[AT] 3020 electrical, continued... "resistance wire"?

Sat Aug 31 08:59:44 PDT 2024

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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