12V front distributor coil resistance.

sorting my way through my 48 8N (again)... the thing stopped running when coolant dumped all over the front of the engine. inside of the front distributor looks dry, ditto the inside of the cap, outside of the coil and cap were all greasy/wet...

Just took the coil off for a proper cleaning (a bit of wd40 and shop towels). checking the coil, I'm seeing 2.7 ohms on the primary, and 13.3 KOhm on the HV/secondary (both measured from the power terminal on top). is that reasonable?

I've got a 2 ohm ceramic resistor in series with the ignition coil, no wirewound thing. it used to run just fine this way, and the inside of the distributor cap and the rotor looked just fine, hardly any sign of tracking on the contacts. the old plugs, however were very black and greasy looking, like it was running way rich. I put new Autolite 734's, gapped to 0.025 in it.

Right now, the fuel tank is disconnected and the hood jacked up to make it easier to service, and I know I'm going to need a new radiator but I want to get it running first so I'm not wasting money on radiator/hoses/pump with a dead motor. before I started to take it apart, it would crank and crank, and barely chuffachufffa a few times if I modulate the choke while cranking before stopping. plenty of fuel smell in the exhaust gas.

(quoted from post at 22:55:55 08/02/20) sorting my way through my 48 8N (again)... the thing stopped running when coolant dumped all over the front of the engine. inside of the front distributor looks dry, ditto the inside of the cap, outside of the coil and cap were all greasy/wet...

Just took the coil off for a proper cleaning (a bit of wd40 and shop towels). checking the coil, I'm seeing 2.7 ohms on the primary, and 13.3 KOhm on the HV/secondary (both measured from the power terminal on top). is that reasonable?

I've got a 2 ohm ceramic resistor in series with the ignition coil, no wirewound thing. it used to run just fine this way, and the inside of the distributor cap and the rotor looked just fine, hardly any sign of tracking on the contacts. the old plugs, however were very black and greasy looking, like it was running way rich. I put new Autolite 734's, gapped to 0.025 in it.

Right now, the fuel tank is disconnected and the hood jacked up to make it easier to service, and I know I'm going to need a new radiator but I want to get it running first so I'm not wasting money on radiator/hoses/pump with a dead motor. before I started to take it apart, it would crank and crank, and barely chuffachufffa a few times if I modulate the choke while cranking before stopping. plenty of fuel smell in the exhaust gas.

Click to expand...

orget all about making coil resistance measurements, as you are not qualified to do such. Look elsewhere for your no start.

(quoted from post at 20:35:02 08/02/20) Forget all about making coil resistance measurements, as you are not qualified to do such. Look elsewhere for your no start.

Click to expand...

gee, thanks. I'll throw out my 45 years of electronics engineering experience and 30 years of old car tinkering as worthless.

I shoulda just googled it, seems like 3.7 ohms is just about right for a 12V 8N front distributor... With the 2 ohm ballast resistor, thats 5.7 ohms total DC resistance. it and the rest of the ignition system was new a few years ago but like I said, the tractor boiled over and spat coolant all over the front of the engine.

there's no sign of oil in the coolant, or coolant in the oil, at least.

i'm waiting for a distributor gasket set before I take the distributor off to check the points and static timing. next stop is a compression test if I can find my old compression tester. I really don't want to have to do a head gasket on this thing. maybe I should just tow it next to my driveway, paint it bright colors, and stencil my street number on it... I'm about ready to give up and get a Kubota or something.

(quoted from post at 23:57:24 08/02/20)

(quoted from post at 20:35:02 08/02/20) Forget all about making coil resistance measurements, as you are not qualified to do such. Look elsewhere for your no start.

Click to expand...

gee, thanks. I'll throw out my 45 years of electronics engineering experience and 30 years of old car tinkering as worthless.

