[AT] 3020 electrical, continued... "resistance wire"?
Stuart Harner
stuart at harnerfarm.net
Sat Aug 31 10:11:03 PDT 2024
Hi Dean,
Working from your last question first. Do you need the wire from the
solenoid to the coil?
That would depend on your ignition switch. IF the run wire is hot while
in the START position, then no you don't need the secondary wire. Most
ignition switches do not power the RUN wire while in the START position.
There are several reasons for this with the most important being to
reduce any extra load on the battery during start and to prevent reverse
spikes from the starter back into any of the other electrical system.
This is easy enough to test. Remove both wires from the coil + terminal
and put a test light on the one from the switch. Turn key to RUN, light
should come on. Turn key to START, light should go out. If it stays on,
you don't need the wire from the solenoid. My bet is it will go out, and
you need the wire from the solenoid.
Next up was the do I need a resistance with the 12V coil? The short
answer is maybe. I am a little surprised that the factory used a
"resistance wire" on the 6V setup, but again, that was to protect the
points primarily and the coil from overheating. Resistance wire is an
older way of dealing with the points than a resistance block. The
problem with resistance wire is it comes in many different amounts of
ohms of resistance per length of wire. So you need to know the total
amount of resistance required, then cut the appropriate length of wire
to get that amount. Wire diameter also plays a part depending on how
much current that wire was expected to carry. I think the use of
resistance wire was eventually replaced by resistor blocks.
Now we get into the question of 12V coils and this is where I get a
little bit fuzzy both from lack of total knowledge of the subject and
fading memory from when I was a parts guy in the 1980's. IF I remember
correctly, Ford and GM used a different coil that did not require an
external resistor. Did it have an internal one? I don't know but I do
remember looking up coils for vehicles and there would be different part
number for "with resistor" and "without resistor". This may have been as
Chrysler was changing to a system that did not require a resistor.
Another fuzzy memory is some coils were grounded through the mounting
strap and only had one + terminal, some had a + and - terminal and were
not grounded through the body.
There may be writing on the coil that says if it needs a resistor or
not. Or if there is a brand name and part number, it could be researched
that way.
In any case, since it has been working fine with no burned points
issues, my instinct would be to leave it alone and just replace the
damaged wire. If it ain't broke, don't fix it. :)
All coils cause a reverse spike on the primary winding as the magnetic
field of the secondary winding collapses. This is why points can arc and
why there is a condenser connected to the points. A coil designed for 6V
will work at 12 and will produce a hotter spark. The stress on the coil
and condenser as well as the heat buildup in the coil will eventually
damage it. If one is converting a 6V system to 12V everything will work
fine as long as the coil and condenser are upgraded to 12V versions OR
some provision is made to reduce the voltage to the old coil. IF the 6V
system also used a reducing resistance it makes things more complicated
and switching everything to 12V is just easier all the way around.
If you are curious, just do the test light procedure and you will find
out how it is set up now. The big question will be how to keep the
darned mice from doing it again!
Stuart
On 8/31/24 10:59, Dean Vinson wrote:
>
> Stuart and Dean, thanks for the quick responses. I’ll go ahead and
> put a new length of wire in there to replace the one that had some of
> the insulation chewed off, on the theory that whoever installed the
> old one knew what they were doing and in light of the fact that it’s
> evidently been installed for a long time and the tractor runs fine.
> But I still don’t really have a good understanding of the whole system
> as currently configured… my technical foundation is a little weak for
> stuff like this.
>
> Stuart, you wrote “The purpose was not to protect the coil but to
> protect the points”, and Dean, you wrote “Starts on 12v runs on 6.”
> Ah, okay—so that’s why there are two wires feeding the coil? One
> from the starter solenoid to briefly provide 12V to the coil to
> generate an especially hot spark for starting, and then after the
> solenoid disengages the separate “resistance” wire from the ignition
> switch provides reduced voltage to avoid burning out the points while
> the engine is running normally?
