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Old 06-02-2010, 08:41 PM   #1
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Ok, who can test an Ignition coil?

I unplug my ignition coil plug wire from the distributor and place a spark plug in the boot. I turned the key on and check the voltage with a meter. It reads 5 volts. Did I do the test Right or wrong. So I removed spark plug and put positive prone in. It still reads 5 volts. The negative prone was ground to negative battery post.

My ohm readings reads .37

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Old 06-02-2010, 10:28 PM   #2
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umm if i read your post right you tried to test your coil output voltage with a multimeter? do you realize that the coil output is upwards of 35000+ volts the input to the coil is supposed to be between 11.75 to 15v.

the OHMS for the coil wire are also way too low, iirc it should come out to be about 1.5k ohms per foot of wire, i may be wrong on that though, but its still a lot higher than what you got.

if i got time tomorrow at lunch ill take pics of how you test an ignition coil.

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Old 06-02-2010, 10:44 PM   #3
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Coils are tricky to test. An ohm meter will only tell you it's open (bad). It will not tell you if it's shorted (bad also).

An ignition coil is nothing more than a step up transformer. Transformers cannot step up or step down a continuous DC voltage but it can an intermittent DC voltage like the one in automobiles.

The engine would have to running in order to test the output of the secondary windings (output or high voltage side) of the coil and you would need a high voltage meter (impracticle). I've always tested the output by pulling the boot off of a spark plug, inserting a phillips screwdriver in that boot making sure it's contacting the metal inside the boot. Place the metal shaft of the screwdriver close to (but not touching) any unpainted metal ground on the engine that's convenient. Have someone crank the engine while you hold the handle of the screwdriver. You should see a big blue or white spark jump between ground and the screwdriver. If you do your coil is working but this test does not tell you if the coil fails intermittently.

In my expierience coils degrade gradually. I've replaced at least three in boats (Mercrusier inboard/outboard that uses a Ford/GM conventional coils) that would go bad while I was out on the water. They would heat up and the engine would start to sputter and act like it was a fuel problem. Once it cooled down or replaced the engine would run fine.

Bottom line if you suspect it go ahead and replace it. They're cheap enough.
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Old 06-02-2010, 10:50 PM   #4
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Quote:
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Coils are tricky to test. An ohm meter will only tell you it's open (bad). It will not tell you if it's shorted (bad also). An ignition coil is nothing more than a step up transformer. Transformers cannot step up or step down a continuous DC voltage but it can an intermittent DC voltage like the one in automobiles. The engine would have to running in order to test the output of the secondary windings (output or high voltage side) of the coil. I've always tested the output by pulling the boot off of a spark pulg, inserting a phillips screwdriver in that boot making sure it's contacting the metal clip. Place the metal shaft of the screwdriver close (but not touching) to any unpainted metal ground on the engine that's convenient. Have someone crank the engine while you hold the handle of the screwdriver. You should see a big blue or white spark jump between ground and the screwdriver. If you do your coil is working but this test does not tell you if the coil fails intermittently. In my expierience coils degrade gradually. I've replaced at least three in boats (Mercrusier inboard/outboard that uses a Ford/GM conventional coil) that would go bad while I was out on the water. They would heat up and the engine would start to sputter and act like it was a fuel problem. Once it cooled down or replaced the engine would run fine. Bottom line if you suspect it go ahead and replace it. They're cheap enough.
That's the problem I'm having. When my park my jeep for 10 minutes,it refuses to start until it cools down to around 150. Another problem my jeep starts then shut down like someone took the key out. Third the rpm gage drops to 100 and races up to 1500 and sometimes stall out while sitting at a light.
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Old 06-03-2010, 07:54 AM   #5
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Looks like you're on the right track with suspecting the coil but then again it could be something else. You'll never know until you replace it. I know this advice sounds like what a parts replacer would do but as I mentioned before coils are tricky. You just never know.

You can probably get one at AutoZone for $30.00 or you can get a fancy Accel or MSD for $60.00.
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Old 06-03-2010, 08:42 AM   #6
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Back in the late 60's the way we used to check the coils on our Harleys was to unplug the coil wire from the distributor cap, have someone hold it and then try to start the Harley. There was no question if the coil was good or bad.
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Old 06-03-2010, 10:38 AM   #7
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what year?
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Old 06-03-2010, 11:01 AM   #8
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Be careful how you test your coil or you will know what a stun guns feel like, but on the brighter side it is not as bad a being shocked by a welder.
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Old 06-03-2010, 03:28 PM   #9
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I wonder if its not your computer, similar symptoms......
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Old 06-03-2010, 06:24 PM   #10
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what year?
89 and I bite the bullet and brought a msd coil.
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Old 06-03-2010, 08:32 PM   #11
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more voltage to your plugs isnt going to help you in any way it will be wasted, unless you intend to run a larger gap on your plugs...it takes very little energy to jump a .035 gap or is it .35 either way a stock coil is more than enough for a jeep. but thats my 2 cents.

Enjoy your jeep!
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Old 06-03-2010, 09:41 PM   #12
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Quote:
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more voltage to your plugs isnt going to help you in any way it will be wasted, unless you intend to run a larger gap on your plugs...it takes very little energy to jump a .035 gap or is it .35 either way a stock coil is more than enough for a jeep. but thats my 2 cents.

Enjoy your jeep!
I'm replacing the same MSD coil already on the vehicle. The original owner put in a Ford distributor and did a Nutter Bypass. Maybe they change the gap also.
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Old 06-04-2010, 07:36 AM   #13
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id bet money since it's an 89 its the ignition module...

i had the same re-occurring issue... i fixed mine by going to a CRT HEI distributor

my other jeep also is an 89 YJ 4.2 auto CRT dist/MC 2100 carb
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Old 06-04-2010, 10:00 PM   #14
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Test the new coil the reading I got was .97. Three times the resistance. Put it on and it fire right up. Didn't shut down. Let it run for 10 minutes to get the temp up to 195 and let it sit and it started right up. Drove it 5 miles and it didn't stall out on me. Maybe this was my problem.
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Old 06-05-2010, 09:39 AM   #15
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Hooray. I was interested in what you'd find. Good for you and good for the Jeep.
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Old 06-09-2010, 09:28 PM   #16
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Quote:
Originally Posted by Vector6 View Post
id bet money since it's an 89 its the ignition module...

i had the same re-occurring issue... i fixed mine by going to a CRT HEI distributor

my other jeep also is an 89 YJ 4.2 auto CRT dist/MC 2100 carb
http://img.photobucket.com/albums/v3...e/DSCN1888.jpg
Maybe your right... order a Ignition module from quadratec. Jeep won't start, cold or warm wiggle some wire around the Ignition module and it turn over and just couldn't stay running.

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