UCD anyone help, magneto/armature

[portuncia 0]

First off I removed the motor from the B-112, I had to leave the lower Oil pan in place, is this typical? Got the shroud off of the motor and found a large rat/mouse nest right on top of the armature/mag, cleared it away found the wire that connects to the M on the starter switch had its insulation chewed off, I had no spark and found this wire to be grounded before taking the motor out, so I figured the rodent had chewed the insulation away and the wire must be grounding out on the motor somewhere however when removing the wire on the armature tab I found the tab also goes to ground. Is this normal was I thought when the starter switch is turned off it pulls this wire to ground and stops the motor, I am trying to find out if my armature/mag is bad.

[portuncia 0]

In other words the tab on the mag is grounded when I believe it should be open in order for there to be spark.

[D-17_Dave 12]

First off, no. You can remove the lower engine oil pan with the engine. Second, I think your seeing the connection terminal block on the front engine cover. This bolt is supposed to have insulators between the bolt/screw and the front engine cover. This is only a junction block but as you thought this circuit should remain ungrounded until you turn the switch off grounding the circuit which will shut the ignition off. If the mouse left the wires intact but chewed a little insulation off you can tape the wires off and continue to use the magneto at least enough to finish diagnosing the rest of the tractors condition. If you can tape the wires satisfactory then concentrate on the points as the source for no spark. With the oil pan in place you can run the engine out of the tractor and help dial it in before going to the trouble of replacing it to just try it.

[portuncia 0]

Well I have the engine out seperated from the oil pan, the tractor doesn't have points, I am pretty sure its a Magnetron coil. When I put one leg of the ohm meter on the tab coming off the Magnetron(tab that the wire hooks to that goes to M on the starter switch, this done with the wire removed) and one on the engine block it indicates that the tab is grounded thus there would be no spark to the plug, I am woundering if the Magnetron is shot.

[UCD 14]

The Ground should be there before the coil to complete the circuit. The ground placed in the circuit from the ignition is placed between the points and coil shorting out the circuit. Current travels from positive side through components to ground to operate. if you interrupt the circuit with another ground before the components it kills the circuit hence the ground from the key switch. Electricity takes the shortest and easiest path to ground. I can't explain it any simpler than that. your coil/mag most likely is good and was being grounded out from the bare wire. Been there done that. Take some shrink wrap and insulate the wire and try it.

[D-17_Dave 12]

The magnetron is a magneticly induced switching device. Other than that it's still a magneto style ignition. The secoundary winding in the mag. goes around the primary winding like a coil. An ohm meter would probably read an ohm load, I'm not shure what rateing. If it's open I'd say it's shot. If it was a dead short to ground I'd say it's shot. If it is a Magnetron the clean the magnet, adjust the air gap and spin the engine hard with a rope or use the starter as a Magnetron needs 300 rpm's to fire.

[portuncia 0]

I am still unsure as with the mag mounted on the engine and the wire removed from the tab. One leg of ohm meter to tab and other touching the block shows the tab is grounded thus the wire to the switch is always grounded. This would tell me that something is wrong with the mag

[UCD 14]

That ground is supposed to be there.

[portuncia 0]

I am still not convinced that this is right how can the magnetron electric ignition produce spark when the wire tab is at ground. I checked to see if there was even a small amount of resistance, several ohms or more but it is a straight short to ground. I am going to try and get a new one. Dont want to put the whole thing back together to have to pull it apart again becasue of a bad part.

[ambler 0]

Do you have the service manual? What is the year manufacture number on the shield? Magnetron is in engines after 1981.

[portuncia 0]

I was just assuming as I saw a post here that showed the old style armature(with points) and the magnetron and mine looks like the magnetron. I had read somewhere that they make them for older models to eliminate the points. All I know is my motor has not points and the guy before me said he did the conversion to the electronic ignition. If they don't have magnetrons for these old 12 hp cast iron singles then what are the armatures that use the electronic ignitions. (Sam, the briggs point type coil will have an equal space between each side of the armature legs and the coil. On a magnetron coil, the space on one side of coil will hold the magnetron "trigger". Earlier magnetron coils had the "add on" magnetron trigger while later coils had it molded into the coil. Those later units will show a metal button in the molded area next to the armature leg, which will face the flywheel. Also, in general, the magnetron coil will have a spade terminal to attach the ground wire to for grounding the igntion, while a point type coil will have a wire to go to the points.) That is an exact description of it but all I can find on the magentron is the number 22. And my spade terminal is shorted to ground so I believe I have a bad on. Might have accidently hooked power to it when fooling with the ignition switch.

[UCD 14]

One side of the coil should be grounded the other side should go to point/switch. With ohm meter set on highest setting read between point/switch side to the ground side. Are you using an analog or digital meter? A low setting might give a false reading depending on meter. It should read resistance, I am not sure of exact value on that coil, but it should be at least 5k ohms but I believe 10k or more. If it reads near zero or zero you have a shorted coil on primary side. Also check between Point/switch tab and secondary wire (spark plug wire) it should read infinity. If not primary side and secondary side are shorted. Between the secondary and ground should read high resistance, if not secondary side of coil is shorted.

[portuncia 0]

From the spark plug wire to ground I read 2550 ohms which is inline with the 2500 to 5000 ohms that B&S has on their website blurb regarding Magnetrons. But I believe this is just reading the resisitance of the secondary coil which has 60 times the wire of the primary. From the grounding spade tab on the magnetron(tab is shown in picture of first post in this thread) I am showing no resisitance with a digital meter, actually took the reading using two different digital meters. Maybe a couple of tenths of an ohm different than shorting the two meter probes together, a short for all practical purposes. If I am understanding this right the grounding tab should show at least the smaller resistance of the primary coil because when the starter switch is turned off thus taking the wire to dead ground it takes the top of the primary straight to ground. So if the secondary reads 2550 ohms the primary should be on the order of 2550/60= approximately 43 ohms? I read 0.4 ohms. BTW thank you for your help and patience.

[UCD 14]

.4 ohms on what scale?

[portuncia 0]

200 ohms with buzzer for continuity, 2K ohms both yield very low readings all other scales read 0's