Battery to distributor resistance

jmerrell@knology.net · Jul 5, 2015

I have a 1949 8n, converted to 12V. Anybody know the resistance values of: A) the terminal board built-in resister, B) the in-line ballast resister, C) the 12 V coil. What is ideal power that needs to be supplied to the plugs?

JMOR · Jul 5, 2015

(quoted from post at 16:19:20 07/05/15) I have a 1949 8n, converted to 12V. Anybody know the resistance values of: A) the terminal board built-in resister, B) the in-line ballast resister, C) the 12 V coil. What is ideal power that needs to be supplied to the plugs?

he 12v front mount coil is typically 2.5 Ohms. Select the other resistor(s) such that coil + resistance will yield 4 amperes coil primary current. That is it.

Geo-TH,In · Jul 5, 2015

Jessie,
If you have two resistors the same size in series, there will be an equal voltage drop
across each. So, if the ballast resistor is equal in size to the coil's resistance,
won't there be 6v drop across the ballast and 6 volts applied to the coil right?

Then to charge a 12v battery the alternator puts out 14+ volts, so then you would have
a 7 volt drop across ballast and 7v drop across coil.

A previous owner converted my jubilee to 12v and when I turn the key on, my coil is
only getting 6v when the points are closed and engine isn't running, ballast is
dropping 6 v. It's worked that way for me with no issues. I would think the larger the
ballast, the less likely you are to burn the points.

Steve@Advance · Jul 5, 2015

Not sure of the value of each resistor, but checking across the resistor(s) and the coil in series should be 2.5 to 3.5 ohms.

JMOR · Jul 5, 2015

(quoted from post at 18:43:43 07/05/15) Jessie,
If you have two resistors the same size in series, there will be an equal voltage drop
across each. So, if the ballast resistor is equal in size to the coil's resistance,
won't there be 6v drop across the ballast and 6 volts applied to the coil right?

Then to charge a 12v battery the alternator puts out 14+ volts, so then you would have
a 7 volt drop across ballast and 7v drop across coil.

A previous owner converted my jubilee to 12v and when I turn the key on, my coil is
only getting 6v when the points are closed and engine isn't running, ballast is
dropping 6 v. It's worked that way for me with no issues. I would think the larger the
ballast, the less likely you are to burn the points.

rue, but if you take that toward the limit, you eventually have only milliamps of coil current (clearly a no start, no run situation) , but points will last forever! 4 amperes is the correct answer.........forget the volts!

Geo-TH,In · Jul 5, 2015

Jessie, If I ever get the time, I'll measure the coil current on my Jubilee. It has a
round coil, not a square one. Do round ones use 4 amps too?

JMOR · Jul 6, 2015

(quoted from post at 21:47:11 07/05/15) Jessie, If I ever get the time, I'll measure the coil current on my Jubilee. It has a
round coil, not a square one. Do round ones use 4 amps too?

he 4 amperes is nearly 'universal' since that is about upper end of where points can switch & survive reasonably well.

jmerrell@knology.net · Jul 6, 2015

One reply says 2.5 - 3.5 ohms. Another says 4 ohms. Which is it? Also, my question was: WHAT IS RESISTANCE OF EACH IGNITION COMPONENT: 1.Terminal block resistor, 2. ballast resister, and 3. the coil itself. I've already burned out one coil and don't want to repeat that.

JMOR · Jul 6, 2015

(quoted from post at 09:29:47 07/06/15) One reply says 2.5 - 3.5 ohms. Another says 4 ohms. Which is it? Also, my question was: WHAT IS RESISTANCE OF EACH IGNITION COMPONENT: 1.Terminal block resistor, 2. ballast resister, and 3. the coil itself. I've already burned out one coil and don't want to repeat that.

ho said 4 Ohms?

Texasmark1 · Jul 6, 2015

In the MF 35 service manual the recommended point break current is 5 amps at the peak....aka when the break occurs. Using ohms law, battery voltage/5 amps = total resistance in the circuit......that's everything from + to - battery terminals through that circuit.

So, considering you are running and the gen is genning, you are getting say 14v for the voltage part divided by the 5 amps of break current = 2.8 ohms.......doesn't matter where it is, coil primary, ballast resistor, wiring resistance, point contact resistance......the total is the total.

Battery to distributor resistance

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