Testing Ignition Coils

[whizkidkyus]

Is there any reasonably priced tester that a person can buy to test ignition coils ( magnetos ) on lawnmowers ( any horse power,any brand ,one or two cylinders ) weed eaters , blowers , chainsaws , and cars ( coil packs ), etc. Or, is there a way to reliable build one yourself at home cheaply from spare parts ? Or, does anyone know of a way to do it with a voltmeter ? I know about pulling the plug wire off or the plug out and shorting them to a ground and watching for spark . I want something that will check them if they are already off the motor. Thank you for any and all help.

Thanks,
Whizkidkyus

 

[JDemaris]

Without a coil tester that tests a coil under load, best way (with a VOM) is to check resistance across the primaries first, and then between one primary and the secondary. Across thye primaries will be low reading (usually 1 to 3 ohms) and from one primary to the secondary -(8,000 to 15,000 ohms). If you get a zero reading (no resistance), or and infinity reading - you've got a bad coil.

 

[whizkidkyus]

By the way , I'm talking mostly about electronic coils .

Whizkidkyus

 

[tech4]

You can google testing ignition coils and come up with a lot of ideas. Basically most coils are just transformers with a low voltage on the primary and high voltage on the secondary. I have seen circuits where a low frequency transistor switching circuit was injected on the primary and look for the spark on the high voltage side.
My experience has been that the coils open which is easily found with an ohmmeter or more common is that there are internal winding shorts that only show up at high voltage and the ohmmeter check will not show up. A simple tester is to use an audio oscillator (set for about 20 Hz) on the primary and a neon bulb on the HV side and the light should glow with about 1 volt of audio signal for a 100:1 coil. You will have to play around with some known good coils to come up with correct signal levels that work.

 

[JDemaris]

What do you see as a difference between what you call an "electronic coil" and others? A coil is usually just two coils of wire - i.e., a primary and secondary circuit -regardless if used in a self-contained magnet-powered magneto, a breaker-switched battery ignition, or a high energy breakerless ignition in a newer car or truck.

 

[PJH]

OK, am I missing something? It's been a while since I've had a small engine coil in my hand, but I can only remember the coils having two leads - the one that is connected to the points, or electronic replacement for the points, and the high lead connected to the plug. I see where a short could be detected, but without the flywheel magnets passing by this coil, I don't see a genuine pass/fail test. For instance, I had a Honda coil that would run perfectly for four minutes, then fail. You could almost set your watch to it. I had a hard time turning loose of $60+ for a coil that worked so good part of the time. New coil fixed it.

I always dreamed about taking an old engine, maybe even a dead one with no rod/piston, belt driven, and rigging some kind of universal coil mount on it to test coils. I know - a hillbilly thought, but my first battery charger, in 1965, was a generator spun by an old washing machine motor.

Skinflint Paul

 

[JDemaris]

There are three connections. #1 is primary, #2 (to the spark plug) is the secondary, and #3 is ground which is common to both.

You can make two tests with a ohm-meter. #1 between the ground and the primary, and #2 between the ground and the secondary.

Most of the time a bad coil can be detected with a simple ohm-meter. But some coils have intermittent breaks in a wire or insulation and only show problems when tested under load.

 

[JMOR]

(quoted from post at 10:39:06 02/06/10) OK, am I missing something? It's been a while since I've had a small engine coil in my hand, but I can only remember the coils having two leads - the one that is connected to the points, or electronic replacement for the points, and the high lead connected to the plug. I see where a short could be detected, but without the flywheel magnets passing by this coil, I don't see a genuine pass/fail test. For instance, I had a Honda coil that would run perfectly for four minutes, then fail. You could almost set your watch to it. I had a hard time turning loose of $60+ for a coil that worked so good part of the time. New coil fixed it.

I always dreamed about taking an old engine, maybe even a dead one with no rod/piston, belt driven, and rigging some kind of universal coil mount on it to test coils. I know - a hillbilly thought, but my first battery charger, in 1965, was a generator spun by an old washing machine motor.

