# How to Test Toro Interlock Module / Circuit



## classiccat (Mar 1, 2014)

This hopefully takes some of the mystery out of testing the interlock / safety circuit on Toro snowblowers starting around the 1981 model year. 

You shouldn't hook-up a voltmeter to the interlock modules...it could burn-out the internal circuit.

Tools you'll need: 

 Spark tester
 2 jumper clips
 continuity meter (for the interlock circuit only...not the interlock module itself)

If the module itself fails, good luck finding one; they're NLA but do show up occasionally on eBay.

If the module passes however the interlock circuit (consisting of harness, handlebar switches, auger limit switch, gear-select limit switch) has bad continuity. Start by cleaning all of the connections and testing individual components. Use dielectric grease on the contacts to preserve them for years to come!


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## Grunt (Nov 11, 2013)

Excellent explanation of how the safety switches work in relation to the infamous module. "I" still want to dissect a failed unit to see what is inside and possibly duplicate the mystery electronics. Thanks Buddy.


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## classiccat (Mar 1, 2014)

Grunt said:


> Excellent explanation of how the safety switches work in relation to the infamous module. "I" still want to dissect a failed unit to see what is inside and possibly duplicate the mystery electronics. Thanks Buddy.


No problem Grunt! I know this mystery has been plaguing you more than anyone!

A dremel is probably the best tool for the bulk delayering and a chemical approach for touching down.

This circuit is tricky b/c you don't want it interfering with the normal ignition; If you don't have that sharp change in the primary current that this circuit is stealing from, the secondary charge will be weak. 

My gut is telling me that since it's a current-driven circuit, it's feeding into the base of a BJT...and one of the 2 n/p junctions of that device gets roasted killing that isolation.

Darn it Grunt, now I'm curious! :icon_cussing_black:


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## Grunt (Nov 11, 2013)

classiccat said:


> Darn it Grunt, now I'm curious! :icon_cussing_black:


My interest is in the "Survival of the species" ideology. If people junk their machines because they won't start, won't operate with the few safety protections there are, an inexpensive way to reproduce the module might keep these old monsters alive for another 30-40 years.


I just thought of an old saying, "Curiosity Killed the Cat". Maybe you shouldn't get involved in this any further. I'd like to keep you around. :kiss:


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## David Lane (Feb 12, 2020)

Excellent for Safety and will forward in the contact to keep care of these steps.


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## SayItAintSnow (Dec 15, 2017)

Grunt & 'Cat:

Gentlemen...you've piqued my curiosity about this, mostly because, even though I have heard of some snowblowers having safety interlock circuitry, I've never acutally worked on one that did. :question:

Not sure I understand what the manufacturer's philosophy was here, but looking at the schematic, is it nothing more than they were trying to insure that you couldn't start the machine if the auger lever or drive lever were set to the _engage_ position? Or is there more to it? :icon_scratch:

At any rate, it would seem to me that a solid state relay of the type that we used to see frequently in old computer equipment, could probably be used as a substitute. There used to be tons of these around from the surplus electronics vendors inventories, of varying voltages and currents.

Typically, these modules would be completely sealed in an epoxy cube or rectangular shaped pack, which would pretty much assure that they were impervious to moisture issues. The only caveat in trying to use one of these for a snowblower, that I would offer, is the question of reliable operation through a range of typically cold temperatures, but I would guess they would be o.k. It'd certainly be cheap enough to try. :wink2:


Many of these modules have quite a range of "control" or "trigger" voltages. For example, you can easily find ones that have trigger voltage of 5-40 vdc, so you don't have to rely on a perfectly stable voltage. You could even run one of these using AC as a trigger, if you first run the trigger voltage through a full-wave bridge rectifier.

