# B&S Magneto Question



## WCarp7 (Nov 9, 2017)

I have a question about B & S magnetos. I have a bad magneto with only 0.5 ohms resistance between the stop terminal and the armature, so that's why I wasn't getting a spark. On two new magnetos, the resistance only measures 1.3 ohms between the stop terminal and the armature which is only 0.8 ohms higher than the bad magneto. Therefore, I was wondering if the new ones are good or not (all too often new parts are bad). I'm wondering what the resistance should be approximately?

The new one doesn't seem to have any visually noticeable electronic module like the older one and it's part number is 590454. See attached photo.

Thanks


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## AVB (Nov 28, 2014)

Electronically triggered magneto coils are impossible to test with ohm meter. 

This due to the construction of the trigger circuit as would be only reading the trigger inductor and not the Darlington transistor/SCR or the primary of the coil's transformer. Now you can test the secondary of the transformer. The kill terminal usually attaches the base of the Darlington; therefore, removing any triggering voltage.

Really the only valid field test is to mount it and spin the flywheel at least 350 rpm and check for spark.

Be careful as not mount it upside down. If you do so the coil will not work.


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## WCarp7 (Nov 9, 2017)

It's interesting that you call the magneto electronically triggered. According B & S Support, it's not electronic---but they may not be familiar with the item. They should be but they are not engine repair Techs.

You mentioned about the trigger circuit but I wonder where it is? It's certainly not at the flywheel side. Is it at the back? As for measuring the resistance of the stop terminal to the armature, no active device (transistor at least) is connected because the resistance does not change with polarity of the ohm meter's leads.

It would be very interesting to see a schematic of the magneto.

As for spinning the flywheel at 350 RPM, I cannot do that by hand and I didn't want to go to the trouble of installing everything, and only finding that the magneto is defective. Having a resistance of only 0.8 ohms higher than a defective magneto (an older style that is definitely known to ne electronic) was why I was questioning whether the new one was good.

Thanks,


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## AVB (Nov 28, 2014)

Strange that you got a response from Briggs as I can't never get them to respond to my questions posted through the Power Portal.

It looks like time for me to order one in of the new style just to take it apart though I fairly certain they just moved things to the rear of the coil assembly. It got to be controlled accurately for proper ignition timing. 

I do know many of the new 2 cycle coils even have a timing advancement circuitry. I have seen where the timing advance circuit shorts out causing either the exhaust to ignite or backfire through the carburetor.

I also know that it don't necessary takes a trigger coil at the front of the coil to trigger the circuit as I use an aftermarket Nova II module on old points systems to trigger the coil and these are remotely mounted and just use the current and voltage from the coil primary winding to trigger it. If I not mistaken these are an SCR circuit setup. I even seen these to fail because I had one Kawasaki engine that nearly set my pants leg on fire by causing one foot flame out of the exhaust. It also nearly the P out of me as I thought someone pull the trigger of a shotgun right behind me.


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## WCarp7 (Nov 9, 2017)

Thanks for your response. I realized afterward that the reason why the resistance I was measuring was not changing depending upon polarity was perhaps because the voltage the meter applies was too low to turn on any active (electronic) components so that was not a good test to determine if there were any active components in it. Apparently, it does have one or more active or electronic components in it after all as I mention below.

Again, according to B & S Customer Service, the magneto is not electronic but it was confirmed today by a B & S Technical Service Rep that it is indeed a Magnetron magneto. So you are correct that it has electronic components (at least one or more).

The Tech Service Rep said that a person can not test the stop terminal by measuring the resistance and that B & S doesn't supply any specs for it. Even he didn't have and couldn't get a schematic of the magneto. Apparently their Engineering department won't let that info out, which is interesting.

I attached a screen shot of the info about the magneto the Tech Rep sent me. Most of the text is quite small but if you click on the image, it will enlarge.


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## AVB (Nov 28, 2014)

Here is a couple possible setups that they could incorporated into the module.


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## WCarp7 (Nov 9, 2017)

Thanks. I wonder what the Stop terminal is connected to since B & S says you cannot tell whether the magneto is good or bad by measuring the resistance from the Stop terminal to the armature? It's only a guess since B & S won't give us any more info about it than they have.