Skinflint Paul

" I don't see a genuine pass/fail test. "

I would say that such is a good statement. As some have said, you can use ohm meters, batteries/light bulbs, audio frequency generators to observe dead shorts, opens, transmission from primary to secondary, but all combined will not ensure a good functioning coil. Even if you built up your electric motor spun old engine hulk with a coil/mag and flywheel, you are still not home free with you testing, as by your own experience, you found one that only after 4 minutes of heating, did it stop functioning.
That is not a unique occurrence. Winding resistance changes with temperature. Insulation my be marginal and will be OK cold, or OK at lower test voltages, or OK until heat induced expansion crushes it a little, and on and on, but when running under the real-world stresses, it fails.
You can get some 'indications', but not a real end to end test, at least not without a lot more of an electronics lab that most would have access to.
I have had EI units in temperature chambers, driven by pulse generator simulating distributor trigger, driving the actual coil, with spark gap and ran temps from 30 below to so hot that I melted the connector shell plastic and it never stopped working for days.....put it back on PU & about an hour later ....dead. Jerk new one out of tool box, re-install and all is well. Go figure? Sometimes the final answer just isn't worth the cost & effort to find.

 

[PJH]

Thanks JD for correcting me - I was aware of the necessary ground, but didn't word it properly.

Paul

 

[tech4]

Usually the common wire is grounded under one of the coil mounting screws.

 

[Noah W]

IGNITION VOLTAGE OUTPUT TEST

Scopes may be inaccessible from time to time, so we at "Bubba's Garage" have come up with this here test for Ignition Output:

1.Hold medium wrench in one hand, Sears wrench preferred for accuracy of test 'cause other smooth wrenches are too slick for throwin'!

2.With engine runnin', grab hold of the plug end of an ignition cable with the other hand and hold on tight!

3.This will more than likely cause you to throw wrench! KV output is based on how far the wrench wuz thrown!

10 feet = approx. 10,000 volts
20 feet = approx. 15,000 volts
30 feet = approx. 25,000 volts
Over 30 feet = Caution! Must be one of them new
dis-is-it ignition systems,
use bigger wrench and retest.

WARNING! No side effects have been reported due to testing at this time (although it has been known to kill chiggers after berry pickin')

Bubba Sez: Electricity will not usually hurt you, instead it is the back of your neck going through the hood latch that really ruins your day!!

 

[John T]

You can use a simple ohms check and it will show if a coil is BAD

HOWEVER, even if that shows its not bad, that dont necessarily mean its good.

The reason is many breakdowns wont happen unless at high voltage and/or operating temperature conditions so while a low voltage ohms check can show if a coil is bad it WILL NOT TELL YOU IF ITS GOOD....

On many standard points and condensor coil ignition systems Id expect the low voltage primary to read maybe .5 to 4 or so ohms (depends on coil) while Id expect the High Voltage Secondary to be maybe 5000 to 10000 or so ohms (depends on coil) IF EITHER READS AN OPEN CIRCUIT ITS BADDDDDDDDD

There are often 3 terminations, the LV Primary, the HV output tower/button,,,,,,,,common ground

You can also bench test jury rig up a low voltage limited current input (Primary to ground) to the primary and a spark plug attached to the HV secondary and each time voltage is applied to the LV Primary but then its removed, the plug should fire (electrode to grounded strap). Still only a few tries at that may make the plug fire but once the coil reaches operating temp a HV breakdown could still occur

John T

 

[Anonymous-0]

This is what you need.. 100 AUD 20 years ago.

 

[Brian Jasper co. Ia]

Whizkid, are we talking "lawnmower" coils or for automotive engines? Briggs and Stratton Magnetron coils are the easiest to test. They have only a kill wire. Remove it and spin the engine. If the spark will jump 1/4 inch, it's good. Ford's coil on plug system is notorious for failure. They're also the easiest to diagnose. Switch the coils around and drive until the check engine light comes back. If the code for the misfire followed the coil, replace it. Testing for ignition modules is usually they work or they don't.

 

[whizkidkyus]

Both kinds of coils .

Whizkid

 

[JMOR]

(quoted from post at 10:18:07 02/06/10) What do you see as a difference between what you call an "electronic coil" and others? A coil is usually just two coils of wire - i.e., a primary and secondary circuit -regardless if used in a self-contained magnet-powered magneto, a breaker-switched battery ignition, or a high energy breakerless ignition in a newer car or truck.

I took apart a leaf blower a couple weeks ago that had a plastic blob on the metal laminated core with only two wires (one to spark plug, one to kill switch, core was no doubt ground). Now, $64 question: what's inside blob besides the two coils? Is there nothing else, some electronics maybe, or what? No points under or around the flywheel & of course, as I said already, no wire to connect to them anyway, even if they existed. Never seen anything published on this type.