Note: This presentation is tailored toward the 1999 to 2004 Ford Mustang 4.6L V8’s coil-on-plug (COP) ignition system–however the fundamentals apply to the testing of the ignition coils on any modern 4-stroke/cycle engine.
I had been seeking a method to test coils, actually COPs (coil-on-plug), for my '03 Mustang and it came to me that the K2543 would be the perfect driver for doing so–so, I bought one…
Here’s a photo of the complete set-up, and a WMV video of it in action–I will provide details below:
[color=#0040BF]Click here to watch the WMV[/color] (if you can’t see the sparks download and play the file, your internet connection bit-rate may be too slow to see them).
Also used is an Ampro T71240 adjustable gap spark tester that I picked up someplace several years ago for less than $10. Any similar unit would work.
Here is the interconnect diagram:
[color=#FF4000][size=150]Warning! (Danger Will Robinson!):[/size][/color]
[color=#FF4000][size=150]Do NOT press and hold the trigger switch for more than 30 seconds as this could cause the COP to overheat (not likely, but it might).
Also do NOT fire the COP more than 10 times in rapid succession, without waiting 3-5 minutes between tests. Doing so 15-20 times or more can and likely will cause the COP to overheat and burn out.[/size]
[/color]
This is because in normal use the coil very rarely, if ever, dumps its full power in one big fat maximum voltage spark. On an n/a engine the maximum gap ionisation voltage is 10kV to 12.5kV, maybe 15kV to 18kV worst case–this drops to less than 3kV to 5kV during the actual spark event.
Running f/i the ionisation voltage rises to maybe 18kV to 22kV, the spark event voltage is only a bit higher than on an n/a engine.
Setup and interpreting results:
The COP should have markings on or about the primary connector indicating the + connection. For the stock, and stock “look-a-like” units, this will be the pin on the right when holding to COP upright and looking at the connector.
This device is best used with an adjustable spark gap tester like that I linked to above. Set the gap to 3/8" or so, then press the trigger switch for 1 second and release.
A good COP should fire across a 3/8" gap with no difficulty–if it doesn’t then it’s weak and failing, replace it.
Next open the gap a bit at a time. 1mm or so is a good amount, and retest. If it fires then keep opening the gap until the COP fails to spark across the gap. Now close the gap in 0.5mm increments until the COP can once again fire the gap.
The gap width is now a solid indicator of the COP’s maximum output voltage.
It takes approximately 3.3kV/mm to ionise an air gap at STP (Standard Temperature and Pressure). So by measuring the maximum gap across which the COP can arc (in mm) and multiplying by 3.3 you can calculate the COP’s maximum real world output voltage.
Using the Accel COP (with 20k miles on it¹) shown in my first post I found the maximum gap across which it will fire was 15.0mm, making its maximum output capability = 3.3 * 15.0 = 49.5kV.
Testing an OEM COP (with 80k miles on it) I found it could fire across a 16.8mm gap = 55.4kV–so much for after market COPs.
¹ - I bought the Accel COPs NIB at a swap meet in Daytona Beach, and ran them for 20k miles or so before within a week’s time two failed–one completely and another was misfiring badly under load.
Using this test device the dead one is indeed dead. The misfiring one will fire a normally gapped (0.054", 1.4mm) plug in air, however its maximum output voltage is less than 6kV (it could only fire across a gap of 1.7mm).