the bottom of the high voltage lug on top of the coil would be reason for replacement. 

3) In the case of internally resisted coils, less than 3.5 ohms internal primary resistance (as measured across the primary terminals) would be reason for replacement. Internal resistance of 3.6 to 4 ohms is preferred. It’s a good practice to measure internal primary coil resistance at least annually. Resistance measurements should be made at approximately the same coil temperature each time since resistance lowers slightly in colder temperatures and raises on warming temperature. Remember to remove all leads from at least one of the terminals before metering.

4) A good induction coil should produce at least ½” to ¾” of arc. Periodically check the strength of your secondary discharge by removing the coil lead from the middle of the distributor cap and holding it ¼” (or so) from the head while someone cranks the engine as if to start. Slowly move the coil lead further from the head until the secondary discharge drops out

5) Coil temperature: While it’s true that heat is not a friend to electrical apparatus, good quality coils are built and tested to operate at temperatures that are considerably higher than we would normally encounter in our application. The coils in our catalog, for example, have been “life tested” for 300 hours at 250 degrees °F.  A coil being too hot to touch is therefore usually not a reliable indication that it is about to fail.     

Typical symptoms of a coil failing:

1)  Quoting from page 3-7 of our MMI Service and Overhaul manual:  “The engine fails in a very “raucous” kind of shutdown, —– back firing, and abruptly stopping and starting”.  This type of failure happens when the breakdown of the insulation within the internal windings reaches a critical point and a short circuit establishes between the windings within the coil and the engine block. There’s also a note at the bottom of page 3-7 of the manual as follows:  “In the case of coil failure, the coil may work again after it cools; but —– the failure will most likely occur again as soon as the coil becomes hot”. We would add here that the shutdowns will probably occur in less time with each shutdown. 

2)  Complete loss of ignition resulting in engine shutdown and/or failure to start.  This failure occurs when the internal primary circuit in the coil opens up for some reason (perhaps due to external mechanical damage).  We list this possibility only in the interest of completeness.  To put the likelihood of occurrence in perspective, we can only recall such a failure occurring in our troubleshooting experiences a handful of times in the past 30 years.

While we try to never turn newsletters into infomercials, if you find yourself shopping for a new coil, we invite you to check out the coil in our online catalog. It has provided us and our customer base with excellent reliability for at least 20 years. It is a internally resisted coil and to this point we are able to use it without the need for adding any additional external resistance.

If you would like to learn more, there is a large amount of information in the ignition section of our Community Forum relative to adding external resistance to ensure that you don’t exceed 4 amps in the primary ignition circuit.