Farmall A no spark

I have an A tractor that I have converted to 12v. The coil I got from the IH dealer said it was for 12V with external resistor. When I am cranking the tractor I have no spark from the coil, if it is still rolling over when I release the starter it will fire and start. If I attach a jumper wire across the ballast resistor it will send a spark while cranking and continue to run when the wire is removed. Is my coil possibly designed for 12v input? It has 2.4 ohms across the primary when cold. The resistor I'm using has 1.3 ohms resistance.

(quoted from post at 11:38:26 02/28/21) I have an A tractor that I have converted to 12v. The coil I got from the IH dealer said it was for 12V with external resistor. When I am cranking the tractor I have no spark from the coil, if it is still rolling over when I release the starter it will fire and start. If I attach a jumper wire across the ballast resistor it will send a spark while cranking and continue to run when the wire is removed. Is my coil possibly designed for 12v input? It has 2.4 ohms across the primary when cold. The resistor I'm using has 1.3 ohms resistance.

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Unless you have a $$$$ meter it may not be reading low Ohms accurately.

Your reading of 2.4 Ohms is in between the 1.6 Ohms or so of the typical 6 Volt coil and the 3 - 3.5 Ohms of a typical 12 Volt coil, impossible to say if the coil is "odd" or the meter reading is not accurate.

If you have an Ammeter capable of reading in the 5 Amps range that is a more accurate way to determine what's going on.

With the battery fully charged and the breaker points "closed", connect the meter to read coil primary current, then switch the ignition "ON" and read the current.

Typically, you want to "see" between 3 and 4 Amps.

If the primary resistor's value is too high and current is at the lower end of that range or below spark will be weak.

On the other hand, primary current approaching or exceeding 4 Amps can cause excessive coil heating and shortened breaker point life.

A couple of other "ideas", check battery voltage while cranking, a 12 Volt battery with a weak cell would still crank that little engine well, but with weak spark voltage, I've had exactly that happen.

Check for full battery voltage at the switch side of the resistor, a "weak" ignition switch or poor connection in the primary circuit could cause voltage drop and weak spark. The points need to be closed while checking this to "load" the primary circuit.

Also, poor resistance-type sparkplug and coil wires, or a missing carbon "brush' in the distributor cap or a mismatch of cap and rotor could be causing a weak spark.

Just some things to check out!

Thanks for the ideas, I just tried measuring the current and got 1.6 amps. If I use a jumper wire on the resistor it is drawing 8 amps, maybe I have the wrong resistor for this coil. I left the trickle charger on it for overnight and I will try it again tomorrow as the battery was discharged some from all the recent cranking. The rotor, cap and points have been replaced already.

I would wire it to by pass the resister for starting.

I'm not sure how to do that with the rod and button style starter. If it had a key start that would be the easiest way to do it.

I have found tractors that are 12v conversions with 6v starter will spin the
engine convincingly at 7 or 8 volts but not have enough voltage to fire the
coil. Charge it up and try again.

2.4 ohms say to me it is likely to be a true 12 volt coil and no resister is needed. I'd pull the 2 wires off the resister and hook them together and try to start it. If it starts as it should leave it that way. If it still doesn't like as it should good chance the starter is pulling to many amps so taking to many amp from the system to power the coil

Hello..guess I will call you Al welcome to YT! Since
you say you are drawing 8 amps with the resistor in
line I wonder what you are seeing if going directly to
the coil. Considering whatever the accuracy of the
instrument you are using. The higher the amp draw
with the resistor in line the less likely the resistor will
be to causing a no start issue. This is in the terms of
you saying you may have the wrong resistor. Old may
be on to something with the coil not actually needing
a resistor. Quite often new coils that do not need a
resistor have it printed right on the can. If all else fails
you could run a momentarily push button to hold while
cranking that powers the coil side. To wire a bypass
wire without that would require to figure out how to
bring a wire out from under the button switch cover
and install a diode to not back feed the starter from
the ignition. It is odd you would have this issue my M
is converted to 12v and I currently have no charging
system on it. I can easily go all day and make 2
dozen or more starts. I run a stock 6 v ..grenade.. coil
and a stock 350 resistor it starts fine. And an M
cranks way harder than an A, but it also has what I
think is an M&W reduction gear on the starter.

Lets take your post one step at a time:

1) The coil I got from the IH dealer said it was for 12V with external resistor.

IFFFFFFFFF thats actually the case ???????? its more like a 6 volt coil so it needs the external resistor JUST LIKE IT SAYS
or it will overheat plus the points burn premature.

2) When I am cranking the tractor I have no spark from the coil, if it is still rolling over when I release the starter it
will fire and start. (may be a bad battery or cables or grounds or connections, see below)

That sounds like there is insufficient coil current WHEN CRANKING at which time battery voltage is reduced so the spark is
weak but when its not cranking and battery voltage is normal it fires okay

3) If I attach a jumper wire across the ballast resistor it will send a spark while cranking and continue to run when the
wire is removed.

Thats fine UNLESS the coil current is excessive (which more ballast resistance will fix) in which case after running the
coil may overheat

4) Is my coil possibly designed for 12v input? It has 2.4 ohms across the primary when cold.

A typical full true 12 Volt rated coil (no ballast required) would be like 2.5 to 3.5 or so ohms (many around 3) HOWEVER
a cheap ohm meter at low range isn't real accurate I CANT SAY FROM HERE YOUR COILS ACTUAL RESISTANCE

5) The resistor I'm using has 1.3 ohms resistance.



6) Again your meter may or may not be accurate. Typical are 1.2 to 1.8 or so ohms


NOTES

Typical coil LV primary currents are around 3 to 4 amps ifffffffff you're only drawing 1.6 THE SPARK MAY BE WEAK WHEN
CRANKING but strong enough to make her run otherwise once started

7) Perhaps your starter is bad drawing excess current drastically reducing battery voltage causing a weak starting spark
???????????

8) Perhaps your battery or ground cables or connections or terminals are bad/resistive dropping too much voltage causing a
weak starting spark ??????????????

9) You need to check voltage ON THE STARTER when cranking and if its below 9 or 10 volts due to the above or a bad battery
or not well charged THAT CAN CAUSE YOUR PROBLEM !!!!!!!!!!!!!!!

10) The goal should be a coil and any needed ballast should total around 3 total ohms so the coil current is in the 3 to 4
amps range and when cranking battery voltage doesnt drop too much below 10 volts........

11) I would check your battery and cables and connections first !!!!!!!!!!!! Any of those may be the problem

Im sure I missed a few things hopefully the other gents can add to this

John T

I am drawing 8 amps when wired without the resistor, but only 1.6 amps when going through the resisitor that's why I'm wondering if the resistor that I used is too high of ohm rating?

There are starter switches with a terminal on the side that in use gets battery
voltage to feed the coil. I think JD used them on some of the 12 volt 2
cylinders.

This site has them listed under JD 60.

Thanks for the help everyone. It turned out to be a bad ignition switch, only getting 9-10 volts through the switch put in a new one and it
works great.