Coils going bad - A coil catastrophe.

Help! I've traced my 8N (front distributor) starting problem to a bad coil. I replaced the coil with a new one. The tractor started right up and ran well for a few minutes. Then it wouldn't restart. I tested and found the new coil to now be bad. I don't want to buy a new coil to then have it go bad too. What could be causing coils to go bad? I have a new voltage regulator (although I don't know what difference that would make), new properly gapped points, and a new condenser (that I checked and found to be charging and discharging properly). Any tips anyone? Thanks in advance.
 
For the most help, post this lower down on the page, in the Ford "N"-specific Forum.

And post the "rest of the story", still 6-Volt, or a 12-Volt conversion, and so forth.
 
No idea, I'm not much of a Ford man. However, for sure if you're applying 12 volts on a 6 volt coil it will overheat and fail in short order.

John T
 
Front mount distributor has a ballast thermister (sp) which as it warms up the resistance goes up and if it is removed or shorted can cause that. Or if you have a 12 volt system and do not have the factory resister and a ballast resister then that can cause a coil to go bad fast also, Or your buying coils made in the land of almost right
 
Thanks. The ballast resistor is only a few years old, but maybe its bad. I figured if I was getting voltage to the coil, the resistor was OK. Cheap enough to change.

Yes, original 6V system, that's never given me a problem until now. Battery is good.
 
Even the wrong voltage won't kill a coil
in a few minutes. It was bad from the get
go. Buy your next one at Napa.
 
(quoted from post at 21:11:50 07/14/19) Even the wrong voltage won't kill a coil
in a few minutes. It was bad from the get
go. Buy your next one at Napa.
........or as is very often the case, he disturbed (accidentally & unknown to him) and fixed enough bad connections to make it run.....at least temporarily. i.e., not a coil problem, but just a generally bad connections design from 1939. A connection relying on pressure and alignment is not a good hard connection............just trouble for ever!
 
How are you determining your coils to be "bad"? Are you measuring the primary and secondary resistance? Is there physical evidence, such as cracking, that the coil has failed? Or is the tractor simply not running and you're assuming the coil has failed?
 
> The ballast resistor is only a few years old, but maybe its bad.

It is highly improbable for the ballast resistor to fail shorted, which is the only way it would cause a coil failure. And in the case of a missing or shorted ballast resistor, the points will usually burn out long before the coil.
 
I'm not sure what I should be reading for the coil, but my primary resistance is 1.4 ohms, and the secondary resistance is 6.9 ohms.

I determined the coil to be bad by removing the coil and distributor from the tractor, connecting 6V to the coil, jumping from the coil primary to the points, and grounding the distributor to the plus side of the battery. With the points closed, and a lead connected to the distributor's body, I should see a spark across the lead and the coil's secondary terminal. Since I don't see a spark, I'm assuming the coil is bad. Am I missing something?

For what its worth, all the electrical connections seem good. I replaced all the wiring a few years back when doing a complete 8N rebuild. All terminals are clean and solid. Besides, I get a consistant 6V to the coil, so I don't think anything before the coil is bad.
 
> I determined the coil to be bad by
removing the coil and distributor from the
tractor, connecting 6V to the coil, jumping
from the coil primary to the points, and
grounding the distributor to the plus side
of the battery. With the points closed, and
a lead connected to the distributor's body,
I should see a spark across the lead and
the coil's secondary terminal. Since I
don't see a spark, I'm assuming the coil is
bad. Am I missing something?

Yes. You have not removed the points and
condenser from the picture. Also the spark
occurs when the closed points open.

What is the voltage drop across the closed
points? It should be less than a tenth of a
volt. Half a volt or more is definitely
bad.
 
You state "I determined the coil to be bad by removing the coil and distributor from the tractor, connecting 6V to the coil, jumping from the coil primary to the points, and grounding the distributor to the plus side of the battery. With the points closed, and a lead connected to the distributor's body, I should see a spark across the lead and the coil's secondary terminal. Since I don't see a spark, I'm assuming the coil is bad. Am I missing something?"

Welllllllllll to begin with the coil fires only AFTER the LV primary (+ to -) conducts current to ground via good and closed points and then the circuit is open. THAT OPENING OF POINTS INTERRUPTING CURRENT IS WHEN IT FIRES.

