[AT] tractor electrical question

Fri Aug 27 06:28:55 PDT 2021

Seeing the voltage change from 6V to 12V when the points open/close doesn’t
make sense, it should stay constant. The voltage should be applied to the
primary side of the coil and the points should be on the secondary side.
Any opening/closing should affect the secondary side. You need constant
voltage applied to the primary side to keep the magnetic field constant.
When the contacts open the magnetic field builds up in the secondary, and
when they close the field discharges to the plugs.

Also the points act as a switch. If you think about a wall switch that
turns on your room lights, the voltage at the lights changes when you open
or close the switch, it’s full or zero, but not half.

Carl

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

> No PO, we bought it new. Put a new harness on 20-25 years ago--its in good
> condition and we haven't hacked into it anywhere.
>
> 6 or 12 volts to the coil (depending on if the pts are open/closed) except
> when cranking, then it maintains 12. It is my understanding the starter
> solenoid is what provides the boost in Voltage when the starter is
> engaged--I verified it is outputting 12 volts when starter engaged and I
> had the wire disconnected from solenoid--this is on the R terminal.
>
> The new coil is marked 12V, nothing else, yet the description on the bill
> was for 6 volt. It was in an unopened CaseIh box. Part number on the box
> and online manuals matches my 50 syear old paper parts book--thats
> encouraging. Trying to cross over the CaseIH part # to see exactly what it
> calls for doesn't really work too well, I kept getting different answers,
> thats why I ordered one from the dealer--not saying its right, but figured
> OEM would be the simplest.
>
> The old coil was probably on there for 15+ years--it only had one wire to
> the + terminal. IH schematic shows 1 wire as well.
>
> I've looked for an external resistor and so far have found nothing.
> Researching online suggests ignition systems using resistor wire--I have no
> idea about such.
>
> I'm willing to go get a coil from the parts house or rob one off a
> lawnmower for testing. I just need this thing fixed ASAP as I don't have
> another tractor to do what it does--and my time needs to be spent on other
> things. Bad how this stuff never happens in the off season when time is on
> your side!!
>
> John Hall
>
>
> On 8/26/2021 10:27 PM, Carl Szabelski wrote:
>
> 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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