Though the plentiful and varied supply of these seems to be drying up from the surplus electronic dealers, the market has shifted to Ebay, where there are still plenty of them to choose from. I would guess that even if you had to add a bridge rectifier, you could get out of the whole deal for less than 15 bucks. :yahoo:


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## classiccat (Mar 1, 2014)

SayItAintSnow said:


> Grunt & 'Cat:
> 
> Gentlemen...you've piqued my curiosity about this, mostly because, even though I have heard of some snowblowers having safety interlock circuitry, I've never acutally worked on one that did. :question:
> 
> ...


I’m afraid that this circuit is not going to be as simple as folks hope (a simple relay is a convenient analogy for explaining operation however they don’t work for starting and killing).
- a NC relay for killing won’t start
- a NO relay for starting won’t kill

The voltage on the primary wire is going to be quite high (several hundred volts).

My guess based on when this technology was implemented (early-80’s) is that it’s borrowing ideas from CDI. It will have standard diodes, zeniers, caps and resistors and then the magic will be possibly from one or more of the the following: SCRs, BJTs, Triacs or even an old school FET for switching.

Probably similar to the patent 3,726,265 by William A. Howard for magneto ignitions.



















We’ll see, I have a failed unit soaking in solvent to try and loosen-up the potting.









^^^ that's the ground plane connection peaking out already


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## JLawrence08648 (Jan 15, 2017)

I've never have come across one.

Can they be disconnected and bypassed?

I've bypass my JD1032.


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## SayItAintSnow (Dec 15, 2017)

classiccat said:


> _I’m afraid that this circuit is not going to be as simple as folks hope (a simple relay is a convenient analogy for explaining operation however they don’t work for starting and killing).
> - a NC relay for killing won’t start
> - a NO relay for starting won’t kill
> 
> The voltage on the primary wire is going to be quite high (several hundred volts)._



Classiccat:


My impression as to how this worked was based solely on the simple schematic that appears at the end of that video. My assumption was that the circuit allows one of two conditions:
1) allow voltage to feed the ignition module
2) cut off voltage to the ignition module

*So, is there more to this than simply preventing the engine from starting when either the auger or drive is engaged? 
*
So the stator on this engine is producing a voltage of several hundred volts? Obviously it's not the voltage going to the spark plug as that would be several thousand, not several hundred volts. Why is it so high?

Pondering this in real time here.......

Wait......Is this setup from before the days of solid state ignition modules?

Lol....and I thought I was confused before.....:devil:

Sorry for all the silly questions, and I hope you don't mind, but I am curious about its purpose and how this is supposed to work when everything is fully operational. :wink2:.


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## AnOldGearhead (Nov 18, 2017)

classiccat said:


> I’m afraid that this circuit is not going to be as simple as folks hope


It looks to me that your second image, along with reading the patent, provides enough detail to almost copy it. 




classiccat said:


> (a simple relay is a convenient analogy for explaining operation however they don’t work for starting and killing).
> - a NC relay for killing won’t start
> - a NO relay for starting won’t kill



Right, but the patent does overcome those problems.


From the blue box at the upper right in the Google web page you linked to, you can download or view the full .pdf (it's easier to read than the Google web page).





classiccat said:


> My guess based on when this technology was implemented (early-80’s) is that it’s borrowing ideas from CDI. It will have standard diodes, zeniers, caps and resistors and then the magic will be possibly from one or more of the the following: SCRs, BJTs, Triacs or even an old school FET for switching.



No magic (your second image explains it) and Toro may have licensed the technology from the patent holder. Or maybe Toro broke the patent and has or had its own patent.




classiccat said:


> Probably similar to the patent 3,726,265 by William A. Howard for magneto ignitions.



I was going to suggest that you might want to build upon that patent in an attempt to break it but, curiously enough, it looks like the patent expired today.


I'm in no way an expert and these are just my random thoughts.


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## classiccat (Mar 1, 2014)

JLawrence08648 said:


> I've never have come across one.
> 
> Can they be disconnected and bypassed?
> 
> I've bypass my JD1032.


