NAA Coil Failure

[mstanfield]

I had a recent mis-fire issue with the tractor (NAA orig. 6volt system)and did a tune-up including plugs, coil, condensor, carb clean, etc...ran GREAT!

Started tractor yesterday: it ran 2-3 minutes and died. No spark to any plugs. Tested 6V power from battery to the coil, thru the switch; all is good. Tested coil, and there is "0" ohms across the terminals.

What causes a coil to go bad in such a short period of time (about 2 weeks)??

I also have some distributor cap terminal notching and realize I need new shaft bushings...could this be related?

I'd appreciate any help or ideas

Thanks!

 

[old]

A lot of the cheap home meters will read that or some other way off reading. Not likely that the coil is bad but very very good chance that the points have burned got dirty or slipped on you. One easy way to check a coil and ya it is not the best way but it works. Hook up the distributor side of the coil to ground then take the ignition side off the coil and with the key on hold the center coil wire close to the block and then hook and unhook the ignition side wire to the coil. I'll bet you find that the coil has a nice spark from the center wire. Also if all is as it should be and the points are closed you should be able to do the same thing and you should get spark but if the points are the problem you will not have spark. I use the ignition wire test a lot to test points on non running tractors. Tells me right away if the points are the problem

 

[Jerry/MT]

?????

Zero ohms is no resistance, OO(infinity) ohms is open circuit. Why do you think your coil is "bad". How good is your VOM calibration?

Are you sure that you didn"t measure zero VOLTS across the coil primary terminals with the key switch ON? That"s possible if the points are open and it doesn"t mean the coil is bad. If the points are closed you should read close to battery voltage across the terminals

 

[Dell (WA)]

mstanfield........as a general rule, roundcan coils are bulletproof. HOWSOMEVERS: nubie shadetree mechanics are not. specially when it comes to ohm meters. Surprizingly enuff, the coil primary is about 1-ohms. Yer primary is from one side terminal to the other side terminal. Many ohm meters are NOT ACCURATE at reading low ohms so it looks like a short or "zero" ohms. You doubt? Try measuring from either side terminal to the center tower terminal. Should read about 6000-ohms.

As fer yer current problem, 2-mins is about standard run time fer BAD condenser. Even NEW condensers can be BAD right outta the BOX. Fish yer old condenser outta trash and re-install it. Condensers don't go bad from use. HTH........Dell, yer self-appointed sparkie-meister

 

[Bruce (VA)]

" I also have some distributor cap terminal notching and realize I need new shaft bushings...could this be related?"

The distributor cap notching could account for the no spark issue, but not the "coil failure". Because it's highly unlikely that the coil is bad.


" Tested coil, and there is "0" ohms across the terminals."

That doesn't tell you jack.

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 # 38). 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.

Put the old condenser back in & see what happens.
50 Tips

 

[mstanfield]

Thanks to all. I've read that condensors can be bad...but replace both points and condensor with the tune.

Just to clarify, the tractor currently has no spark, even when cold. I'm gonna swap condensors, and also test for coil spark...

Also will test for 6000 +/- ohms...

I don't think the meter is a cheapo; $40 digital deal from Sears...but who knows....

Any other advice is appreciated...need to work the old tractor...had to use the my '39 Farmall H (spare).....

 

[Bruce (VA)]

Do you have battery voltage across the points when they are open? Verify the gap on the points at .025. Then, 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. (I always spray my feeler guage blade off w/ contact cleaner.) Make sure you have voltage across the points, as in past the insulator on the side of the distributor. That is a very common failure point on sidemounts, along w/ the attached copper strip. It's hard to find a short there because it is usually an intermittent . So 'wiggle' the insulator & the copper strip a bit when you are doing your checking. If you find the short there, the Master Parts catalog lists everything you need on page 154. You can make the strip and you could also make the insulators as well. But, somethings are just easier & in the long run cheaper to buy. Get the strip, 12209, screw 350032-S, 12233 bushing & 12234 insulator & just replace it all. If you just replaced the rotor & lost spark, put the old one back in. Insure that the rotor fits firmly on the shaft & that the little clip is there. Make sure the distributor cap is not cracked & doesn't have carbon tracks. Check continuity on the secondary coil wire. Make sure it is firmly seated in both the cap & the coil. In fact, replace it temporarily w/ a plug wire. Next, remove the secondary coil wire from the center of the distributor cap, turn the key on & crank the engine while holding the end of the wire 1/4" from a rust & paint free spot on the engine. You should see & hear a nice blue/white spark. If not, you have a bad coil or condenser. Just put the old condenser back in to eliminate that as a possibility.

