Need for ballast Resistor in 12V conversion

[Dave in CT]

I am performing a 12-Volt conversion on a recently purchased Super H. I have Bob Melville's wonderful "Farmall M and Super M 12 VOLT CONVERSION" which I believe will serve in my case as well. I'm just looking for confirmation on a slight alteration to the usual conversion procedure.

I'm replacing the stock 6-volt coil with a 12-volt Standard brand UC16T coil which has "USE WITH EXTERNAL RESISTOR" printed on the side (it also has "1306" printed on the side). I thought the external resistor was to reduce the 12 volts to 6 volts so one could use the old 6-Volt coil. Will I still need that resistor (which I have) when using this coil?

Thanks for your time.

 

[old]

Yes you need a resister the coil as you said says it needs one simple as that

 

[AlinMO]

Why not just use your old coil with a ballast resistor? Use a resistor for a 1966 Dodge.

 

[Janicholson]

The coil you purchased is a s volt coil and needs the resistor a way to increase the spark during startup is to run a bypass wire from the starter side of the starter heavy switch through a diode that allows 5 amps to go from the starter to the ignition coil, but not back to the starter. This is connected to the coil side of the ballast resistor. Don't use a pertronix ignition with the bypass, not needed then. Jim

 

[Dave in CT]

Not sure what you meant by "s volt coil" - a typo? The 1306 also has a big "12V" printed on it. I was hoping to just use a 12V coil and not bother with the resistor but, I'll do whatever you recommend - you obviously have the experience.

I also have the diode as well so I can wire that in, if needed.

Also not sure what "Don't use a pertronix ignition with the bypass" means.

 

[the tractor vet]

If it say USE then use When i do a conversion i have never changed a coil i install a resistor .

 

[Jim.ME]

(quoted from post at 12:28:02 11/04/23)
Not sure what you meant by "s volt coil" - a typo? The 1306 also has a big "12V" printed on it. I was hoping to just use a 12V coil and not bother with the resistor but, I'll do whatever you recommend - you obviously have the experience.

I also have the diode as well so I can wire that in, if needed.

Also not sure what "Don't use a pertronix ignition with the bypass" means.

Yes the s is a typo, it should say six.

If you wanted to use a 12-volt coil that does not need an external resistor you need one that says No external resistor required, or such on it.

A diode is used to prevent back feed from an alternator excitation circuit, not in the ignition.

Pertronix ignition is a brand of electronic ignition. A bypass is a circuit from the starter solenoid to the coil which provides a full voltage bypass of the resistor, during cranking, to improve spark.

 

[Janicholson]

The new Standard Brands coil is the same as a six volt coil (same actual primary resistance, or close). If you choose, now or later, to use electronic ignition, the bypass i mentioned could put the ignition module in danger, follow the kits instructions at that point. (just future thinking) Jim

 

[used red MN]

NAPA carries a a resistor.
One resistor option

 

[BarnyardEngineering]

Ok, here's the reason WHY a "12 Volt" coil is in reality a "6 Volt" coil and needs a resistor.

You'll find that on many 12V gas powered vehicles, there is a wire that runs from the "I" terminal on the solenoid and bypasses the ballast resistor. This compensates for voltage drop during cranking. So instead of 3V, and a weak spark (or no spark), the coil gets 10V and puts out an extra-strong spark.

They're called "12 Volt" coils to prevent countless arguments at the parts counter. "I need a 12V coil for my 1968 Dodge Charger." "Here's a 6V coil." "But it's a 12V system." "The book calls for a 6V coil." "It's a 12V system that doesn't make any sense. Look again." "It only calls out this one part number, a 6V coil." "Stupid parts guy."

 

[RTR]

Not that anyone else here hasn't, but ole JOHN T can give a good detailed explanation.

It is basically a piece of ceramic coated wire that reduces the voltage down to keep from burning the points as I understand it. If it clearly says "use a resistor" , then use a resistor. Get a Chrysler-style ballast resistor from parts store. Just make up a random 70s Chrysler or Dodge model.

This post was edited by RTR on 11/06/2023 at 01:58 pm.

 

[Pete in Holland MI]

A coil with a resistor in line is another piece of equipment, and another junction point to go bad. Get a coil set up for 12v with an
internal resistor. Do it right the first time.

I use a NAPA 905 on my conversions (4 so far) with no resistor and no issues.