Hey JLaw! You can disconnect it (along with the safety features) as long as you have a way of grounding the kill wire (most of our machines do this with the throttle).




SayItAintSnow said:


> Classiccat:
> My impression as to how this worked was based solely on the simple schematic that appears at the end of that video. My assumption was that the circuit allows one of two conditions:
> 1) allow voltage to feed the ignition module
> 2) cut off voltage to the ignition module
> ...


The schematic at the end of the video is to demonstrate the relationship of the microswitches, on/off switch, handlebar switches and interlock module.

Think of the interlock module delivering current to the safety circuit…and it can only deliver current if there is continuity through the safety switch circuit.

Starting: 
- Auger and traction are off and neutral respectively
- Note: It's possible to start the machine with auger or traction engaged if the handlebar switches are held down. So you shouldn't hold the handlebar switches down when starting.

Operating:
- either of the handlebar switches must be depressed when auger &/or traction are engaged.
-- wheel clutch models have 2 handlebar switches so you can use either hand to work the clutch

Unintentional Kill:
- engaged tractor &/or auger without a handlebar switch depressed.



SayItAintSnow said:


> So the stator on this engine is producing a voltage of several hundred volts? Obviously it's not the voltage going to the spark plug as that would be several thousand, not several hundred volts. Why is it so high?


The stator is not producing power but rather the charge coil within the electronic ignition module. 

When the flywheel magnet passes the charge coil, it induces a current that charges-up a large capacitor.

Then as the flywheel magnet passes a trigger coil, a “switch” (silicon controlled rectifier) allows this capacitor to discharge into a transformer that steps-up from a ~ 200V (Tecumseh coils) to 20kV for spark.

I believe that this interlock module is stealing some of that power from the Charge Coil / Capacitor to power-up the safety circuit.

EDIT/NOTE: Briggs Magnetron / electronic ignition is different (not CDI) however still the general concept of drawing current generated on the primary winding. 



SayItAintSnow said:


> Pondering this in real time here.......
> 
> Wait......Is this setup from before the days of solid state ignition modules?


I'm fairly certain that toro only implemented these with electronic ignition.

@Grunt, we're getting close! Soaking in simple acetone and carefully scraping with a razor knife is clearing away the potting material.









A little capacitor is peaking out there:


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## JayzAuto1 (Jul 25, 2016)

GAfternoon,


"The stator is not producing power but rather the charge coil within the electronic ignition module.

When the flywheel magnet passes the charge coil, it induces a current that charges-up a large capacitor.

Then as the flywheel magnet passes a trigger coil, a “switch” (silicon controlled rectifier) allows this capacitor to discharge into a transformer that steps-up from a ~ 200V (Tecumseh coils) to 20kV for spark.

I believe that this interlock module is stealing some of that power from the Charge Coil / Capacitor to power-up the safety circuit."


This topic is discussed on a couple other Forums.... That is the consensus, that the Interlock Module Steals a small amount of current from the ignition coil when running. If the safeties are not in the proper position, that exciter current will be grounded thru the safeties and kill the engine spark. It seems SO Simple...... Yet at last check, it has not been duplicated cheaply OR successfully. Although those Interlock Modules aren't used very much anymore. It did work well....usually until some well meaning frustrated operator tried to 'By-Pass' it !!!

The last one I was able to purchase, was NOS, and priced at $55, and that was 3-4 years ago. Haven't needed one since, luckily!!!

I do have 1 good used one left. I try to grab them from machines that are junked.

Patiently waiting for The C/C to find The Answer to this one!!!!


GLuck, Jay


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## classiccat (Mar 1, 2014)

JayzAuto1 said:


> GAfternoon,
> 
> 
> "The stator is not producing power but rather the charge coil within the electronic ignition module.
> ...



You and me both Jay! 

Well, we'll at least have some clarity on the module for Tecumseh CDI igntions; the module (41-8601) I'm reverse engineering is from my '89 824. 

