Deere A Coil. internal or external Resistor.

primerk5

Member
The coil that came with my tractor was super rusty. I cleaned it up and it started to leak. My Dad had an old NOS Standard products 12 volt coil that is marked "use with external resistor" I don't see any resistor in the wiring kit i bought from the Brillman Co.

What is the proper coil for this? with or without resistor built in.

for what its worth, I didn't see one on my original either.
 
This is an easy one:

"12 volt coil that is marked "use with external resistor"

THAT MEANS EXACTLY WHAT IT SAYS: If used on a 12 volt tractor "USE WITH EXTERNAL BALLAST RESISTOR" like it says or the coil will overheat and the points burn up sooner. Its in reality a 6 volt coil, but if used on a 12 volt tractor, you use the external series voltage dropping (12 to 6) ballast resistor so the coil only sees the 6 volts (other 6 dropped across ballast) it was designed to handle. If used on a 6 volt tractor DO NOT USE ANY EXTRA BALLAST.


"What is the proper coil for this? with or without resistor built in."

If its a 12 volt tractor, you can EITHER use a full true 12 volt coil (may be labeled "12 Volts" or "12 Volts NO Ballast required" OR IF YOU USE A 6 VOLT COIL (like you mentioned) then you need the external series voltage dropping (12 to 6) Ballast Resistor.

NOTE if you x ray or dissect a coil that Billy Bob and Bubba say has an internal resistor, YOU WILL LIKELY NOTTTTTTT FIND A DISCRETE STAND ALONE RESISITOR TUCKED AWAY SOMEWHERE INSIDE THE CAN.

If I forgot anything (likely lol) the other fine gents will hopefully add to this.

John T
 
We use 12V coils marked "use with external resistor" on all our tractors with alternators.

The photo below shows an external resistor.

a161113.jpg" width="650"


Take a look at a few "external resistors" on our tractors.

"15" one of our Model 70s.

a161107.jpg" width="650"


"1011" one of our Model 70s.

a161108.jpg" width="650"


"Vernon" one of our Model 70s.

a161109.jpg" width="650"


"BW" our Model 40T.

a161112.jpg" width="650"


Hope this helps.
 
Great Pics. You're right, if its a 12 volt tractor and the coil says "use with external resistor" YOU INDEED NEED ONE (to drop 12 volts down to 6) or else the coil would overheat as its only intended for 6 volts. The alternative would be a full true 12 volt coil NO BALLAST REQUIRED.

John T
 
There was no ballast resistor on my tractor when i got it. So, your saying I could use the new one i have and put a ballast resistor on it or get a 12 volt one with a resistor built in?

My tractor is originally 12volt with a generator.

Sorry, but i got somewhat confused and I am trying to retain the configuration my tractor was in when i got it.
 
For goodness sakes and for the last ^}+#={*]^[#{^#+#= time. There IS NO such thing as a 12V coil with a resistor inside it
 
"So, your saying I could use the new one I have and put a ballast resistor on it YES or get a 12 volt one with a resistor built in? YES

CORRECT Either will work on a 12 volt tractor:

6 Volt Coil (labeled "For use with External
Ballast") and then you add one:

12 Volt Coil (Labeled "12 Volts" or "12 Volts
NO Ballast Required)

You got it

John T
 
Sorry but not 100% true. American Motors from 1960
to 1962 with their six cylinder engines came in two
versions of ignitions. One used a Prestolite
distributor and a Chrysler designed coil with an
internal ballast resistor. That is the only such
coil I know of that was eve used with a 12 volt
system but does indeed exist. the other system was
a Delco coil with external resistor that GM had used
from 1955 to 1963.
 
I have a 51A that is pretty much original. There is no ballast resistor on it. BUT...it is 12v positive ground. To the best of my knowledge, coils for pos or neg ground have different internal connections. Both have + and - terminals but I believe the primary and secondary coils will both be connected to the ground side. Either will work but the incorrect one will give a weaker spark. Anyone care to add to this?
 
JD I have a 62 Rambler with a coil that does not need a resistor. When my sis was using it in college (late 60's) some gas station mechanic in Cedar Falls replaced it with a coil that does need a resistor. Points weren't lasting very long till I figured it out. Jim
 
Glenn;
You're gonna have to take it up with the manufacturers. As long as they put {for use with external ballast} and {no ballast required} on the boxes they are implying there is a ballast resistor inside even though we know there isn't. Can't fault folks too much for listening to the manufacturers.
 
Youre on track. A coil will still produce a spark at either polarity, but its more efficient if used correctly, i.e. for a Pos Ground tractor the + goes to distributor while if a Neg Ground the - goes to distributor.

Once more FOR OTHERS BENEFIT (sounds like you have it) on a 12 volt tractor use EITHER:

a) Full true 12 Volt Coil which may be labeled "12 Volts" or "12 Volts NOT for use with Ballast Resistor" or "12 Volts No Ballast required"

b) A 6 Volt Coil which may be labeled "6 Volts" or "For use with Ballast Resistor" or "Ballast resistor Required" PLUS the series voltage dropping (12 to 6) Ballast Resistor JUST LIKE IT SAYS.

This is so simple but some (unlike you) still have a problem with it.

John T
 
Mornin M Man

" Can't fault folks too much for listening to the manufacturers. "

I AGREE This is so simple but some (not you) have such difficulty for some reason?


IF IT SAYS FOR USE WITH OR REQUIRES A BALLAST THEN USE ONE If you don't the coil will overheat and eventually fail. If it says that, it needs the external series voltage dropping (12 to 6) ballast so the coil sees the 6 volts it was designed to handle.

IF IT SAYS NO BALLAST REQUIRED THEN DONT USE ONE Otherwise the spark will be weak. If its a 12 volt coil and says no ballast required its designed to operate at 12 volts nominal (lower when cranking but still at 14 charging volts) SO DONT USE A BALLAST THAT REDUCES COIL VOLTAGE DOWN TO 6 OR SO.

So simple

John T
 

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