Farmall 400 burning up points

I recently bought a Farmall 400. Im am using a 12 volt battery. Postive ground. Im have a 12 volt coil with the built in resistor. I have brand new condensor, rotor and cap. The tractor will run for a half hour then die. The point always are burned. I have a .020 gap on the points? Any help is greatly appericated.

Take a close look at your coil. It probably says 12v R which means to be used on a 12 volt system with a external resistor. At any rate, take a ohm reading of the primary windings. If it is less than two ohms it is a 6 volt coil, 3 to 4 ohms it is a true 12 volt coil.

The points can handle about 3.5 amps for a normal load. Yours res being subjected to much more current than that. Thus the coil is not correct. The coil will have between 4 and 5 ohms of resistance if it is indeed a 12volt coil. (disconnected at one small terminal and measured across the small terminals.) If it is more like 2 to 3 ohms, it is a resistor required coil.
I realize you indicated it was correct, but there are not really any resistors inside coils, they are just wound with more smaller diameter wire. If it is low resistance as above, put a resistor from a mid 1960s Chevrolet between the ignition switch, and the coil. Jim

I agree with the other fine gents. The points must be switching more current then they are designed for (around 4 amps is a typical maximum current)

ORRRRRRRRRRRRRRR

The condensor is bad even though its "new"

Id stick an ohm meter on the low ohms Rx1 scale on the coils LV primary between its little + and - terminals and it should measure around 3 to 4 ohms. If so the coil is NOT the problem. HOWEVER if it reads like 1.5 to 2 or a bit over ITS A 6 VOLT COIL AND NEEDS AN EXTERNAL SERIES VOLTAGE DROPPING (12 to 6) BALLAST RESISTOR otherwise it overheats and the points burn up quickly.

1) Check coils LV primary resistance to insure its a 12 volt coil, if not replace it

2) If the coil passes TRY A NEW CONDENSOR

Of course the condensor needs wired to the points terminal and well grounded

John T

I agree with the other fine gents. The points must be switching more current then they are designed for (around 4 amps is a typical maximum current)

ORRRRRRRRRRRRRRR

The condensor is bad even though its "new"

Id stick an ohm meter on the low ohms Rx1 scale on the coils LV primary between its little + and - terminals and it should measure around 3 to 4 ohms. If so the coil is NOT the problem. HOWEVER if it reads like 1.5 to 2 or a bit over ITS A 6 VOLT COIL AND NEEDS AN EXTERNAL SERIES VOLTAGE DROPPING (12 to 6) BALLAST RESISTOR otherwise it overheats and the points burn up quickly.

1) Check coils LV primary resistance to insure its a 12 volt coil, if not replace it

2) If the coil passes TRY A NEW CONDENSOR

Of course the condensor needs wired to the points terminal and well grounded

John T

Your coil is for a NEG ground system. Ground goes to the distributor,so if it is wired + on the distributor, then your built in resistor is not working at all, you have it on the output side not the input side as on a NEG ground system. I believe you need a new coil and ballast resistor(separate).

If you have or can find a circa 1940's and 1950's tractor coil containing an actual discrete "Resistor" hidden away somewhere inside the can Id LOVE some information on it cuz Ive (tractor dealer, farmer and electrical engineer for yearsssssssssssss for that vintage tractors) NEVER SEEN OR HEARD OF A SINGLE ONE and Ive seen, bought n sold a ton of them. The problem is many lay persons mistakenly refer to 12 volt coils as "Internally Ballasted" while the fact is theres no internal stand alone discrete "resistor" hidden away somewhere inside the can on "most" the old tractor coils in the forties and fifties etc. NOTE there were some very early auto applications that had coils with a divider inside (had like a ring on the outside of can) that did indeed have a discrete "resistor" contained insdie the can butttttttttttttt Ive NEVER seen such a coil on ANY 40'S 50's vintage tractors 6 or 12 volt THATS WHY IM ASKING YOU FOR INFO IM NEVER TOO OLD TO LEARN

NOW AS FAR AS WHERE ANY RESISTOR IS IN A SERIES CIRCUIT AND WHY THE CURRENT DONT CHARGE RERGARDLESS WHERE ITS LOCATED IN THE CIRCUIT (This goes to the issue of he may have too much coil current which is causing premature points burning)

A series low voltage primary ignition circuit consists of the source (battery),,,,,,,,,, the coils LV primary winding + any ballast,,,,,,,,,,the points (switching device),,,,,,,,then back to ground and the opposite battery post......The current in that LV primary ignition circuit = E/R and the current is the same regardless where the ballast is located!!!!!!!!!!!!

