[AT] tractor electrical question

Thu Aug 26 19:27:12 PDT 2021

If you’re reading 6V at the coil on a 12V system with the engine off and
the ignition switch in the on position, then you’re dropping 6V from the
starter relay to the coil. That would indicate you have a ballasted
ignition system which has a ballast resistor in the wire going from the
relay to the coil. There should be a second wire attached to the + terminal
of the coil that provides 12V during a start by temporarily bypassing the
ballast resistor through the ignition switch when you start. The reason the
new coil is marked 6V is because that’s what you need. The coil should only
need 6V when the engine is running. Running at 12V will cause the coil to
overheat and eventually fail. Applying 12V during a start for a short time
doesn’t hurt it.

If you don’t have the second wire, then you probably have a coil with an
internal resistor. That requires 12V to the coil at all times, 6V will be
dropped internally through the internal resistor. Maybe the PO did some
rewiring and left a ballast resistor in while putting in a coil with an
internal resistor. If you do have an in-line ballast resistor and a coil
with an internal resistor, then you have too much resistance and you’re
dropping too much current going through the coil, which gives you a weaker
spark. That may be why putting 12V directly to the coil makes it run
better. You’re overcoming the extra resistance with twice the voltage.

The first thing you need to do is determine which type of system you have
(ballasted or non-ballasted), and if you have the right coil (internal
resistor or not). From looking at the wiring diagram I sent earlier it
doesn’t seem to indicate you should have a ballasted system since it
doesn’t show a second wire at the coil. But again, maybe the PO made some
modifications?

Now if you’re dropping 6V at the starter relay, then the relay or the
connection to the relay is suspect. That’s why I suggested testing the
starter relay to see if you were dropping voltage there.

Once you have a definite idea of what type of system you have, it should
make finding the problem easier.

Carl

On Thursday, August 26, 2021, John Hall <jtchall at nc.rr.com> wrote:

> points, plugs, cap, rotor and coil are all new. I verified with my 50 year
> old parts book and it has the same part# for coil as what online parts at
> Case is showing. The new coil has 12V on the outside, but the bill had 6V.
> Box was sealed up, but it came from overseas, so a lot of variables there
>
> Point gap was verified when I had the distributor off and plate out so I
> could make sure weights were free and springs not broken.
>
> I had my voltage regulator with me yesterday. When I tried to check at
> coil with engine running rough, it showed OL for whatever reason. I've
> checked so much stuff lately that I'm not sure but I think I got 6 and 12
> volt readings depending on where the points were once I shut the engine
> off. I tried checking the voltage with engine running at the fuel solenoid
> for reference and it was floating all over the place from 4 to 13.5V. This
> was observed with the engine hot and running rough. With engine not running
> it holds steady a little over 12.
>
> Last night I also found that the wire to the coil and the wire to the R
> terminal on starter solenoid can read 6V (little less actually) while the
> wire going to the fuel solenoid in the carb was reading 12V.
>
> John Hall
>
>
>
> On 8/26/2021 6:35 PM, Howard Pletcher wrote:
>
> It's not unheard of, particularly since many of those parts now come from
> overseas.  To me, swapping with another if you can find something to swap
> with would be the easiest way to prove or eliminate that as a possibility.
>
> Make sure your points setting is close to spec and they are clean as that
> would affect the ignition.  Have you looked at your plugs, not thinking
> that's the problem, but if they are fouled or badly gapped, they could have
> an effect.  You also might run it until it heats up and needs the 12V to
> see if it still shows 6V to the coil.  If not, then it's somewhere else in
> the circuit.
>
> Many of the other things suggested have no connection with the ignition
> circuit.
>
> On Thu, Aug 26, 2021 at 5:12 PM John Hall <jtchall at nc.rr.com> wrote:
>
>> Howard, you are leaning toward new coil and old coil are both bad?
>>
>> John Hall
>>
>> On 8/26/2021 3:47 PM, Howard Pletcher wrote:
>>
>> No, what he is reporting is just what should be happening.  With the
>> points open, no current is flowing through the primary circuit and there is
>> 12V at the + side of the coil.  When he bumped the starter and closed the
>> points, the current flowing through the resistor in the primary circuit
>> results in a voltage drop so that there is now 6V on the coil as intended.
>>
>> The wire from the starter to the coil does send 12V directly to the coil,
>> but this is only engaged while the starter is cranking in order to produce
>> a hotter spark for starting and is not connected to 12V  the rest of the
>> time.
>>
>> The fact the coil seems to require 12V to its terminal when hot points to
>> an internal problem in the coil as discussed by Dell in the Yesterday's
>> Tractors article.
>>
>> Howard
>>
>>
>>
>> On Thu, Aug 26, 2021 at 2:53 PM Carl Szabelski <c.s.szabelski at gmail.com>
>> wrote:
>>
>>>  By the way, I believe it’s not the coil and is related to the cranking
>>> motor. Especially since the voltage drops when you bump it like you said.
>>> Something in the motor may be partially shorted or corroded, causing a
>>> voltage drop to the coil. The coil should always see 12V whether or not
>>> there is a resistor, internal or external. The resistor is essentially
>>> after the coil winding and drops the voltage after the winding, not before
>>> it. If the engine is running fine when you directly connect 12V to the
>>> coil, the coil is working like it should and is good.
>>>
>>> Carl
>>>
>>>
>>>
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>
>
> --
> Howard
>
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