how to test a coil

[Powerstroker]

First off, I am probably the least mechanically inclined person on this board. I have a 1944 2N front mount distributor. I have no fire and a friend who has done a lot of mechanical work thinks the coil might be bad. I cannot find any shop around here that can test it so was wondering how you fellows do it. Thanks for any help you can give.

 

[Mad Farmer]

With simple equipment you can tell if the coil is bad (multimeter).

But not easy to tell if it's good.

Other's fill you in on testing with a multimeter.

 

[HCooke]

I have multiple tractors, so I take the easy way out and swap coils with a running tractor. It is rarely the coil.

 

[8NChris]

To test a coil you take primary and secondary resistance readings across the hot (+) and trigger (-). Then across the hot (+) and coil wire. I always look up the manufactures specifications because they can fall in a window that would be good for one but out of spec for the other. Some say a yellow spark is fine but yellow is not operating at it's peak. If you don't have an ohmmeter line up your cap/rotor on any cylinder. Pull that plug and ground it. With the negative coil wire disconnected and have the positive wire at battery voltage. Ground the negative side with a screw driver. If the coil is good you'll see the plug fire. You can use this method with the coil wire post and some kind of gap between ground. Once you get the principal of the ignition down you can isolate and test the other things that can kill spark like burned points, condenser or a break down in the trigger wire. If you suspect a weak hot coil wire load it with the appropriate voltage head light.

 

[Tim PloughNman Daley]

You can static test a front mount coil but won't tell you if it is good or bad -that requires very expensive test equipment. Before you start pulling things off, get your Essential Manuals and wiring diagrams and verify it is correct first. We need to know if you are wired as the OEM 6V/POS GRN setup or if it is a 12V switch over job. Non-starting issues are mostly due to incorrect wiring. Start with the battery. Is it good? Have it tested not simply put on a charger. Know how and why the front mount works. It mounts one correct way. 9N/2N (they are the same after 1940, year doesn't matter) 6V uses front mount, ballast resistor, coil, 1-Wire/3-Brush generator, roundcan cutout circuit, 1-Wire starter motor, ammeter, ignition switch, and the pushbutton starter switch. If 12V gen and cutout are removed and 12V alternator installed. Have battery tested, if it won't sustain a full charge under load, it is bad and needs replacing. While battery is off, disconnect lights (can reconnect later when system is wired right) and use a VOM set to Continuity, NOT a test light, to verify wiring. The Ford N is about the easiest machine to work on and you can do it, just need to know the how's and why's.


FORD TRACTOR 9N & 2N, AFTER S/N 12500, OEM 6V/POS GRN WIRING:

<img src="https://i.imgur.com/QWaC1aah.jpg">​

<img src="https://i.imgur.com/CJedLEch.jpg">​

FORD 9N-10505-B CUTOUT –USED AFTER S/N 12,500 TO S/N 258504 ON 9N & 2N MODELS ONLY:

WIRING PICTOGRAMS by JMOR; OEM 9N & 2N ELECTRICAL SETUP:

FORD 9N/2N ESSENTIAL OWNER/OPERATOR/PARTS/SERVICE MANUALS:

Tim Daley(MI)

 

[Bruce (VA)]

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.”

As others have said, 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."



So, what problem are you having that leads you to suspect a coil failure?
75 Tips

 

[Powerstroker]

Ok, a bit of history. A while back the tractor would not even click when the starter button was pushed. I had the battery checked and it is good. I pulled started the tractor and after that it wouold start with the starter for a while. Then the last time it started it ran rough and would not smooth out and finally died. Would not fire after that.

 

[jimtrs]

(quoted from post at 20:01:49 07/30/20) ..... I have no fire and a friend who has done a lot of mechanical work thinks the coil might be bad.

If your friend did proper and thorough evaluating &amp; testing, then yes, your coil my be bad.

If your friend [i:b400c935db][b:b400c935db]"just thinks"[/b:b400c935db][/i:b400c935db] your coil may be bad because your tractor won't start or has no spark, then put the coil at the bottom of the list as potential problems.

