cliffyk

I have been pondering ways to make a practical device for testing COPs, under actual operating conditions, for some time; and a couple weeks ago while looking at a sales flyer from an electronics supplier I have dealt with I ran across the perfect core component. I had seen this device in the past but as it is decidedly "old-hat" it had slipped my mind (gimme a break, I'm old).

Here's a photo of the complete set-up, and a WMV video of it in action--I will provide details below:



Click here to watch the WMV (if you can't see the sparks download and play the file, your internet connection bit-rate may be too slow to see them).

The core component, with the heat sink in the center of it all, is a Velleman Electronics K2543 CDI (Capacitive Discharge Ignition) driver module that can be had as a kit from Arcade Electronics for $20.31 (currently on sale).

This kit is designed to retrofit a classic Kettering (points/condensor/coil) ignition system to a CDI driver, however it can also serve here to drive a COP when a simple normally open momentary contact switch is used to simulate the distributor points.

I have also used an Ampro T71240 adjustable gap spark tester that I picked up someplace several years ago for less than $10. Any similar unit would work.

Shown in the photo and video is one of my five remaining good Accel COPs, none of the 3 bad ones will fire at all even though they all check out OK with a low voltage Ohmmeter.


If anyone is interested I will draw up an interconnect schematic, it works quite nicely to test an individual COP, or to compare a set.

cliffyk

I wanted to add that this tester goes beyond just seeing if the COP can fire a lousy 0.054" plug gap, and in fact can destroy a COP (by overheating) in less than 90 seconds by making it fire repeatedly at its full potential.

In practical use a COP is never called upon to deliver more than 20kV to 25kV tops, repeatedly, in even the highest compression/chamber pressure engine. This tester, when using an adjustable gap spark device will make a COP use it's full potential of 40kV+--doing this repeatedly, over a short period, will burn out even the most robust COP.

The breakdown voltage of air at STP is around 33kV/cm, in the video above the gap is about 0.55", or 1.4cm. So the voltage being demanded of the COP is 46kV or so...

H0SS302

NICE. You should see about fabbing up a box for an all in one unit n selling em man.

stangalator

so would/should the stock system fire a bigger gap then .054? neat idea btw.

Crazycarnut

Cliff you are the man.

cliffyk

It could, however there is no need to do so--a 0.054" gap produces a plenty enough "big-***" spark to get things going. It is also important to keep in mind that this is an ambient air test--as combustion chamber pressures rise, due to normal operation, higher compression ratios, or forced induction the voltage needed to ionise the plug gap also rises.

The best case condition is normal ambient air. The good news is that normal or close to normal ambient air conditions are easily reproducible; this makes this device great for comparing COPs within a set, or one brand to another.

What this tester can do is measure a COPs full potential. If you use the adjustable gap device to determine the maximum gap a COP can fire (carefully because you can burn out a good COP with this set-up) you will have determined a measure of its maximum potential output voltage.

This is what makes this test device a true test of a COP's condition. Testing a COP with a typical low voltage ohmmeter does not validate the unit's high voltage integrity. Electricity will always find its way to "ground" through the path of least resistance, however that path may and often will be different at high voltages (and high frequencies) as are typical in an gap ionisation/spark event.

A low voltage ohmmeter is incapable of testing this; as I said above all three of my failed Accel COPs will pass a simple resistance test--but when tested using the new fixture one will not fire at all, and the other two can barely fire across a 0.025" gap.

sailorflavor

Nice Cliff I like it! Can you get that schematic? And I have been thinking of a way to test fuel injectors as well (a fuel injector test stand) any ideas on that one Cliff?

cliffyk

Here is the interconnect diagram:



Warning! (Danger Will Robinson!):

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.


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.

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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).

Sxynerd

You may also right click, "save target link as", download the file, and then play it. (Mac users, do what you do.)

There are issues with linked videos & ubuntu so saving the link may be the best option for you guys as well. (Me too.)

cliffyk

Sailor, I never answered your injector question; what parameters are you looking to test?

In the past I have used a pressurised container of solvent to back and forward flush injectors (make sure the switch used to run the injector is in a tight enclosure), and used an oscilloscope to record the pulse waveform to compare injectors in a set--this will show any mechanical binding on opening and/or closing as surges or sags in the pulse's trace.

You could also capture the fluid over a timed period at a known pressure, and measure it to determine flow rate.