I'll still need a dead module from a briggs (41-8610) and there is 1 year (1981) that Toro used interlock 41-8600 on the points Tecumseh engine. 

So net on the Toro interlock modules:

826: 38150 (Briggs powered): 

1980: No interlock module; grounds the magneto directly
1981: 41-8610 (magnetron electronic ignition)
1987: 41-8610 (magnetron electronic ignition)

824: 38080 (Tecumseh powered):

 1980: No interlock module; ground the magneto directly
1981: 41-8600 (Magneto points ignition); you can still buy this module on partstree.
1987: 41-8601 (Electronic CDI ignition)


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## classiccat (Mar 1, 2014)

Ok guys, I think I got this finally nailed down. Patience, acetone and a hobby razor knife were the only tools required.

I mirrored the circuit board backside to ID components.









Single ceramic capacitor...








^^^ the only thing throwing me is the "P" suffix on the top-line; that's not a normal tolerance value.

3 resistors: 









SCR is indeed the magic sauce...and finding a replacement is going to be a little tricky...especially since some of the label pulled off. @Coby7, i believe you have a background in electronics... you probably have some good cross reference tables?








^^^ i was lucky in that the "sticker" was still stuck to the potting material and mostly intact! So i mirrored it and lined-it up to get an educated guess on the missing alpha-numerics:

overlaid:









Equivalent Circuits: 


















I'll need to double check my handywork (my wife is pulling me away from my desk  ) but this circuit operation makes sense...and is quite slick in its simplicity.

Finding that SCR that works with these voltages and temperature ranges could be challenging. That cap is rated to 500V...Tecumseh charge coil will be in the 200V range i believe.

Now I need one from a Briggs-powered Toro!!!


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## Coby7 (Nov 6, 2014)

Ceramic capacitors are usually high in voltage tolerances. SCRs are also very tolerant as far as specs. So if yours as failed you should be able to find a suitable replacement. Is it shorted out?


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## Grunt (Nov 11, 2013)

classiccat said:


> Ok guys, I think I got this finally nailed down. Patience, acetone and a hobby razor knife were the only tools required.
> Now I need one from a Briggs-powered Toro!!!



Thank you CC, that is an amazing amount of detective work. Your attention to detail is unmatched and your wife's tolerance to your investigation is greatly appreciated. Thank you Mrs. CC. 
:rock:


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## SayItAintSnow (Dec 15, 2017)

'Cat & JayZ'

Gentlemen, thanks for taking the time with the explanations of the theory of operation of this device. Yes I can see this is a fair amount more involved than I initially thought.....:wink2:

The circuit itself looks simple enough though, and I have to say 'Cat, isolating that circuit board was borderline "archeological" level retrieval...You are clearly a much more patient guy than I am!

I wonder if you sent the same photos you have of that SCR to customer service at outfits like Mouser Electronics or Digi-Key if they would be able to help you out with a suggestion. Even if that semiconductor is no longer manufactured, there's gotta' be something out there that will be a good substitute. :icon_scratch:

Great job!!! :icon-clapping-smile

BTW...what model Toro was that? I want to make sure I have nothing to do with any of those in the future .....:devil:
.


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## classiccat (Mar 1, 2014)

Coby7 said:


> Ceramic capacitors are usually high in voltage tolerances. SCRs are also very tolerant as far as specs. So if yours as failed you should be able to find a suitable replacement. Is it shorted out?


Yeah these class 2 ceramic caps are all over the place with temperature... trade-off of the high permittivity dielectrics.

It's only job in life is to hold a potential on that SCR "gate" between pulses (when the interlock circuit is open).

I haven't desoldered the components yet for final "postmortem"...my $$ is on the SCR however it's plausible that the CAP took a dump.

There are 2 other markings on the other side... green and white lines.


