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How do you know is a coil is bad


 
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thommoos
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PostPosted: Wed Jan 02, 2013 11:05 am    Post subject: How do you know is a coil is bad Reply to specific post Reply with quote

I have an 8n front distr. will not start. resistor 3 ohms. check points to enusre the ar gap @ .015. I had distr grounded with coil attached an not spark. Points and condensor new.

Is there a way to check to see if coil is bad? it has continutiy from screw to coil.

Am i missing something?
 
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JMOR
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PostPosted: Wed Jan 02, 2013 11:15 am    Post subject: Re: How do you know is a coil is bad Reply to specific post Reply with quote

thommoos wrote:
(quoted from post at 15:05:34 01/02/13) I have an 8n front distr. will not start. resistor 3 ohms. check points to enusre the ar gap @ .015. I had distr grounded with coil attached an not spark. Points and condensor new.

Is there a way to check to see if coil is bad? it has continutiy from screw to coil.

Am i missing something?
6v or 12v coil? 6v or 12v battery? If on the bench, with cap on, you won't see each of the 4 sockets spark if you rotate distributor shaft in wrong direction.
 
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Bruce (VA)
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PostPosted: Wed Jan 02, 2013 11:15 am    Post subject: Re: How do you know is a coil is bad Reply to specific post Reply with quote

Coil problems are difficult to diagnose. For starters, round coils are pretty robust & square coils aren’t (because of the difference in insulation used), but neither one will hold up to a poorly done 12v conversion that allows too much current to the coil or leaving the key on (see tip # 3Cool. Too much current creates heat which melts the insulation. Insufficient resistance in a 12v conversion will do the same thing. Rarely do coils just “go bad.”

There are a few ways to see if a coil is bad, but it’s not possible to determine if a coil is good w/o some expensive testing equipment. If you detect a dead short or high resistance in the coil w/ an ohm meter, it’s bad. If it’s cracked, it’s bad. If a sidemount coil w/ battery voltage to the primary will not jump a ¼” gap from the secondary wire to the block, it’s bad. But, here is the hard part: even if you do not detect a short, even if it will produce a spark, even if it’s not cracked, that doesn’t mean the coil will work when it’s hot & under a load. So, it’s a process of elimination. If the tractor starts & runs fine for 30 minutes or an hour then cuts off & refuses to re-start, and you checked for spark at the plugs & it had no spark at all, AND you have the correct voltage at the coil that’s a good sign that you have a bad coil. Let it cool off, restart it & if you have a good spark, odds are it’s a bad coil. But, even then, you might end up w/ a spare coil on the shelf!

Bottom line.......coils do go bad, but I'll venture a guess that 75% of new N coils sold today are sold to folks who do not understand how to diagnose a poor spark problem or how a coil works. So, for those who don’t know any better, in a no spark situation the first suspect is usually the coil……and, more often than not, it isn’t the problem.

Or as one regular around here humorously suggested: "Well, it is like this...I don't know or really understand what that black thing does & I am suspicious of the unknown, so I think the problem is the black thing."

Before you check anything else, make sure you have the correct voltage at the top of the coil. It should be battery voltage w/ the points open & about half that w/ the points closed.

Assuming that the bushings & advance weights are ok, & that you have correct voltage to the coil, the most common electrical failure (no spark, weak spark) points on the frontmount are:

1. The insulator under the brass concave head screw & where the copper strip attaches. (it’s fiber & will wear out; poke & prod w/ your meter leads to make sure it still works)

2. The pigtail at the bottom of the coil not making contact w/ the concave head brass screw inside the distributor. (With the coil on, the pigtail must firmly contact the brass screw. No contact = no spark.).

3. The copper strip is broken or grounded to the plate. (look very carefully for cracks & breaks)

4. The condenser wire grounding to the plate or side of the distributor.

5. The tab on the bottom of the coil not making contact w/ the brass button on the cap. (With the cap on, the tab must firmly contact the brass button. No contact = no spark.)

6. Incorrect positioning of the spring clip on the plate causing the pigtail to ground. (the open part of the clip goes between 7 & 9 o’clock on the plate. That puts the straight part of the clip opposite of the timing screw at 3 o’clock)

7. Incorrect seating of the coil on the distributor due to a loose bail or no gasket.(the coil must not move at all; if it does, replace the gasket or bail. Or stick some cardboard under the bail).