I shoulda just googled it, seems like 3.7 ohms is just about right for a 12V 8N front distributor... With the 2 ohm ballast resistor, thats 5.7 ohms total DC resistance. it and the rest of the ignition system was new a few years ago but like I said, the tractor boiled over and spat coolant all over the front of the engine.

there's no sign of oil in the coolant, or coolant in the oil, at least.

i'm waiting for a distributor gasket set before I take the distributor off to check the points and static timing. next stop is a compression test if I can find my old compression tester. I really don't want to have to do a head gasket on this thing. maybe I should just tow it next to my driveway, paint it bright colors, and stencil my street number on it... I'm about ready to give up and get a Kubota or something.

Click to expand...

ight be a good plan for you!

(quoted from post at 23:55:55 08/02/20) sorting my way through my 48 8N (again)... the thing stopped running when coolant dumped all over the front of the engine. inside of the front distributor looks dry, ditto the inside of the cap, outside of the coil and cap were all greasy/wet...

Just took the coil off for a proper cleaning (a bit of wd40 and shop towels). checking the coil, I'm seeing 2.7 ohms on the primary, and 13.3 KOhm on the HV/secondary (both measured from the power terminal on top). is that reasonable?

I've got a 2 ohm ceramic resistor in series with the ignition coil, no wirewound thing. it used to run just fine this way, and the inside of the distributor cap and the rotor looked just fine, hardly any sign of tracking on the contacts. the old plugs, however were very black and greasy looking, like it was running way rich. I put new Autolite 734's, gapped to 0.025 in it.

Right now, the fuel tank is disconnected and the hood jacked up to make it easier to service, and I know I'm going to need a new radiator but I want to get it running first so I'm not wasting money on radiator/hoses/pump with a dead motor. before I started to take it apart, it would crank and crank, and barely chuffachufffa a few times if I modulate the choke while cranking before stopping. plenty of fuel smell in the exhaust gas.

Click to expand...

Coil should have 3.5-4 ohms resistance across + and - posts. Resistors added or minused to get in that range.

" 45 years of electronics engineering experience"

Which means you understand Ohm's Law.

The ideal running current range for that square coil is 3.5 ohms on the primary. Anything more than 4 amps will eventually harm the coil. But......the higher
the resistance, the more difficult it will be to start. That's why Ford use the oem ballast resistor ONLY, .3 ohms cold, 1.7 ohms hot.


So do the math: oem ballast resistor + additional resistor + primary coil resistance / running voltage

Do the math running/not running and see what you get.

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.” Especially when cold.

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

it was running a little rough, and my son was using it to push stuff up into a slash pile for burning later in the winter... it boiled over (the radiator is toast), and blooshed the whole front of the engine in coolant. he burned out the starter motor trying to get it running, so I put a new 12V starter, solenoid, and new battery cables (old ones were in poor shape). I also had added a 10W solar battery charger so the battery wouldn't die when left parked for months.

before I started to take it apart, it would crank and crank, and barely chuffachufffa a few times if I modulate the choke while cranking before stopping. plenty of fuel smell in the exhaust gas.

when I pulled the original plugs they were black and fouled, so I gapped and put in a new set of plugs, same symptoms, occasional chuffa-chuffa-chuffa-wheeze (stop) after cranking if I have the throttle and choke just so, no sustained running.

my next step (today) will be to pull the distributor and go through the points, make sure they are gapped and static timed correctly (seen the diagrams)

then I plan on pulling hte 'new' plugs and doing a compression test on all 4. while I have the plugs out and battery back connected, I'll also check the spark.

(btw, my electronics engineering experience encompasses DC, AC, RF, and digital... I well know the difference between DC resistance and AC impedance and reactance, and I know about things like dwell and advance on older breaker point based ignition systems such as 1960s VWs).

~3.5 ohms, is that for a 12V system?

