jd 40 12v conversion

valley iron · Dec 10, 2018

just baught a jd 40s and previous owner had converted to 12v.everthing seems to work fine except coil is 12v with no internal or external resistor.figure i should cut voltage down for points. should i go with 12v coil with internal resistor or add external one to extend life of points?

Hayfarmer · Dec 10, 2018

for long term new coil, 12 v with internal resister may be better. Resister will also work but they sometimes burn out after a while. One thing to check is if it has regular points and condensor or EI

valley iron · Dec 10, 2018

good point! so used to somebody elses cobbled mess that overlooked that they actualy might have done it right. will pop cap off and take a look.

rvirgil_KS · Dec 10, 2018

First, you need to determine what coil you actually have.

A direct 12 volt coil-no external resistor required will have about 3 ohms resistance across the + and - terminals. A 6 volt coil, or a 12 volt external resistor required will have about 1 1/2 ohms across the + and -. By adding a 1 1/2 ohm resistor in series with the 1 1/2 ohm coil will give 3 ohms total so it will work on 12 volts.

Modern replacement coils for old tractors do no have an added resistor inside the can in series with the coil primary. 1 1/2 and 3 ohm coils are wound with different wire to give the desired resistance. Both are one continuous coil of wire.

The question is which coil do you have. Sounds like it might be the correct 3 ohms. An ohmmeter would tell.

Or if in doubt get a NAPA IC14SB coil or equivalent. It is a direct 12 volt no resistor required

valley iron · Dec 10, 2018

thanks for advice. if it dosent have EI i will go with that napa coil.

buickanddeere · Dec 10, 2018

Google the part number on the side of the coil or measure the coil's primaries . Instead of guessing it is 1.5 or 3.0 ohm.

John T · Dec 10, 2018

You say the coil is 12 volt no internal or external resistor??????????? So is it 12 volt or not??? How is it labeled??

First of all it likely DOES NOT HAVE ANY STAND ALONE DISCRETE INTERNAL RESISTOR WHATSOEVER. If its a 6 volt rated coil on a 12 volt tractor THEN YOU DO NEED AN EXTERNAL BALLAST RESISTOR or the coil would overheat and the points burn quickly.

I doubt you will find any 12 volt coil that has an actual internal resistor ????????? They did make coils with internal resistors but Ive never seen one on an old tractor !!!!!!!!!!!

If its a 12 volt rated coil the LV primary resistance measured between the little + and - terminals would be around 2.5 to 4 ohms, many are 3 ohms. And if so no external ballast resistor is required.

If its a 6 volt coil it would be around 1.2 to 2 or so ohms, many are 1.5. If that's what you have you need an external series voltage dropping (12 down to 6) ballast resistor.

I would use a full true 12 volt rated coil (maybe what you already have???) and not worry with any ballast

John T Long retired n rusty Electrical Engineer

jd 40 12v conversion

valley iron

Member

Hayfarmer

Well-known Member

valley iron

Member

rvirgil_KS

Well-known Member

valley iron

Member

buickanddeere

Well-known Member

John T

Well-known Member

Similar threads