8. Water/moisture inside the cap due to gasket failure or the absence of a gasket. (the cap AND coil have gaskets)

9. Dirty/corroded/burned/incorrectly gapped or misaligned points.

10. Burned rotor, cracked/carbon tracked cap.

Unless the coil is cracked or shows a dead short, chances are it's fine; square coils rarely fail cold. Pull the distributor & do a continuity check.

First, make sure your meter/light works (don't ask....)

You can change points everyday & it will not fix bad bushings. If you are having trouble w/ points failure, check the shaft. If you detect movement, chances are it needs new bushings.

Inspect the points; if they are pitted or burned, replace them. Next, dress the points by running a piece of card stock or brown paper bag through them. New points sometimes have an anti-corrosive dielectric coating on them & old points can corrode or pick up grease from a dirty feeler gauge or excessive cam lubricant. Make sure the points align correctly. Proper alignment is also critical to longevity. Look at the points when they are closed; both sides should mate evenly. Then, check the gap at .015 on the high point of all 4 cam lobes.

Now, follow these steps:

1. Coil off, cap off, points open. One probe on the brass screw & the other on both sides of the open points. On the side closest to the cam, you should have continuity. Not on the other side! If you do, you will also have continuity everywhere because the points are grounded.

2. Coil off, cap off, points open. One probe on the brass screw & the other anywhere on the body of the distributor. You should have no continuity! Now, rotate the tang on the distributor....as the points open & close, you have continuity (closed) and lose it when they open.

3. Coil on, cap off, points open. One probe on the lead on the top of the coil, the other on the cam side of the open points. You should have continuity!

4. Coil on, cap off, points open. One probe on the lead on the top of the coil, the other anywhere on the body of the distributor. You should have no continuity!

At this point, I just put the distributor, coil & cap all back on the tractor as a unit. The reason I do this is because it is real easy to get the cap or coil mis-aligned trying to put it back together one piece at a time & the result is something gets broken or you get a ‘no spark’ problem.

It's possible to put it back on wrong & break it. Look at the slot on the end of the cam shaft. What ever angle it happens to be, turn the distributor tang to match it. Make sure you can tell the wide side from the narrow side on both the cam & distributor! (close counts) Then place the distributor on the front of the engine, gently push it in place & slowly turn the distributor body until you feel the tang slip into the slot. Rotate the distributor body until the bolt holes line up. Then, hand tighten the two bolts until the distributor body is flush w/ the timing gear cover.

Finally, double check your firing order & plug wires. It’s 1-2-4-3, counterclockwise. It’s very easy to cross 3 & 4.

Post back w/ results & any other questions.
75 Tips

 
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old
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PostPosted: Wed Jan 02, 2013 1:36 pm    Post subject: Re: How do you know is a coil is bad Reply to specific post Reply with quote

Some points when new have protective coating that needs cleaned off of them and many new in the box condensers are bad from the get go because they come from the land of almost right. If you have the cap off and power hooked up and open and close the points by hand you should both hear ans see a spark. If you do not and oh by the way if you goof you will feel the spark and that is if the points are good or bad. and more likely if the points need cleaned since you will take there place. Not a nice way to find the problem but it is a way to find it none the less
 
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russb wa
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PostPosted: Wed Jan 02, 2013 1:58 pm    Post subject: Re: How do you know is a coil is bad Reply to specific post Reply with quote

Have a look
here

Edit: I am just learning along with ...

Looks to me a VOM should be able to check continuity, from the power side to the other two posts, being the center post and post that goes to the points iirc.

I don't think there is any way to know for sure a coil is good, but you might be able to know if it is bad. Hope this helps.
 


Last edited by russb wa on Wed Jan 02, 2013 2:14 pm; edited 1 time in total
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old
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PostPosted: Wed Jan 02, 2013 2:47 pm    Post subject: Re: How do you know is a coil is bad Reply to specific post Reply with quote

I have tried checking them with a VOM but have found that the numbers where all over the place. Used ones new ones etc never found any 2 that would match. Two problem are that to 2 coil made by different companies and those like form the land of almost right and one from the U.S. do not match as for ohms. And then you have the fact home meters are never calibrated so they in them self throw a monkey wrench into the mix. Have 2 coils sitting right here and both show different readings on them
 
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Dell (WA)
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PostPosted: Wed Jan 02, 2013 6:07 pm    Post subject: Re: How do you know is a coil is bad Reply to specific post Reply with quote

thom........it should be intuitively obvious, an ohm-meter can ONLY check for continuity, but can NOT check for shorted turns. The weird 4-nipple squarecan coil insulative tar MELTS and shorts out a couple of turns and guess what??? NO SPARKIES!!!

