GT1800 No Kill

[heartbt 0]

Alrighty, I'm stumped. DVM in hand, I set out to figure this out. It's more a frustrating nuisance, than a problem, but the kid wants to start mowing the lawn (yay!, kinda.) and this needs to work for safeties to work.

engine is a Briggs L head model 41073-0107-18F. No points, magneto ignition.

There is the fixture on the coil with the push crimp style connector on the top (bottom installed)with a black wire that goes to the isolated screw under the air cleaner. The isolated screw, is indeed, isolated (open to ground). From that push crimp on the coil, there are also two bare wires that wind around to a screw that goes right into the plates of the coil. All of these wires are continuous to ground. In other words, they all connect at that push crimp, and in turn ground to the motor thru the plates and the engine mount for the coil. Is this right?

Now here's the thing that confused me. I removed the screw that grounds the two bare wires to the plates, confirmed that they were now open, and I lost spark.

The wire from the isolated lug to the ignition switch is good, Open in run, grounded in off. The ignition switch is properly grounded.

What is going on? I know the coil needs ground to generate, but to kill, it needs to ground the Primary? (a reference, with color pictures, and arrows showing what is what would help me with identifying these things. I know the principal, but am lost on the specifics, or so I'm finding.) I just don't get it. Learn me something guys!

[Save Old Iron 1,566]

No points, magneto ignition.

I know the coil needs ground to generate, but to kill, it needs to ground the Primary?

Correct.

There should only be two "wires" on a magneto

The spark plug high tension lead

The "kill" wire.

Ground the "kill" wire and the tractor will not start.

Lift the kill wire from ground, the magneto can now function and generate a spark. Whether the tractor starts or not depends on many other things.

:D

[heartbt 0]

OK, so, let me just push my knowledge further.

Primary winding is the "plates". They get excited as the magnets fly by and needs a ground to get that way. Correct?

Secondary is the wire bundle that ups the voltage, and is induced by the primary. (I'm familiar with Faraday's law) These are isolated in the RUN state, and grounded during the kill state. Correct?

I have 2 high tension wires to the plugs (twin cylinder). One wire from the coil in the push lock clip that goes to the isolation lug (kill wire). TWO bare wires, on the coil, that SEEM to go to the plates (primary) and the Push lock clip. SO, putting the theory into this coil, THOSE two bare wires, should ground out the secondary, and kill the engine. BUT, if they are not grounded, I get no spark. I've never had this much head scratching getting a motor to run!

I'm going to go and check if the high tension wires are continuous to either of the bare wires. If a coil wasn't $50, and this darn thing didn't spark, I would go buy a new one. It's hard to justify that tho, when it just wants to keep working!

[Save Old Iron 1,566]

I dug up a scan from an old Sears tractor I had years back with that same setup

the schematic is attached below

My mistake - yes - there are more than just 2 wires because of the twin configuration.

But from a functional aspect - there are only 2 things going on here - spark on the 2 high tension leads

and the need to ground BOTH the "kill" wires to shut down the magneto spark. One "kill" wire will connect to the block ground, the second "kill" wire will be grounded thru the M (MAGNETO) terminal on the ignition switch when the switch is in the off position.

Attached Image (Click thumbnail to expand)

[Don1977 604]

You may have the wrong switch. All the Briggs switches I have seen had the ground lug grounded to the out side metal on the switch. I had the wrong switch on my Briggs and used a toggle switch with one leg grounded to stop the engine had it mounted in the hole for the electric clutch. I just had to turn off the toggle before starting the engine.

[heartbt 0]

Save old Iron - Yes. That much I am good on. And now it is more a matter of curiosity, and how is the magneto wired itself. After some closer examination, there's even MORE mires, on the coil itself, and another wrapping that I'm not sure what it does.

So I've decided that if kid wants to mow, he needs to buy a coil! I'm not going to buy a new one so he can abuse the tractor and take up the only time I'm allowed to sit on my butt and drink a "pop".

Don, It is indeed an aftermarket; different switch. However, it is still ground at off and open at run, so it should be working correctly. Also, if I ground the wire from the coil, it will not die. I have to choke this machine dead, or screwdriver from plug to ground.

2nd wheel horse tractor, Who knows how many other kinds, and I finally have a tractor that I have to kill with a screwdriver or choke to death. Irony.

[Save Old Iron 1,566]

So its a magneto autopsy you want

well, the diagram below is probably real close to what you have ( twin cylinder will have 2 secondary (yellow windings)

the kill switch will ground both ends of the primary coil of wire so no voltage can be induced in the primary coil when the flywheel magnet runs past the primary coil in the magneto

no primary voltage induced , no secondary coil spark across the spark plug

Attached Image

[heartbt 0]

Love the diagram, that's how i picture it to work. But there's just a couple few more wires on this coil than that. Next time i have the covers off, I'm gonna snap a pic and dig up this thread. I would really like to know what is going on with this thing.

[Fiddlestix 0]

... and another wrapping that I'm not sure what it does.

[Save Old Iron 1,566]

Picture is always worth a thousand words.

[heartbt 0]

Sound like that doodad is the thing. Triggering module. Whats its function, besides obviously triggering?

[Save Old Iron 1,566]

The electronic trigger is easily created from the previous drawing.

Change the primary coil into a charge coil to charge up the "charge capacitor" instead of dumping its charge directly into the primary of the magneto. Instead of instantly causing a spark to be created, the primary charge now waits in the charge capacitor. The charge has no where to go because the "trigger" contacts are not closed.

The "trigger coil" now has to wait for some stimulation, be it from a magnet inside the flywheel or possibly a metal post on the outside of the flywheel. Depending on the engine, something disturbs the current flow in the trigger coil and the trigger coil closes the trigger contacts. The voltage stored in the "charge capacitor" now has a pathway to dump its charge thru the primary winding of the magneto - this creates the spark.

Attached Image (Click thumbnail to expand)

[Fiddlestix 0]

Have a read here...

Magnetron explained

[Save Old Iron 1,566]

excellent article - thanks

[Fiddlestix 0]

I am not sure exactly what engine is involved but it is possible that this is a Magnetron conversion with the wee little removable module. These instructions give a bit of description, pictorially speaking...

Magnetron Retrofit Installation Instructions

As a bit of food for thought, I thought I read somewhere that Tecumseh placed the grounding "off" switch on the one leg of the trigger coil... no trigger volatge, no primary collapse, no spark.

[heartbt 0]

WOW! Now that is the ticket! More there than I can get my head around quickly. Yep that's what I'm seeing though. I'm going to digest this and see what is actually happening. I'll get back after the weekend if I come up with anything "great and novel" thanks a ton for the Learning!

Fiddlestix, it's a/an 18 horse briggs horizontally opposed twin with flat heads (L heads are what they call them). I put the particulars in the opening. I'm new to this model as well, mostly working on verticals in this size, Horizontals on the larger, like the onans and such.

On a side note, if anyone has the relevant pages from from the Briggs manual on the steps to adjust the valves, I would be extremely thankful again. Leakdown tester says my exhaust valve on the front cyl. is opening earlier in the stroke than the rear.