I cannot find any shop around here that can test it

You probably won't find one.

so was wondering how you fellows do it.

As said, replace with a known good one. if all works well, then the coil was bad. If no change, then coil is probably good.

But again, put a coil problem further down your list of potential reasons for no spark.

If you indeed have no spark, there is some troubleshooting to do to determine why.

Some of the more common reasons would be:

No continuity through points when closed (think corrosion)
No power to coil (think wiring, key switch)

Each of these common problems have multiple reasons why the problem can exist.

 

[Mad Farmer]

(quoted from post at 17:59:33 07/30/20) To test a coil you take primary and secondary resistance readings across the hot (+) and trigger (-). Then across the hot (+) and coil wire. I always look up the manufactures specifications because they can fall in a window that would be good for one but out of spec for the other. Some say a yellow spark is fine but yellow is not operating at it's peak. If you don't have an ohmmeter line up your cap/rotor on any cylinder. Pull that plug and ground it. With the negative coil wire disconnected and have the positive wire at battery voltage. Ground the negative side with a screw driver. If the coil is good you'll see the plug fire. You can use this method with the coil wire post and some kind of gap between ground. Once you get the principal of the ignition down you can isolate and test the other things that can kill spark like burned points, condenser or a break down in the trigger wire. If you suspect a weak hot coil wire load it with the appropriate voltage head light.

I've got to do my dist soon.

Does anybody have proper resistance ranges for primary and secondary on a stock front mount 6V coil?

 

[Mad Farmer]

I can't find coil specs in IT or Ford Service manuals.

 

[Jim L WA]

This may help. From the top of the coil to the pig tail should be around 1 ohm for 6 volt and 2-3 ohms for a 12 volt. From the top of the coil to the tab (cap contact) should be around 6 K ohms.
Also, with the coil mounted on the distributor, key on, you should see 6 volts or more at the top of the coil with the points open and about half of that when the points close. That's not a test of the coil, but a test to see if the points are working.

 

[JMOR]

(quoted from post at 17:12:41 07/31/20) This may help. From the top of the coil to the pig tail should be around 1 ohm for 6 volt and 2-3 ohms for a 12 volt. From the top of the coil to the tab (cap contact) should be around 6 K ohms.
Also, with the coil mounted on the distributor, key on, you should see 6 volts or more at the top of the coil with the points open and about half of that when the points close. That's not a test of the coil, but a test to see if the points are working.

espite this published spec, I'm with JimL WA in finding "around 1 Ohm" to be very common. Tough to measure such low values, but sometimes, I measure voltage &amp; current and calculate Ohms.

 

[Mad Farmer]

Jim and JMOR

Thanks for the numbers on the coil and voltage drops

MF

 

[Mad Farmer]

I went through my front mount distributor. Thanks to all here for suggestions/ideas on doing this

The 6V coil gave about 1 ohm to the points and 36K to the cap , too much for the latter?

I cleaned up everything and replaced points cap rotor. Took it all the way down and removed shaft and cleaned up advance weights.

THe front bushing was more worn than I'd like but I need to have things running so put back together. Will order a bushing from here.

Runs much better with more power. But still cuts out/dies when it gets hot. I guess my coil is bad.

Local TSC had a coil in stock but it was a POS!!! Top cover was so loose it rattled , and pig tail too stiff, $44.

Local Napa has Echlin ($97) and Balkamp ($45)coils available Monday. I'm going to try the latter.

 

[Btzj02]

New YT 6 volt coil Bench tested before install as follows:


Bat Term to Dist Terminal 1.2 Ohms
Secondary Coil to Bat Term 4.3k Ohms
Secondary Coil to Dist Term 4.3k Ohms

JB

 

[Btzj02]

Crap, didn't read close. This is a side mount 8N coil!

 

[Mad Farmer]

I,m going with Echlin coil, old time tractor fixer said most others were a POS!