1948 8n 12v front distrib. weak spark

Had my tractor almost 4 yrs. Never an issue starting. This summer it all started, now I cannot get to the bottom of the problem. Most everything I have in the electrical area is new. Takes being hooked up to the car with jumpers to get it to start, and it won't restart after it runs for a bit. Runs fine.

I'd put in new plugs this summer and it started instantly. A few weeks later, back to the problem. So I have weak plugs.

Is there a possibility I should take OUT the ballast resistor?
Here's an interesting thing that happened today; I pulled 1 plug to test, turned over motor a bit, stopped. Forgot to turn key off. A minunte later, my resistor was smoking. Clueless as to what's up.

" A minunte later, my resistor was smoking. Clueless as to what's up."

Check out tip # 38.

You left the key on. Too much current caused the resistor to smoke.

You ask if you should take it out.....that depends on how many you have & of what value. And it also depends on the internal resistance of your coil.

At a minimum, you must have the OEM ballast resistor w/ that front coil.

Technology & materials being what they were in the 30's, that square coil would melt if it ran on much more than 4 amps for any length of time. (as I said, see tip # 38 for an example). In order to get a hot spark at the same time the starter was drawing max current from the battery, a ballast resistor was added in the ignition circuit. What that did was add about .3 ohms of resistance in the circuit, added to the 1.5 ohms of the coil. That got you 3.5 amps or so at start up. As the voltage increased when the engine was running to about 7.5 volts, the resistor heated up, adding more resistance in the circuit. 1.0 ohms hot, plus 1.5 ohms of the coil got you down to 3 amps or so to keep from melting the coil. The same rule (actually, Ohm's Law) applies to a 12v circuit. I= E/R. Current equals voltage divided by resistance.

It used to be before the "Land of Almost Right" started making coils that you could count on a 12v frontmount coil as having 3 ohms of internal resistance & the 6v coils as having 1.5 ohms or less. Thus, thanks to Ohm's Law, you could calculate what additional resistance you needed in the circuit to limit coil current to 3.5 amps. So, you will need to measure the internal resistance of your coil & see what it is. A digital multi-meter has two probes & a switch. Set the switch on resistance. Put one probe on the top of the coil & the other on the pigtail at the bottom. It will give you a reading in ohms. Lets just say it reads 3.0 ohms. Your OEM ballast resistor (which you must use) is about 1 ohm hot. A coil a 3.0 ohms, plus the ballast resistor at 1 ohm (hot) gives you 4.0 ohms resistance in the circuit. Your 12 volt alternator puts out 14.5 volts. You need to determine current (amps). 14.5 v divided by 4.0 ohms gets you 3.6 amps; that's ok. But, and this is the problem......what if the coil is only 2 ohms? Do the math. 14.5 volts divided by 3.0 ohms gets you 4.8 amps! Not good! And, if the coil is less than 2 ohms (and some are) it will fry quickly. So, to get it to 3.5 amps, you need another resistor in the circuit. Either that, or keep spare $30 coil around.

Or, you can not worry about measuring the resistance; just use an ammeter per the other picture & directions that JMOR put together.

Leave the plugs alone. They're fine. But, if you do indeed have too much current going to the coil, your points might not be.

Have you replaced the points, condenser, rotor & cap, and set the timing in the 4 years you've owned the tractor?

If not, therein lies your problem....failure to observe tip # 40.


50 Tips

once you post the answers to bruce's questions we can likely get you up and going easilly.

I have replaced most things in the past yr or 2 or 3, and tried timing, but, my distributor tangs appear the same-cannot tell which is the wide one.

I will have to go through the entire electrical system remeasuring everything, I admit-I did some of this during the summer and that's when I found the cracked coil and replaced it, later found my points shot and replaced those (do think I left the key on once).

I suspect I may have to have 2 resistors in line...not just 1.

The red wire in your pic is the wire from the coil?

12v / 2.5 ohm or so, coil.. or 6v 1ish ohm coil?

measure the tang.. there is a CLEAR offset.

soundguy

12v square coil. There may be a diff on the tangs, but they are close in measurement.

Trust us.........they are different. Buff the tang out w/ a wire brush. Measure it. You'll see the wide side and the narrow side.

" 12v square coil" usually means all you need is the OEM ballast resistor. Measure it & find out for sure.

Means you need to pull the distributor to set the timing, I'd just replace the points. If the points in there now are good, then you will have a spare set on the shelf should you leaave the key on again.
50 Tips

If you have about $125 put on an electronic ignition. I had an old girl that defied all my efforts so I converted and haven't look back. Good Luck.

Bruce-new points a month ago-the old ones looked like a donut. I have the expensive plug wires and as I mentioned, had put in new plugs also late summer, battery is 4 yrs old, cables all good. New coil this summer and new resistor also. Ignition key is good also.

I sense it's in the amps in the circuit. Seems late summer, when I was too busy to focus on it, I put in the coil, used it 2x and both times, couldn't get it started on it's own-had to jump it and ran it without shutting down for fear it wouldn't start. I have the long type resistor, not the square one, all 12v system.

Tomorrow will check it and I think my coil last time I checked was not 2.5Amps (was less) and thus, my problem.