>
> I’m familiar with the resistors in the little ceramic blocks, since my
> Super M has one, but it’s an after-market add-on since that tractor
> left the factory with a 6V battery and was at some point converted to
> 12V. Following the same logic, it’d make sense to me if the 3020 had
> a physical resistor in front of the coil, since they were designed
> from the get-go for 12V batteries but still used 6V coils. But the
> service manual wiring diagrams don’t show any resistor, just a
> “resistance wire,” and this little four-foot section of #14 stranded
> wire doesn’t provide any resistance to speak of.
>
> Oh, wait… okay, maybe that makes sense after all, because I think this
> four-foot wire is not OEM and I think the coil on the tractor is not
> the OEM 6V coil but is rather a 12V replacement. A 12V coil wouldn’t
> need a resistor to avoid burning the points, correct? So 12V to the
> coil is okay all the time and I don’t **need** a resistor, but I need
> that 12V to come from the ignition circuit since the 12V from the
> starter solenoid is only there when starting. Sound right? And if
> so, do I strictly need the wire from the solenoid? (Not that I’m
> intending to remove it… just trying to think through how the system
> works as currently configured).
>
> Thanks very much,
>
> Dean Vinson
>
> Saint Paris, Ohio
>
> *From:*AT <at-bounces at lists.antique-tractor.com> *On Behalf Of *Stuart
> Harner
> *Sent:* Saturday, August 31, 2024 9:31 AM
> *To:* at at lists.antique-tractor.com
> *Subject:* Re: [AT] 3020 electrical, continued... "resistance wire"?
>
> The only experience I have with coil resistors was on older Chrysler
> products that used a block type resistor usually mounted near the
> coil. The purpose was not to protect the coil but to protect the
> points. Running 12V through the points would cause them to burn
> quicker than running only 6V.
>
> The resistor was wired in such a way that when starting there was 12V
> to the coil as it was tapped off of the solenoid wire, but when you
> put the key back to run, the coil was powered through the ignition
> switch, through the resistor and was only getting about 6V.
>
> The resistors were potted in a ceramic block and mounted in the open
> as they would generate heat from dissipating that excess 6 volts. If
> the resistor failed the engine would start but die as soon as you
> dropped the key back to run. It was fairly common, easy to diagnose
> and easy to fix by replacing the resistor. Like any light bulb, they
> just burned out at random times.
>
> It sounds like your 3020 is kind of wired that way. If you are seeing
> no resistance on the wire, it may have been replaced in the past or
> the resistor has been removed. It will run, but will be hard on the
> points. If it was a resistance wire it would have been left out of the
> harness to dissipate the heat.
>
> Hope this helps,
>
> Stuart
>
> On 8/31/24 00:11, Dean Vinson wrote:
>
> Tinkering with the 3020 yesterday and I opened up the dashboard
> panel to see what was behind it, and found a rosy colored
> mousenest decorated with the insulation from a formerly-red wire
> stringing back through there. There’s a three-inch section of wire
> where the copper strands are exposed, oxidized dark but still
> intact with good continuity.
>
> I can replace the wire easily enough, but I’m trying to get my
> head around its function. Near as I can tell from the wiring
> diagram it’s a “resistance wire”. It’s one of two wires connected
> to the input side of the coil, the other one coming from the
> starter solenoid. Service manual wiring diagram shows it coming
> from the ignition switch, but this one comes from a circuit
> breaker mounted below the ignition switch. It’s outside the
> harness so presumably a replacement for an original wire that
> failed at some point.
>
> The battery is a 12-volt and the parts manual says the coil is a
> 6-volt. Resistance across the coil tests at 2.8 ohms, and 12V
> applied at the input side tests at 12V at the output side, both of
> which suggest this coil is a 12-volt rather than 6 (according to
> some websites I found with tips on how to tell the difference).
>
> So if the “resistance wire” was intended to protect the 6-volt
> ignition system from the 12-volt battery, but the OEM coil was
> replaced with a 12-volt, do I need to replace the wire or can I
> just remove it and call it good?
>
> Dean Vinson
>
> Saint Paris, Ohio
>
>
>
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