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## classiccat (Mar 1, 2014)

Grunt said:


> Thank you CC, that is an amazing amount of detective work. Your attention to detail is unmatched and your wife's tolerance to your investigation is greatly appreciated. Thank you Mrs. CC.
> :rock:


You're very welcome Grunt (from the both Mrs. CC and myself!) She's extremely tolerant of my antics!

It's always fun getting back to my roots; I was actually an industrial electrician (_after my grunt days back in the 90's_). I'm now an E.E. but the devices my team and I fabricate require a scanning electron microscope (SEM) to see. :nerd:

Next step is to source these components and make some for my Toro brothers!


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## classiccat (Mar 1, 2014)

SayItAintSnow said:


> 'Cat & JayZ'
> 
> Gentlemen, thanks for taking the time with the explanations of the theory of operation of this device. Yes I can see this is a fair amount more involved than I initially thought.....:wink2:
> 
> ...


yeah we'll find that component! I actually started looking at the online mouser catalog this morning...good idea ripping them and digikey an email to see if they can come up with an equivalent. :thumbsup: 

As i said earlier, '81 through I'd say '89 or '90 use these modules...IMHO, their best years! I've also read that there are some early Ariens models that also have an interlock module...supposedly less prone to failure.


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## toromike (Aug 20, 2018)

Great job of reverse engineering the interlock module @classiccat! The schematic makes perfect sense. Thank you for all your work.


classiccat said:


> I'll need to double check my handywork (my wife is pulling me away from my desk :smile: ) but this circuit operation makes sense...and is quite slick in its simplicity.


I spotted two typos, in the parts lists next to the schematic diagrams, R2 and R3 are specified as 10k, they are correctly shown as 1k resistors next to the pictures of the resistors. 


> SCR is indeed the magic sauce...and finding a replacement is going to be a little tricky...especially since some of the label pulled off. @*Coby7*, i believe you have a background in electronics... you probably have some good cross reference tables?





> Finding that SCR that works with these voltages and temperature ranges could be challenging. That cap is rated to 500V...Tecumseh charge coil will be in the 200V range i believe.


I looked through my old data books (before the internet we had data books) and could not find an SCR713-1. I previously looked at available SCRs while doing my own research. A MCR100-6 or -8 (400/600v) looks like it would work in this circuit and they are inexpensive (-6/-8 $0.57/$0.52 at Mouser). Operating temp is not a problem.


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## classiccat (Mar 1, 2014)

toromike said:


> Great job of reverse engineering the interlock module @classiccat! The schematic makes perfect sense. Thank you for all your work. I spotted two typos, in the parts lists next to the schematic diagrams, R2 and R3 are specified as 10k, they are correctly shown as 1k resistors next to the pictures of the resistors.
> I looked through my old data books (before the internet we had data books) and could not find an SCR713-1. I previously looked at available SCRs while doing my own research. A MCR100-6 or -8 (400/600v) looks like it would work in this circuit and they are inexpensive (-6/-8 $0.57/$0.52 at Mouser). Operating temp is not a problem.


Sounds like we've got another sparky in the group! 

uh...I was just seein' if you guys were payin' attention! :grin: images updated...good catch!

There are literally $1.50 worth of circuit components...most expensive is probably the connector. 

We can fabricate pots out of a piece of EMT and I think i have a half-jug of ferric chloride PCB etch solution from radio shack still on the shelf. :thumbsup:


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## Coby7 (Nov 6, 2014)

Did you try just cleaning all the crud on the board with alcohol and tooth brush. It looks pretty raunchy.


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## classiccat (Mar 1, 2014)

Coby7 said:


> Did you try just cleaning all the crud on the board with alcohol and tooth brush. It looks pretty raunchy.


Yeah i could've spiffed it up more for the camera after removing an inch of potting material but the excitement got the best of me!

Before i start desoldering, I'll definitely clean that crud out.