(quoted from post at 14:26:30 08/03/20)
it was running a little rough, and my son was using it to push stuff up into a slash pile for burning later in the winter... it boiled over (the radiator is toast), and blooshed the whole front of the engine in coolant. he burned out the starter motor trying to get it running, so I put a new 12V starter, solenoid, and new battery cables (old ones were in poor shape). I also had added a 10W solar battery charger so the battery wouldn't die when left parked for months.

before I started to take it apart, it would crank and crank, and barely chuffachufffa a few times if I modulate the choke while cranking before stopping. plenty of fuel smell in the exhaust gas.

when I pulled the original plugs they were black and fouled, so I gapped and put in a new set of plugs, same symptoms, occasional chuffa-chuffa-chuffa-wheeze (stop) after cranking if I have the throttle and choke just so, no sustained running.

my next step (today) will be to pull the distributor and go through the points, make sure they are gapped and static timed correctly (seen the diagrams)

then I plan on pulling hte 'new' plugs and doing a compression test on all 4. while I have the plugs out and battery back connected, I'll also check the spark.

(btw, my electronics engineering experience encompasses DC, AC, RF, and digital... I well know the difference between DC resistance and AC impedance and reactance, and I know about things like dwell and advance on older breaker point based ignition systems such as 1960s VWs).

~3.5 ohms, is that for a 12V system?

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kip all the middle stuff & insert ammeter in series with coil, apply the 12.5v battery voltage to the system and measure coil current. Cold it should be around 4 amperes (engine stalled, points closed) and drop off a bit as components warm up.

92-1/2% of all non-starting issues are due to incorrect wiring whether 6V or 12V, however most are due to mucked up 12V switch over jobs. Many guys can't figure out the OEM 6V/POS GRN system and guess that a 12V setup will correct their non-starting issues. If it won't start on 6V why should it start on 12V??? Not knowing is no excuse. The OEM ESSENTIAL MANUALS have all the wiring info for the 6V system and should be used as reference when converting. On the FORD 8N Front Mount Distributor, a generator, voltage regulator, ammeter, ballast resistor, ignition switch, coil, distributor, starter motor, safety start push button, and battery are used. Lights were never factory items and often get added to the muck-up. remove them til you get wiring all correct. As stated, forget about what you are trying to measure. So, on your 12V switchover job, is the Ballast Resistor still in the circuit? It should be. Is the coil 6V or 12V? If 6V do you have the external 1-OHM ceramic resistor added in line? It should be. Is the generator removed and an alternator installed with a belt tension device and correct fan belt? It should be. Is the Voltage Regulator removed from the circuit altogether? It should be. Is the battery 12V and tested under load at a shop? It should be and don't think charging it will ensure it is good. Have the correct batty cables as well for 12V. Knowing how things work and won't work is part of root cause problem solving. Guessing is not an option. Verify wiring is correct, battery disconnected, no shortcuts, tested via VOM set to continuity, NOT by a test light and NOT by colors. Tuneup on distributor must be right. Points set to .015" and timing is set one way per manuals. Test unit on your bench with your VOM. Firing order is 1,2,4,3. When wiring is satisfactory, connect battery but don't turn over engine. Test wiring thru coil and ballast resistor on VOM. Do it right the first time.

FORD 8N TRACTOR WIRING DIAGRAMS:
PRIOR TO S/N 8N-263844:

CORRECT FORD 8N ELECTRICAL SYSTEMS - 6V & 12V:

FORD 8N TRACTOR w/FRONT MOUNT -12v CONVERSION:

*PICTOGRAMS courtesy of JMOR

FORD OEM BALLAST RESISTOR -USED ONLY WITH FRONT MOUNT DISTRIBUTOR:

HELP: The LH resistor Terminal Post connects to the COIL Stud Terminal Post as you face the back off dash. OEM Wiring is RED.

EXTERNAL 1-OHM CERAMIC RESISTOR - USED ON FRONT MOUNT AND
ONLY IF COIL IS 6V:


MAJOR CAUSES OF ACCIDENTS:

FORD 8N TRACTOR ESSENTIAL OWNER/OPERATOR/PARTS/SERVICE MANUALS:

Tim Daley(MI)

"...forget about what you are trying to measure." I'll have to disagree with you, here. Coil current measurement is an end to end, all inclusive test covering switches, wiring, points, resistor(s) and coil. One measurement and you learn whether no further piece by piece digging is needed in these areas or not. If not move on and use time more productively than part by part digging/verifying that which is shown to be fine with one simple single measurement.