Post back w/ results; I'll be interested in what the problem was.
50 Tips

 

[souNdguy]

any meter under a couple hundred buclks and industrial quality ( ie.. ANY consumer grade meter ) is what we call a cheapo. you need lab grade or industrial equipment to do some of the real fine measurements. luckilly.. you can get by with a cheap analog most of the time..

 

[BillM (OH)]

Never thought of that one Old - great way to check a round coil - just fooling it into thinking the points are charging it & breaking - cool!!!

 

[36 Coupe]

The round can coil is far from bullet proof.The coil on the NAA is the same one used on Ford cars from 49 on.Ive changed these coils out on the road during the 50s.A spare coil was kept in the service truck.Friends 49 and 52 Ford cars had coil failures out on the road.

 

[old]

Ya and it tests the point to see if they need to be cleaned or replaced. Been using that for well over a decade to find out if spark is the problem with one not running and have fixed many in just a few minutes doing that

 

[Bill Crowell]

You are reading a dead short across the coil terminals with your ohmmeter because the ohmmeter is a DC device but the coil is an AC device. A dead short to DC across the coil primary is normal because the coil is never exposed to DC. In runs on the AC generated by the points and condenser. A coil has impedance to AC. Impedance is to AC what resistance is to DC.

 

[LeoinMI]

You may want to take a look at the copper strip that goes from the points to the wire leading to the base of the distributor. Check it for cracks or the wire being broke. Either one with result in no spark. This happened to me on my NAA. I replaced the copper strip to get good contact and it ran fine after that.

 

[teddy52food]

You are full of bs.

 

[Dell (WA)]

Jerry.........so I should have called the NAA roundcan coil semi-bulletproof. And when you compare the typical 2000rpm tractor engine with 5000rpm V-8's, yeah, sometimes the roundcan coil ain't upto the job. Usually, its the POINTS that cause weak sparkies, NOT the coil. ........respectfully, Dell sparkie-meister

 

[Bill Crowell]

You want to believe this as a psychological defense mechanism because you are insecure that don"t understand the basic principles of electricity.

 

[36 Coupe]

Bill is right and you are wrong.The points produce and alternating voltage.Transformers wont work on DC.

 

[trewgtregg]

Do you have battery voltage across the points when they are open? Verify the gap on the points at .025. Then, 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. (I always spray my feeler guage blade off w/ contact cleaner.) Make sure you have voltage across the points, as in past the insulator on the side of the distributor. That is a very common failure point on sidemounts, along w/ the attached copper strip. It's hard to find a short there because it is usually an intermittent . So 'wiggle' the insulator & the copper strip a bit when you are doing your checking. If you find the short there, the Master Parts catalog lists everything you need on page 154. You can make the strip and you could also make the insulators as well. But, somethings are just easier & in the long run cheaper to buy. Get the strip, 12209, screw 350032-S, 12233 bushing & 12234 insulator & just replace it all. If you just replaced the rotor & lost spark, put the old one back in. Insure that the rotor fits firmly on the shaft & that the little clip is there. Make sure the distributor cap is not cracked & doesn't have carbon tracks. Check continuity on the secondary coil wire. Make sure it is firmly seated in both the cap & the coil. In fact, replace it temporarily w/ a plug wire. Next, remove the secondary coil wire from the center of the distributor cap, turn the key on & crank the engine while holding the end of the wire 1/4" from a rust & paint free spot on the engine. You should see & hear a nice blue/white spark. If not, you have a bad coil or condenser. Just put the old condenser back in to eliminate that as a possibility.

Post back w/ results; I'll be interested in what the problem was.
50 Tips

does it matter which wire goes where on the coil. it has 2 posts

 

[Jim.ME]

trewgtregg
"does it matter which wire goes where on the coil. it has 2 posts"

Are there markings like a +and - beside the small posts? If so + is the positive and - is negative. The wire that matches the system ground goes to the distributor. If it is not marked what brand and part number coil did you get? Is your tractor 6 volt or a 12 volt conversion?

This is a 2012 thread and Bruce passed away in February of 2025.

The FORD forum is best for you NAA questions. I am guessing it is an NAA from some of your past posts.

Edited to add quote.