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## Coby7 (Nov 6, 2014)

I work on GM clusters all the time. You would be surprised what corrosion does in odd places.


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## classiccat (Mar 1, 2014)

Coby7 said:


> I work on GM clusters all the time. You would be surprised what corrosion does in odd places.


I wouldn't be surprised at all after owning an '01 Silverado for 17years. :laugh:

You're a yammy guy so a little backstory here. '80s era toros are plagued with problems with this module.

I had a "dead" one from my '89 824 so I caved (under peer pressure) to excavate for reverse engineering / postmortem.










I didn't intend on putting it back into service although the potting removal went well, i could probably patch this back-up for another 20 years of service.


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## toromike (Aug 20, 2018)

classiccat said:


> Sounds like we've got another sparky in the group!


Yes, I've had an interest in electronics which started in the 60s. My interest runs to analog, digital, and RF systems. I have designed, built, installed, maintained and repaired several amateur radio FM repeaters for clubs, and I was involved with designing and building upgrades for the oldest continuously operating NTSC television repeater in the country. I still operate a couple of FM repeaters, and an NTSC television station at my house.


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## classiccat (Mar 1, 2014)

I dug out the old solder wick, board holder and breadboard to see what failed on this module. :nerd:

The cap was measuring almost 3nF (spec'd for 2nF). A screwed-up cap would be a kill issue...not a run issue.

The SCR is working / latching fine ...at least at 9V.


















I guess I'll have to throw the rest of the circuit together on the breadboard and see if i can run my 824 with it. :grin:


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## KennyW in CT (Feb 24, 2015)

Next step is to source these components and make some for my Toro brothers![/QUOTE]


Now if you could only make one for the Module that goes on Briggs engines for Snapper RER's you'd make a fortune! Factory is 70 bucks, lowest I've seen is 40 bucks.


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## Brent (Jul 31, 2019)

I was following this interesting thread ten months ago and was just thinking about it. Has anyone made any more progress? It was valuable work being done.


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## classiccat (Mar 1, 2014)

Brent said:


> I was following this interesting thread ten months ago and was just thinking about it. Has anyone made any more progress? It was valuable work being done.


I have all of the components from mouser and my Toro824 is out of summer storage; I'll throw the circuit together on a breadboard and make a "Will it Run" video  .


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## asavage (Dec 20, 2021)

A year on . . . whatever happened on this front?


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## classiccat (Mar 1, 2014)

asavage said:


> A year on . . . whatever happened on this front?


Like a fine wine, the interlock module project (_for Tecumseh-powered Toros_) is aging to perfection 

In all seriousness, thanks for the reminder and thread bump. I've got a few days off and running out of projects (_only thing left is to service a leaky TPMS valve stem on my truck_.)

if anyone has a dead interlock module from a Briggs-powered toro, please DM me!


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## classiccat (Mar 1, 2014)

Teaser


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## classiccat (Mar 1, 2014)

Moment of truth! 






Note: This is the equivalent circuit for the 41-8601 only (Tecumseh Electronic CDI ignition). I need to get my hands on defective modules 41-8600 (Tecumseh magneto/points) and 41-8610 (Briggs-powered with magnetron electronic ignition).


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## classiccat (Mar 1, 2014)

I'm now "tooled-up" to fabricate these things!

Laser toner circuit pattern transferred to a copper clad board.









Etched away the exposed copper:









stripped away the toner:









drilled 









removed excess board to fit in the enclosure










my favorite part...soldering:









I have to get some micro screws to fasten the board (eventually I'll fill this with potting material):










the angle piece is what I had on hand...future builds I'll probably just take some stock straight off of the back...it also serves as the ground connection...very important for this circuit.










mounted on my '89 toro 824:









It's alive!! Started on 2nd pull and all kill functionality works perfectly!


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## 1132le (Feb 23, 2017)

great stuff mr cat
however i detest that setup on my 38080
would take alot of getting use too


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