NOTE IM NOTTTTTTTTTTT saying an external ballast isnt normally located between the ign switch and coils input versus between coils output and distributor (where points are) Im ONLYYYYYYYYYYYYYYYYYY saying the series loop coil and points current is equal to E/R REGARDLESS where the ballast is located... IE if the points are switching too much current, that amount of current (E/R) isnt gonna change cuz you chnage the location of the resistor in the series loop circuit SO DONT ANYONE HAVE A CALF

Again with all due respect and NOT wanting to fight or argue ONLY LEARN,,,,,,,, if you have info on tractor coils that have an internal stand alone discrete "resistor" tucked away inside the can PLEASE INFORM ME !!!!!!!

ENGINEERING AND TRADE INFORMATION

What engineeres and technicians refer to as "Wire" still has some degree of "ResistANCE" BUT ITS NOT COMMOMLY REFERRED TO AS A ResisTOR ITS CALLED WIRE !!!!!!!!!!!!

A "ResisTOR" conducts current just like wire does but its called in the trade a "Resistor" NOT WIRE Well DUHHHHHHHHHHH

If you go to an electronics shop and ask for WIRE they sell you wire, if you ask for a resistor they sell you a resistor I DONT KNOW ANYONE WHO WOULD WANT TO BUY WIRE GO INTO A SHOP AND ASK FOR A RESISTOR DO YOU????????

Hope this helps

Best Wishes, God Bless Yall
Thanks vets, Happy Memorial day

John T

Boy, this internal resistor continues to come up doesn't it. Don't know where people ever got this idea as I had never heard of it until these forums. I would agree with all you stated and then add the part about having coil polarity reversed. I have seen a ton of vehicles running just fine with reversed polarity. Is it correct, well no, but the difference it makes is miniscule. The actual polairy is reversed at the firing of the gap on the spark plug. Theoretically it takes a slightly higher voltage to fire if reversed. But, some engines, (wasted spark type) fire one plug negative polarity and the other positive and no one questions this. I'm not an engineer and do get things bassackwards sometimes but I will stand on this one.

Youre exactly right about reverse polarity I was ONLY trying to explain to him how the steady state DC current in a series circuit doesnt change if a resistor is located before or after the points... (and besides theres no actual by definition "Resistor" in there anyway, just wire with its small ResistANCE ) I gave him enough info without adding another can of worms to discuss polarity since his post wasnt really about that.

People are free to call coils internally ballasted or Billy Bob ballasted as far as Im concerned, call em what they like lol (As an engineer I prefer to go by trade terms) I just try to explain to lay persons if they disect one of them do NOT expect to find whats defined and normally understood as a stand alone discrete "Resistor" hidden way somewhere, ONLY coils of wire which, not being a prefect conductor, yes contains some degree of resisTANCE

The data Ive seen was the firing voltage required to arc jump current across a plugs gap was a few thoudand higher if the polasrity wasnt correct. Youre right, they still run at wrong coil polarity but the ignition isnt as efficient when its backwards...

Good info, thanks for the post fun chat

John T

The tractor is running fine now. I put a resistor in between the ignition switch and the negative side of the coil. I put another new coil on it, the coil is a 12 volt that says on the side requires external resistor. i put a spark plug wires on it. I replaced the condenser, points which i gaped to .022, rotor, cap, and spark plugs. I also put a new wire from the positive side of the coil to distributor. Thanks for the help
Its a learning experience i never worked on gas tractor before.

It states on the side of the coil does not require external resistor. Im not a electrical expert im just a general machinist by trade. I never worked on a gas tractor before.

If itis positive ground you are good to go. (point gap is .020" though)
If it is an alternator with negative ground, the coil needs to have its negative side hooked to the distributor. It will run as is, but is happier with the correct gap, and polarity. Jim