1948 8N 12v front dist-Summarizing coil information.

Trying to nail this all down; might be helpful to others. So, my 12V coil reads lets say, 1 on the 1K Ohm scale. My stock ballast resistor reads say 1 also, hot. That's 2. Given that, and the known of my alternator outputting 14.8V, we can take 14.8V / 2 ohms and get 7.4 amps which is way over what we'd normally want, correct? And, if I put 2 more resistors inline, making 3, that's then 4 total ohms, which, when we divide that into 14.8V, we get 3.7A, which is pretty close to what I'm hearing said as an ok rating for running my tractor.

All that said, at 7.4 A is gonna burn my coil up, correct?

I guess the big shocker is that coils 'used to' run 2.5Ohms, and now are <1? Are others finding this problem? I don't really want to put the new round coil on my '48 despite the fact that it'll probably fix the problem, I prefer the stock setup...

You need a good meter to read in the 1-2 ohm range. I would recommend converting to a real 12 volt no resister needed round can oil filled coil as shown on Hobo's web site. That is what I did and then you don't need to worry about what size resisters to use. To little resistance and you can burn up the coil and/or points. To much resistance and your tractor will not start. The 12 volt round can coil solves both problems.

You're making this waaaaay more difficult than it needs to be.

Keep it simple. Get a "12v" coil from a reputable source. It will be 2.5 ohms or greater.

2.5 ohms, plus 1 ohm (hot) on the OEM ballast resistor gets you 3.5 ohms. Do the math.

You're at 4.1 amps. Close enough, because the actual value (red hot) of the OEM ballast resistor is really 1.7 ohms. So, your current when running will be between 3.4 & 4.1 amps.

If your meter measures 1 ohm on a "12v" coil, you have a faulty meter or a "6v" coil. Unless you paid close to $100 for that meter, chances are the problem is the meter.

So, get a "12v" coil, an OEM ballast resistor & hook it up. Then, measure current, not ohms, per JMOR's picture.

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(quoted from post at 12:03:48 07/22/11) Trying to nail this all down; might be helpful to others. So, my 12V coil reads lets say, 1 on the 1K Ohm scale. My stock ballast resistor reads say 1 also, hot. That's 2. Given that, and the known of my alternator outputting 14.8V, we can take 14.8V / 2 ohms and get 7.4 amps which is way over what we'd normally want, correct? And, if I put 2 more resistors inline, making 3, that's then 4 total ohms, which, when we divide that into 14.8V, we get 3.7A, which is pretty close to what I'm hearing said as an ok rating for running my tractor.

All that said, at 7.4 A is gonna burn my coil up, correct?

I guess the big shocker is that coils 'used to' run 2.5Ohms, and now are <1? Are others finding this problem? I don't really want to put the new round coil on my '48 despite the fact that it'll probably fix the problem, I prefer the stock setup...

Click to expand...

Fact: you can not accurately measure 1 or 3 ohms on a METER'S X1k OHM SCALE.

I agree-and, hope you understood that on my 1K scale, I had been seeing .8 on my cheapo meter, which, works for DC V but I won't use it for other stuff. I do have a nicer clamp on meter, it's $109 new, but I got it used on Ebay. Ok, so, I just checked my coil and now, I get 2.2Ohms with the good meter and 0 with the cheapo, which is why I won't use that one anymore. At 2.2 Ohms, I can live with that...and, sorry for the long discussion, just wanting to be sure. Agree about the round coil, but, rather have 'stock', despite the fact that a round coil would be best...

Thanks all.

Oh, and can I then read the amps in my circuit when the tractor is running? I'd think I would use the 40A setting and touch the coil with red and frame with black to get an accurate A reading?

(quoted from post at 12:58:20 07/22/11) I agree-and, hope you understood that on my 1K scale, I had been seeing .8 on my cheapo meter, which, works for DC V but I won't use it for other stuff. I do have a nicer clamp on meter, it's $109 new, but I got it used on Ebay. Ok, so, I just checked my coil and now, I get 2.2Ohms with the good meter and 0 with the cheapo, which is why I won't use that one anymore. At 2.2 Ohms, I can live with that...and, sorry for the long discussion, just wanting to be sure. Agree about the round coil, but, rather have 'stock', despite the fact that a round coil would be best...

Thanks all.

Oh, and can I then read the amps in my circuit when the tractor is running? I'd think I would use the 40A setting and touch the coil with red and frame with black to get an accurate A reading?

Click to expand...

"Oh, and can I then read the amps in my circuit when the tractor is running?"
You could, but there are at least 2 problems with that, 1) it is not a steady DC current, it is a pulsed/complex current waveform, so unless you have a true averaging meter, it will be a meaningless number, 2) I am more than certain that you have no idea what a reasonable "engine running" reading to expect, even if you have a true averaging meter. So why try it?


"I'd think I would use the 40A setting and touch the coil with red and frame with black to get an accurate A reading?" Absolutely NOT! If you have a 'clamp on' DC current meter, you make no connections....just clamp it around the insulated wire. If you set a "wired" ammeter to 40amp scale & touch hot coil wire with red & frame with black, that simply shorts the coil input to ground & would read current through the resistor to ground. Tractor will not run since there will be no coil current. Bruce & I have long ago given you instructions & pictures on how to measure the current. Since you don't know this stuff, just stop being creative & follow the instructions from folks who know what they are doing. Not trying to beat you about head & shoulders, but you will only mislead yourself or harm some equipment.

Bruce-tried to get an amp reading-can't get one that makes sense on my meter. Frustrating. Points closed, key on, I get 2.2 or something crazy.

It could be your meter or the way you are trying to take the measurement. But, as I said earlier, just install an OEM ballast resistor & 2.5 ohm coil & don't worry about it. If you want to check it, follow JMOR's instructions.
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