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## AVB (Nov 28, 2014)

Since the collector of the final transistor is grounded to start with they would be grounding the emitter side to kill the circuit figure one and vica versa in the second circuit.


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## WCarp7 (Nov 9, 2017)

The resistance was the same though as I mentioned---very low, only 1.3 ohms from the Start terminal to the armature, no matter the polarity of the ohmmeter leads so there has to be something more than just an active component connected to the Stop terminal. It would be interesting to see the schematic of the magneto.


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## AVB (Nov 28, 2014)

Let see I if can explain why you see basically the same readings both ways.

First the ignition coil is nothing but step up transformer which has trigger to rapidly collapse the magnetic field in the primary to induce a voltage in the secondary.

From what I gather looking at the diagrams you are primarily reading the transformer primary winding resistance minus few milliohms. This something that most DVMs are not designed to register this accurately. It takes a specialized meter that is capable registering this low and I haven't seen one since my school days.

Let's say you have 2 ohm inductor paralleled by a 470 ohm resistor. And the frequency so does the affect the resistance of the inductor (ie: the primary winding) but you are reading this at DC (0 HZ) with your ohms meter. When even active components are added things get a lot more complicated depend config and the frequencies involved.

Formula 
Rparallel = 1/(1/R1+1/R2+1/R3 ...)
 
Once calculated you see readings than less than the smallest paralleled resistance in the circuit. In the example above your reading would be 1.99 + few more 9's. Of in the circuit magneto there is more resistances that are paralleled so the reading would be even lower. 

Can get fairly complicated as you got to the figure in all the series and parallel resistances at whatever the operating frequency is involved; of course, most small engines don't come anywhere near the frequencies that I got to deal with in the amateur radio field. For a small engine this can be as high as 250 Hz or more when dealing two cycles. (RPM / 60). I have seen a few running peak rpms close to 18,000 rpms.


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## WCarp7 (Nov 9, 2017)

Yes, I'm aware that things connected in parallel with the Stop terminal will affect the resistance that's why it would be helpful to see the schematic of the particular magneto. The defective magneto's Stop terminal resistance to the armature was 0.5 ohms and the new supposedly good one measured just 0.8 ohms higher which isn't much difference.

Both of my Fluke DVMs---even an old one that I have has a low ohms range. My Fluke 289 will measure to 0.001 ohms.

I plan on trying to get the engine started within a few days, then I will know if the new magneto is good, for sure.


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## AVB (Nov 28, 2014)

That the way Briggs has us techs to test them.

I think we will ever know the exact wiring and components due to the encasement. It like those days when the manufactures would grind off the numbers on IC chips to keep the repairman from buying the chip at a lower cost. Another way was to apply another layer with custom part numbers or cut off legs so you couldn't add functions. Manufacture have gotten very good at hiding things but if given enough time it could be figured out though it may not be exact.

With this being less than 19.00 + shipping from my aftermarket supplier and I can get them the next day. I would not fret much about having to get a free replacement since they provides me with a one year warranty if defective upon installation so I can stock them without worry.

Personally it will next Spring before I order a couple of these as the shop is virtually shut for the year. Only a handful chainsaws coming now. I am going in the hole keeping it open this time of year.


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## WCarp7 (Nov 9, 2017)

I thought I'd mention I installed the B & S magneto and it appears to work fine.

You mentioned a price of $19 plus shipping for an aftermarket magneto. Is that equivalent to the B & S 590454 or some other one?


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## AVB (Nov 28, 2014)

WCarp7 said:


> You mentioned a price of $19 plus shipping for an aftermarket magneto. Is that equivalent to the B & S 590454 or some other one?


This is my dealer cost to get one thru Stens and yes it is equivalent to the 590454. Stens PN 440-467 Retails for 33.85.

End users can get these thru Wal-mart for as low as 23.04 with free shipping but there are not the Stens ones but one from China. Just takes a lot longer to get something from California then my one day delivery out South Carolina.

Everybody is wanting to buy direct and expect the shops to put them on for free. No wondering a lot shops are just closing up around here. I personally know of three shop that closed this year here. Even I am ending the year on a loss. We have to make a profit to stay in business. 