Not a Guru here for sure .

Anybody know if a Megger , megohmmeter would be of any practical use testing a square coil . We use Fluke hand held amp clamps at work for diagnosing sewer pumps and controls . We have a quality ? megger I can use if it would help . I don't know the brand from memory but it is not one of those little cheap hand help units with 5 or 6 led lights . It is a larger box with a digital read out . Yea I know this description is vague , just didn't know if it had any practical application . I have a box of a dozen old square coils . I think of them as junk and plan on gutting a few for round can conversions , then I saw how much a replacement is and thought I don't want to trash a good one if i can help it . :shock:

(quoted from post at 20:18:32 08/04/20) Not a Guru here for sure .

Anybody know if a Megger , megohmmeter would be of any practical use testing a square coil . We use Fluke hand held amp clamps at work for diagnosing sewer pumps and controls . We have a quality ? megger I can use if it would help . I don't know the brand from memory but it is not one of those little cheap hand help units with 5 or 6 led lights . It is a larger box with a digital read out . Yea I know this description is vague , just didn't know if it had any practical application . I have a box of a dozen old square coils . I think of them as junk and plan on gutting a few for round can conversions , then I saw how much a replacement is and thought I don't want to trash a good one if i can help it . :shock:

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believe that the internal connection of the design , where primary and secondary are internally connected, makes meggers usefulness unlikely.

OK , Thanks for the reply . I bought an old beater that I will probably roll into the shop and try to get it running . ( didn't really want them , rough pair of 2N's , one with a Sherman step up for $200 ) I should be able to sort things out fairly easy while the hood is off . The wife has been wondering what is in all of those packages lately . Ebay is great if you are in no hurry . If a front mount distributor comes up for under $25 I'll get it . I don't know what I will do with a bunch of extra distributors , but I know I probably wont be writing in with a tractor stuck in the field that wont start .

getting back to this after a few weeks (had to evac for the CZU fire in the santa cruz mountains, but back home, then suffered through a heat wave, then days of massive smoke).

so I just hooked my DC ammeter up to the coil circuit, and when the engine starts and runs for 10-15 seconds, I'm only seeing about 0.85 DC amps average (yes, I understand its a AC circuit). every time I've tried to get the static currenbt with it off, the points haven't been closed, I'll try that again in a bit when the battery gets a chance to recharge..... ok, repeated short cranks, and it finally stopped with the points closed, was only 2.5 amps or so static current, so I bypassed the 2 ohm resistor, now I'm seeing around 4.5 amps static cold. cranked it again a few more times, first at zero throttle with choke, then about a third throttle and it fired and this time ran for nearly 30 seconds before it died. with the resistor bypassed, the running ignition current was about 1.8 amps.


history to date... son overheated, dumped coolant all over ignition, AND fried the starter... in process of sorting that out, I discovered some baked wiring, so rewired most all the tractor, this time using a shorter piece of high temp 10 gauge for the alternator wire, and marine grade ring terminals with the thermal shrink sleeves that glue onto the wire insulation.

cleaned up the old coil and distributor, had to replace the points plate because the hold down screws were stripped, and the original points were flopping around.

replaced the radiator, hoses, thermostat

new battery

new main battery leads

new starter motor, 3-terminal solenoid, new ignition switch, reverted to using original starter button.

replaced rotted old fuel bowl assembly, replaced filter screen inlet in carb. reset carb to both jets 1 full turns from bottomed out and idle speed screw backed out and turned in just enough that I think its touching (kinda hard to see)

full 12V negative ground conversion (GM/Delco '1 wire' alternator, 12V coil, 12V starter+solenoid).



current symptoms, will crank and crank, with the right combination of throttle and choke, will start and run 10-30 seconds then stumble and stall. is likely running rich as I smell and see a lot of gas fumes in the exhaust.