If you want to test the coil independent of any points or condensor or distributor problems: Insure good full hot battery voltage is present on the Input (usually when IGN switch is ON),,,,,,,,,,,unhook the output to the distributor (eliminates any points or condensor or wiring problems),,,,,,,,,,,,,,,use a jumper to connect the coils output to frame/ground then remove it EACH TIME THE COIL CONDUCTS (+ to -) AND THE CIRCUIT (jumper from coils LV primary output to frame/ground) IS OPENED THE COIL SHOULD FIRE. A plug wire on its HV output terminal to a grounded plug should fire when circuit opens (like points opening).

Typical no fire is caused by faulty (pitted or burned or oxide or film coated) points not fully closing and then opening. They cant have any protective film/coating on them as some come new from the factory CHECK FOR THAT.

DISCLAIMER Im NOT a Ford man so no idea what or where or what connections you have access to in order to test per the above BUT THE THEORY IS STILL TRUE. I see way moreeeeeeeeee more points or condenser or ballast or wiring problems then coils

John T
 
(quoted from post at 14:54:13 07/15/19)
(quoted from post at 11:28:01 07/15/19) are you sure that the 6.9 Ohms is not 6.9K Ohms, that is 6,900 Ohms?

Sorry, reading was 6.9Kohms.
f you re-install that coil, WITH GOOD CONNECTIONS, I think there is a very good chance that it will work.
 
That sounds more like an expected HV Secondary winding while the LV Primary may be 1+ to 4 ohms.

NOTE a coil can easily pass the low energy low voltage ohm meter continuity checks BUT STILL BE BAD AS IT HAS HV BREAKDOWN ONCE WARMED UP !!!!!!! The ohm meter can only tell if its total bad open but cant insure its a good coil.

See my coil test method below independent of any points or condenser...?..But even that can pass yet it still fail under temperature and load. Be sure its NOT any points or condenser or wiring problem before moving to the coil.

John T
 
It is getting increasingly difficult to get quality replacement points and condensers. I finally gave up and installed a Pertronix conversion in my 4000 and haven't had a bit of trouble since. YT sells a <a href="https://www.yesterdaystractors.com/Ford-8N_Electronic-Ignition-Conversion-Kit-12V-Negative-Ground_1247XT.html">Pertronix kit for your 12 volt front distributor 8N</a>, although it's a bit pricey.

If you do decide to switch, it's important to use resistance plug wires with the Pertronix kit.
 
John - thanks again for the help. I retested the coil and, sure enough, I'm getting a spark off the secondary. You helped me identify my mistake in how I was originally testing. I still have the starting problem, but at least I know its not the coil.

Although the points I have are cheap new ones, I'm about to order a set of much more expensive Blue Streak points, just to make sure everything is as good as it can be.

Any chance my battery might be just on the weak side? I measure 6V, and it cranks the engine fine, but its not the newest.
 
Measure the voltage on the battery while you crank, that will indicate the health of the battery.

You indicated this was a 6V battery, in that case there should not be a ballast resistor.
 
You ask "Any chance my battery might be just on the weak side? I measure 6V, and it cranks the engine fine, but its not the newest."

Sure a weak battery can cause a poor spark ESPECIALY under cranking conditions when battery voltage is reduced. Depending on Condition of battery,,,,,,,,,,Condition of and current draw of starter motor,,,,,,,,,,,,,Faulty or too small cables or connections or switch/solenoid voltage drops,,,,,,,,,,,,,Condition and compression of engine and how hard to crank,,,,,,,,,,,,,,,WHILE CRANKING Id expect the battery voltage to drop to maybe 4.5 to 5 or so volts but if much below that the spark may be weak. At 67 degrees a full charged 6 volt FLA battery at rest and stabilized would be 6.3 volts and again when it cranking (subject to all the above) it might drop to 4.5 or a tad less to 5 volts.

A shop can do a Load Test for free on your battery, you might want to take advantage of that if you suspect a weak battery !!!!!!!!!!!!!

If you can pull a plug and observe its spark WHILE CRANKING it should be visible and blue and NOT any faint thin wimpy hard to even see yellow spark.

Good quality points gapped correct with a good condenser (I've seen many bad out of the box grrrrrrrrr) and sufficient coil voltage (say 4.5 to 5 when cranking with no ballast and a 6 volt coil) ought to do the job

DISCLAIMER again I'm NOT a Ford man and no idea if any ballast is used and if so its resistance and voltage drop or the correct coil input voltage and if there's any by pass when cranking SO ASK A FORD MAN NOT ME

John T
 

There is a resistor mounted behind the battery. Its listed as a YT Part No: A8NN12250A (for older 8Ns). That's what I'm calling a ballast resistor.
 
I appreciate everybody's input and advice. I'll be incognito for awhile while I catch up on my diagnostics homework. I'll be back though to let everyone know what I found. Thanks again.
 

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