(quoted from post at 21:38:43 12/10/11) Bruce-new points a month ago-the old ones looked like a donut. I have the expensive plug wires and as I mentioned, had put in new plugs also late summer, battery is 4 yrs old, cables all good. New coil this summer and new resistor also. Ignition key is good also.

I sense it's in the amps in the circuit. Seems late summer, when I was too busy to focus on it, I put in the coil, used it 2x and both times, couldn't get it started on it's own-had to jump it and ran it without shutting down for fear it wouldn't start. I have the long type resistor, not the square one, all 12v system.

Tomorrow will check it and I think my coil last time I checked was not 2.5Amps (was less) and thus, my problem.

Click to expand...

Eriklane, just because you replaced your points a month ago, they may have slipped and need setting or if you used some of the cheap points the rubbing block may be shot.

About a year ago I bought one of the cheap $12.00 tune up kits, points, condenser and rotor (parts store was out of blue streak etc) the points lasted 2 months, not to mention the rotor was broken when I opened the package.

Also as Bruce said you need to check your timing.

trouttman

As others have said check the points--If it was running good before I"d bet you do not have a timing problem..Some folks want to mess with the timing even if you just add air to the right front tire....

We just fixed a weak spark on my neighbor's 8N by replacing the plug wires with solid wires. Are your new wires solid or resistance?

Erik, I know this stuff can get confusing sometimes, so that's why I try to give the 'back story' to whatever I suggest. Maybe it's TMI. But look at what I told you yesterday:

" You ask if you should take it out.....that depends on how many you have & of what value. And it also depends on the internal resistance of your coil.

At a minimum, you must have the OEM ballast resistor w/ that front coil. "

I just didn't leave it at that; I explained WHY you need the OEM ballast resistor.

This is what you replied:

" I have the long type resistor, not the square one, "

Understand the problem now? You do not know what the internal resistance is in the circuit AND you do not have an OEM ballast resistor either.

The same mis-communication has occured over your points. You said: " Forgot to turn key off. A minunte later, my resistor was smoking." I said: " But, if you do indeed have too much current going to the coil, your points might not be." And, as a matter of further explanation, I told you to check out tip # 38. You also said: " new points a month ago"

Once again.....w/ this explanation, do you see the problem? It does not matter if you put new points in yesterday if you left the key on & smoked the resistor 10 minutes ago. You most likely burned the points.

Bottom line........

Replace the points, set the timing, install an OEM ballast resistor. Measure the current to the coil or the internal resistance of the coil to make sure it draws more than 2.5 amps & less than 4.

Post back w/ results or more questions.
50 Tips

" If you have about $125 put on an electronic ignition. "

All that will do is add another level of complexity to the problem.

The tractor won't run now. He does not know why. The EI is only going to be as good as the system it’s installed on. All you get by putting an EI on an N w/ these problems is a poorly running N w/ an EI instead of points.

He will be much better off fixing the problem he has & considering EI later.
50 Tips

Let's start here-the new 12v square coil is showing 1.6 ohms.

I put the expensive sparky wires on 2 yrs ago. The points I put in 2 mos ago were from Napa.

Erik........would it surprize you to learn yer new
12V squarecan coil is really a 6V-coil??? New 12V
squarecan coil should read about 2.5-ohms.
...unless... yer el-cheapo ohm-meter is inaccurate. ALWAYS use the MANDATORY "infamous
ballast resistor" whether yer coil is the original
6-volt or the modern 12V-coil.

Remember, the weird 4-nipple frontmount points are
gapped 0.015". (and points slip)

You do know ittza 2-bolt, 15-min job to remove yer
4-nipple frontmount dizzy and change or adjust the
points on the kitchen table, don't you??? Just un-
snapple yer capple and letter dangle. installation
is a reversal of the removal ...except... finger
start yer 2-bolts, install yer rotor and gently
twirl rotor until dizzy OFF-SET drive tang drops
into the OFF-SET camshaft drive slot. then tighten
the 2-bolts and re-snapple yer capple. Simple, eh?

Always "polish" the INVISIBLE corrosion from
between the points by clamping 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. Understand??? ........Dell, yer
self-appointed sparkie-meister

My 'el cheapo' meter is the same on in the picture posted earlier in this post.

I have 2 resistors now in the circuit and it started right up.

I have a coil that is showing 1.6 ohms, needed 2 resistors in series versus 1. It now starts.

" I have 2 resistors now in the circuit and it started right up. "

I'm not sure what you're getting at here.....
50 Tips

I rechecked the timing today and found it off and suspect that was why it wouldn't start. Popped to life on one turn over. I'd had the screw/plate way too high, and mine needs to be almost to the bottom position.

So, a longstanding problem and my lack of detailed knowledge is fixed. The 1.6 Ohm coil is where it started and I didnt' realize that when my old coil went bad (I think I left the key on) and the points burned, the new coil was low resistance and me, being no sr. electrician, just figured off the shelf coils are good to go. Lesson learned.

What I don't get is this-if 1 resistor in circuit is not enough, and 2 is required, due to the low coil resistance, does that make for a weak spark?