[AT] tractor electrical question

Fri Aug 27 23:43:43 PDT 2021

Greg,

You are absolutely correct. If using a really high input resistance reading
device such as a VOM, it will not draw enough current to impact the voltage
reading. So here is what is happening.  When the points close the voltage
read is 6V because the Ballast resistor is equal to the DC resistance of the
coil.   When the points are open , no current flows so the voltage goes to
12v because there is no current clow through the ballast resistor so the
voltage is equal on both sides of the ballast resistor.  Basic Ohms Law.

Dean VP

Snohomish, WA 98290

"Socialism is a philosophy of failure, the creed of ignorance, and gospel of
envy, its inherent virtue is the equal sharing of misery."

..Winston Churchill...

From: AT <at-bounces at lists.antique-tractor.com> On Behalf Of Easley, Greg A.
Sent: Friday, August 27, 2021 10:07 AM
To: 'Antique Tractor Email Discussion Group' <at at lists.antique-tractor.com>
Subject: Re: [AT] tractor electrical question

Ohm's Law

Points open = no current flow = no voltage drop

Points closed = current flow = voltage drop

From: AT <at-bounces at lists.antique-tractor.com
<mailto:at-bounces at lists.antique-tractor.com> > On Behalf Of Carl Szabelski
Sent: Friday, August 27, 2021 10:53 AM
To: Antique Tractor Email Discussion Group <at at lists.antique-tractor.com
<mailto:at at lists.antique-tractor.com> >
Subject: Re: [AT] tractor electrical question

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John,

Found this article that might help explain the difference between ballasted
and non-ballasted systems, and hopefully, help you with you issue. Still
can't see how the opening/closing of the points is affecting the voltage
input to the coil since there shouldn't any affect on the primary side of
the coil. Even if there was an internal short between the primary and
secondary, that wouldn't cause a 6V drop. Whatever is causing the voltage to
drop has to be in-line with the coils + terminal. It would have to somehow
be tied into the primary side and untied as the points open/close.

The points, condenser, rotor, spark plugs and coil secondary essentially
form a separate circuit from the rest of the tractor's electrical system,
except of course, for the common ground. There are no external wires that
tie into any other circuit, and there should be no affect on the primary
side to cause a voltage drop. Something on the primary side is apparently
being tied in and out to cause the drop. Why it's associated with the points
is weird.

Carl

http://classicmini.weebly.com/choosing-a-coil.html
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reserved=0>  

On Thursday, August 26, 2021, Carl Szabelski <c.s.szabelski at gmail.com
<mailto:c.s.szabelski at gmail.com> > 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
<mailto: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
<mailto: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
<mailto: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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