How To Stress Test Ignition Systems

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cubguy's dad
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How To Stress Test Ignition Systems

Postby cubguy's dad » Sun Mar 08, 2009 12:26 pm


This topic is in the process of revision. Please be patient. Thanks CBoK Editors

Ignition System Stress Testing

Next month will mark my 31st anniversary of professional service in the Heavy Equipment and Automotive repair industry. While a lot of things have changed in this field the basics have remained the same. In my experience there are four things that an engine needs to run, Spark, Fuel, Compression and Air. (I will post my lesson on Air later)

I personally have learned not to get bogged down in the percentages of how well or how bad a component works or performs (save that for academia) as it will sometimes lead you in the wrong direction. Don’t get me wrong, I like the theory behind the operation and performance of the various systems and I can discuss these topics at length. But in my business time is money so the system or components being tested comes down to simply pass or fail.

Diagnosing a No Start/Run condition on an engine comes down to what has failed or can not perform up to a certain level that will make the engine run. Let's take a look at the Ignition System.

The first test concerning the Ignition system is does it have spark? The better question is does it have GREAT SPARK? I learned this lesson from Dale Stump when as a young man I helped him work on Evinrude Outboards one summer. Evinrude magneto coils were noted for shorting out internally but still produce enough voltage to jump the spark plug gap when cranking. Testing for spark by using the spark plug would give you false results about the health of the ignition system. I don't know how many coils I replaced when the owner said "I don't know why it doesn't run, I have spark". The following is what Dale showed me about the difference between spark and great spark. He showed me this stress test of sorts for the ignition system.

Basic ignition spark can jump a .030 spark plug gap or make the hair stand up on your arm as you hold on to it (2,000 volts?). Either of these tests will not demonstrate whether the ignition system itself will produce enough voltage for starting or operation under full load. These tests done by themselves will not lead to a definitive conclusion.

However, great spark can jump a .250 or up to a .315 gap (approx. 15,000-20,000 volts) I have seen a few magnetos jump a .375 gap but those are the exception rather than the rule. If your Ignition system will deliver enough voltage to jump a .250 (1/4") gap at all the spark plug wire ends, then your ignition system passed. This test will show whether or not that the ignition system is the cause of your hard starting or engine performance problem. If the ignition system has enough capacity to pass this test it will be able to perform under any condition that the engine will encounter. It will have essentially PASSED.

Caution, breaker point ignition, magnetos or early electronic ignitions are relatively low voltage upwards of 45,000 volts and caution should be observed because of the electrocution hazard. However, the ignition system consisting of coil on plug or coil near plug produce over 120,000 volts and fire the plug upwards of 3 times in milliseconds. This is dangerous heart stopping voltage and extreme care should be exercised when working on these modern ignition systems.

The equipment needed to stress test an ignition system is inexpensive and will produce reliable results.

Store bought Lisle tester from the local auto parts store $5.98.


My first tester made with an old spark plug, clip and a hose clamp


My current tester that is in the service truck, this one is a little easier to use and was about $10.00


Kyle's homemade one consisting of a piece plastic electrical conduit, stud and nut/bolt. It is made from the same concept as the tester shown above


Testing the ignition on Kyle's 47 cub with my old tester. This old spark plug is opened up to about 3/16" and the magneto on this tractor showed great spark.


Using my variable gap tester to max out Kyle's ignition system, Kyle's magneto was able to produce enough spark to jump a 1/4" or a .250 gap at cranking speed. That is Great Spark.


Testing for Great Spark is easy and will produce reliable results that will improve your accuracy to diagnose the actual problem.

Mark LaFollette.

editor's note:

This How To is still in the formative stages and there will be a couple revisions. In regards to the location of the spark plug tester, please understand that the tester is located here for demonstration purposes and demonstration purposes only. I am sure that all will understand the risk of testing a spark plug close to a source of an ignition and I would hope that all will take appropriate measures to test the spark safely bearing in mind the consequences of failing to adhere to good safety practices.

We should have a proper replacement pic in the near future.

The gauge from the service truck with the markings on it is supposed to represent approximate voltages. In the picture the tester is set at the 10,000 volt line or a 1/4 inch opening. The 20,000 volt line represents a 3/8 inch gap. My old "spark plug" tester has a 3/16" gap and I used it for years, but I like the adjustable testers so I am able to find max system voltage.

Here is what I found over the years that different ignition systems should put out. These figures should work for all testers store bought or homemade.

Magnetos, 6 volt breaker point battery ignition and small engine ignition systems should be able to consistently jump a 1/4 inch or .250 gap (nice blue spark)

12 volt breaker point battery ignition systems are a little stronger and should be able to jump a 5/16 to 3/8 inch gap.(nice blue spark)

Early electronic ignitions are stronger yet and will jump well over 1/2 to 3/4 inch gap.

The current ignition systems in today's cars have dangerously high voltages. 120,000 volts plus and testing these systems should be left to the experts.

I look for the spark to consistently jump for 6 times or more at cranking speed, this puts maximum load on the ignition system. Also adjust the gap up or down on you're adjustable tester to find the maximum voltage that your tractor will put out. Do this when the tractor is running good with no problems and use it for a reference later on when you do have a problem. This will avoid any guess work later.

Stress testing an ignition system will quickly tell you if the ignition system can put out maximum voltage and whether or not it is the source of your current problem.

Here are some new photos showing the testers in operation.



Experiance is knowing what NOT to do the next time.......
1937 John Deere A (Big John)
1953 Farmall Cub (LiL Red)
Lots of projects.

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