99 MTD lawn tractor with 18.5 V-Twin engine no spark

[jimigunne]

It turns over but no spark at all. I thought at first that the single wire going to magneto was switched 12V, but then
figured out it is supposed to be grounded to kill the spark/engine, or grounded to enable the spark/engine. Which is it? I measured
2 ohms to frame ground on the yellow wire going to the magneto, with it disconnected from magneto. Also with the wire disconnected,
I measured .2 ohms to gnd. right at the magneto terminal. It seems that it must be open to gnd. to run, and because the magneto
terminal is itself grounded out, it must be bad? So magneto is no good? I would really like to have a schematic for this mower. Where
can I get one? The electrical system is a mess.

 

[Fish]

To give you any help, we will need the model numbers of both the engine and the tractor. The engine's should be around one of the valve covers by one of the spark plugs, the tractor's should be around/under the seat.

It is critical that you know what you are doing, as these ignitions have nothing to do with the voltage from the battery or charging system, and
not knowing what you are doing can and will burn up some very expensive ignition modules.

Post what you have and we can go from there.

Fish

 

[Carscw]

Go to mtd parts.com and put in this number it is for a 98 but is the same as yours. You can pull up a full view diagram

14AV834P401

(( cowboy up and get over it ))

 

[jimigunne]

Go to mtd parts.com and put in this number it is for a 98 but is the same as yours. You can pull up a full view diagram

14AV834P401

(( cowboy up and get over it ))

OK. Got wiring diagram downloaded, but it does not help with immediate problem of no spark, since indeed it gets no voltage from battery. I located a replacement ignition coil from internet supplier, from the pictures I could see that it is the exact same one, with two spark leads....it appears in every way identical.While I had the old ign coil off, I put a magnet near the magnet on the flywheel and there was strong attraction/repulsion there. I have no instructions about what to set the gap between ign coil and flywheel at....but I set new one to where it was rubbing initially and then backed off a hair. I left the control wire off, as it is supposed to work with no ground, right? Still, there is no spark. (I fixed up the starting circuit with new solenoid and it cranks really good now!) But no spark even with new ign coil! It seems there is only one possiblity....they shipped me a bad ign coil. As I said, I read 0 ohms to frame ground at the control terminal on old one, which is why I thought it was bad. So I read control term to gnd. on the new one, and it ALSO reads 0 ohms to ground! What is up with this?

 

[Fish]

If yours has the solitary coil, with 2 spark plug wires, then your engine is not a "V-Twin", which is why I was requesting the model numbers so
we know what you have. If you leave the small kill wire disconnected, and mount the coil per instructions, and use the cardboard card supplied
with the coil to set the gap, then you should have spark.

If not, the new coil is bad, the magnets have lost their pull, or else you have already burnt up the new coil.

The small wire to the coil is the kill wire, as when ground is supplied to this wire and thus grounds out the coil and safely shuts off the engine.

If at any time 12v happens to get supplied to this terminal, it will destroy a brand new coil.