I just returned a 95 Kawasaki Mule that was taking a long time to repair as the customer was just pushing so hard for it return. Actually was here only 1.5 months. I not even 100% sure it is firing on both cylinders. I was having problems getting the wrong parts from Kawasaki distributor. I had just got it running Monday afternoon so I could it field tested for the second time as the propeller so knocking so bad the first time out. U-joint didn't have a single bearing left.

Plus the Mule was just plain worn out which was a major problem as I had to repair the drive train propeller shaft u-joint just to get back to the engine problem. He didn't want to spend the money on new shafts plus it would required a new front pinion gear assembly due the yoke being rusted on so bad.

I would like to had a little more time with it but the customer was pushing like I could control how fast I could make UPS to ship things and still keeps costs down. I hate it when I get customers get like that. They don't understand how much wear they put on these UTV especially running them in rice patties.

I think I did fine considering I had disassemble the front final drive while it was still in the machine as everything was so rusted in place. I had virtually zero clearance to get the pinion assembly out and in. I couldn't even put the retaining studs back in, instead had to use regular bolts with Loc-tite. The studs had to be removed so I turn the part 90 degrees for removal and insertion as it had clear the frame. Would have rather done the job right and repair things as if it was my own machine.

It was the 1 mm shim that got shipped wrong as they sent a .8mm one and it took another 1.5 weeks to get the replacement.

People do buy things they can't afford to have fix. And sometimes they just get burned buying and selling these too.

Sometimes I wonder why I even willing to learn how to fix these ATVs and UTVs. Tools are so expensive to obtain. On this Mule one tool was over 220.00 (cost) just to get a clutch off and it is no telling when I going to use it again. But I got start repairing basically junk first to get to the better stuff. 

That how it was with the lawn care equipment. People would bring the worst possible problems just to see if I was any good. I even had one customer that in a riding mower one time in a pickup load of boxes because he could figure how to put it back together. Griped like heck because I charged him 50.00 to put it back together. I should charged for all my time instead feeling sorry for him.

Luckily I do have one customer that buys and sells these so those are not in super bad shape though it still takes 1 to 2 weeks sometimes to get them back up and running. Lots drive train problems but it is wonder they even survive what they are put through. He knows exactly what I got to go though as he at one time did the same work; just not able to do it anymore due physical problems.

Look like they would take better care of something that cost 8-10K or more. The last Cam-Am ATV I repaired was sold for 9500 and it was used. The Mule are selling around 2000 as old as it is.

On this Mule I made less 100.00 in profit for 8 hours of work and that didn't include the cost the tool so I actually lost 120.00 if you count the tool.


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## AVB (Nov 28, 2014)

Now to answer your question what is inside the coil.

1 ea 13mm x 34.5mm single sided circuit board.
4 ea SMT transistors.
8 ea SMT resistors, inductors, and/or capacitors. 
(SMTs were destroyed upon board removal.)
1 ea D13003 transistor.

Primary Winding reading is 1.5 - 1.7 ohms.
Secondary Winding reading about +- 6 Kohms.

The kill terminal is directly connected to the non-ground side of the primary; hence, explains why you or anyone else can NOT test the primary trigger circuit.

This info was acquired through the destruction of the new OEM version of this coil. It also explains the configuration differences as they have gone to SMT trigger version.


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## WCarp7 (Nov 9, 2017)

That was interesting what you mentioned about the repair issues. I understand there can be quite a few problems associated with repair now-a-days---including getting parts and customers. I see that at Walmart.com, a person can get an equivalent 590454 B & S magneto for only $11.57 but I wouldn't think it would be very good.

Thank you for providing the info about what you found in the magneto. Since the non functioning magneto's old stop terminal resistance measured only 0.5 ohms to the armature, either the primary was shorted and or there was a shorted semiconductor.


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## AVB (Nov 28, 2014)

I suspect that the output transistor shorted the collector-emitter (CE) junction.

Here is the reasoning.

Coil's primary winding is about 1.5 ohms. The output transistor's emitter is in series with a 1.0 ohm 1/2 watt resistor which is paralleled with the primary winding. Calculations shows that with the transistor CE junction shorted the resistance would be around 0.6 ohms.

Now I also know more about this Magnetron coil. Sometimes a die grinder and a pair pliers can reveal secrets.:grin:


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## WCarp7 (Nov 9, 2017)

Now I know more it too. It was an interesting issue to deal with. Thanks


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