Howsomevers; make my "secret" sparkie tester from enny ol'sparkie by bending the ground electrode out for a full 3/16" gap and with the ignition switch ON, snap yer closed points (0.015") open'n'shut. Does the sparkie JUMP the 3/16" gap with blue-snott authority ...or... is it a weak yellowish-orangish poor excuse???

Surprizingly enuff, the biggest cause of weak sparkies is the INVISIBLE corrosion between the points. Me? I clamp a clean $1-bill between the points and pull. Iff'n yer really cheap, tear a strip from HEAVY brown paper grocery sack and use that. Simple, eh? .........Dell, yer self-appointed sparkie-meister
 
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Hobo,NC
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PostPosted: Wed Jan 02, 2013 7:24 pm    Post subject: Re: How do you know is a coil is bad Reply to specific post Reply with quote

russb wa wrote:
(quoted from post at 01:58:27 01/03/13) Have a look
here

Edit: I am just learning along with ...

Looks to me a VOM should be able to check continuity, from the power side to the other two posts, being the center post and post that goes to the points iirc.

I don't think there is any way to know for sure a coil is good, but you might be able to know if it is bad. Hope this helps.


You nailed it a DVOM are a VOM can only tell ya if its bad it can not tell ya if its good Crying or Very sad...

Any one that has a DVOM that reads all over the place needs to realize they need a GOOD meter not HF junk... Good meters don't do-dat unless the battery is shot...

On a side note I use these when I am working in/at the shop if not then shade-tree it like Dell...


http://www.pkneuses.com/www.pkneuses.com/cont.htm
 
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souNdguy
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PostPosted: Thu Jan 03, 2013 5:41 am    Post subject: Re: How do you know is a coil is bad Reply to specific post Reply with quote

3 ohm resitor?

really?

jump power to the coil and check.

put a test lamp inline with coil and rotate engine see if lamp blinks

post back
 
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Dean
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PostPosted: Thu Jan 03, 2013 12:16 pm    Post subject: Re: How do you know is a coil is bad Reply to specific post Reply with quote

Bingo.

Nor can one test for insulation breakdown at high potential (a common failure mode for ignition coils) without special equipment, e.g., Hi-Pot tester.

Bingo, again regarding ignition contacts.

I keep a stack of IBM cards in the shop to burnish ignition contacts. Doing so will usually clean the contacts sufficiently for proper operation without damaging the plating as use of a point file will do.

Dean
 
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JMOR
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PostPosted: Fri Jan 04, 2013 8:27 am    Post subject: Re: How do you know is a coil is bad Reply to specific post Reply with quote

Unless you have a $10,000?? coil tester, then the good coil test for a front mount distributor N is to install your suspect coil on a known good running N & see how it is running after a few hours of working. Still good? Then your coil is good. Installing the known good running coil on your tractor isn't nearly as sure-fire a test. Why? Because if it runs good after a couple of hours, you condemn your coil.......perhaps it is still perfectly good? How is that? As has been discussed at length, the actual successful installation of a front mount coil is a bit of an art. Real easy to end up with a no-connect or a short. Not easy/obvious to detect....take some time & know-how. You could install the same coil twice & have it be fine once & bad the second time! Bruce walks you through how to install with some certainty. I wouldn't be at all surprised to find (it it were possible to do) that half of all front coils ever replaced were perfectly good coils. Just removing & re-installing may have been just as good a fix as the new coil.........but he will never know. Bruce is wise in telling you to VERIFY all the coil/distributor connections with coil/distributor assembled on the bench and the installed as an assembly onto the engine.
 
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forddoc
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PostPosted: Sat Jan 05, 2013 4:49 pm    Post subject: Re: How do you know is a coil is bad Reply to specific post Reply with quote

Wow I didn't realize the coil tester that Snapon made back in
the sixties is worth ten grand, good thing I got two. I even test
new outta the box, two min test takes less time than on and off
again and back to parts shelf for another one. Had two bad
new ones from napa in the last year.
 
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