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Hi All. I just got done cleaning up a bad mouse infestation in my old Toro lx500. They chewed one of the plug wires in half and a lot of the insulation on the white kill wire that runs between the two coils leaving a fair amount of exposed wire. I replaced both coils and replaced the section of white kill wire that was damaged. Now the machine will start right up and runs great if that kill wire is disconnected from the second (or both) coils but of course, in this condition the mower continues to run even if you get off the seat. That's super dangerous IMO and I'd like to make it work correctly. If I plug the white kill wire back into both coils it will not start. I've checked the 20A fuse and cleaned the seat switch (which is really just a spring steel contact) but I'm not sure where to go next. I have a multi-meter and a base understanding of how to use it but these types of electrical issues do tend to vex me. Any help or suggestions of next troubleshooting steps would be welcome and appreciated. Thanks!

Please post the engine info. It depends on the engine setup. Some have a pair of diodes in that wiring harness, others the diodes are in the coils. But with you replacing both coils, the diodes may have been the section that was replaced or you still have an electrical short in the kill circuit somewhere.

I would start by tracing the kill wire from the coil back to the switch. This is a problem all of us who work on electrical problems hate as it is slow and tedious. Patience and a friend will be of some help, as you are looking for a place where the insulation is rubbed through or bad connections. Number one rule when working with electricity, DOUBLE CHECK EVERYTHING AND DON’T ASSUME ANYTHING.

Rivets said:

Number one rule when working with electricity, DOUBLE CHECK EVERYTHING AND DON’T ASSUME ANYTHING.

Click to expand...

How True..

Hey Guys. Thanks for the replies. The engine is a Kohler SV720 (23 hp Courage) I'm OK with the tedium and I'll get it done but to be honest I'm not really sure exactly how to trace that kill wire back to the switch, or which switch is even in question here. If anybody has the time/patience for a little more specific direction I'll get out in the driveway and try again tonight (in the snow) because this is starting to make me nuts!...

Okay this engine's coils have the steering diode built-in the coils. There are two safety switches involved the seat and brake. Both are required to be depressed to prevent the engine kill function. According to wiring diagram you would tracing the yellow from the engine back to the ignition switch. It also goes to the brake switch. Then it continues as a yellow wire with white tracer to the seat switch. There also a yellow wire from the seat switch to RMC module.

Wiring info service manual. Your mower wiring is near back of the manual.
https://www.toro.com/getpub/38101

Here is a manual which will help you out. Has a wiring diagram of your unit and will give you a better understanding of how you systems work. I know I shouldn’t do this, but I’m assuming you have a VOM and know how to use it. The switch I’m talking about is the key switch. With the key in the run position, PTO switch OFF, and the seat switch closed, you should get zero continuity between between the kill wire and ground. Remove kill wire from coil to test this. If your engine dies while in use, some where in this circuit you have a short to ground, killing the engine. That short is what you are looking for. https://www.toro.com/getpub/34847

Looks like Star beat me to it.

Just trying something here as the redirect isn't working for me Rivets.

Test link

EDIT: aHHH I see the problem it is adding HTTPS to my links and my Firefox setup don't like it. Oh well Internet Explorer works.

When working on the kill wire I like to use a test lamp with the clamp on the + side of the battery because you are looking for a short to ground.
Always pull the kill wires off the coils because as little as 9V dc at 0,01Ma will fry most f the control circuits in the trigger chip.
Next thing to do is make up a 1/2 dozen patch wires with a male spade at each end so you can jump all of the safety switches .
Having done that if you still have a ground short you are looking at a bare wire & not a defective switch .
Over the ( few ) years I have been doing this most times it is a wiring short & not a bad switch.
This of course assumes you can work out which switches are open or closed in the run position.
IF you are confused the ask as we all have the same diagrams at our disposal ( thank you Toro )

OK fellas. Those Toro guides are great stuff. At this point I've checked all the switches and searched thoroughly for bare wires and I can say I'm 99% sure that they're good. It still won't start with the white kill wires hooked up to the spade terminals on the coils. But....As a test I hooked up the kill wire on the second coil only and it will start (the machine actually runs great but for this electrical problem). Interestingly when get off the seat while it's running I can clearly hear that left cylinder cut out (even thought the engine is remains running). So that kill wire / safety switches are working as they should on the second coil but when the first coil (right hand side) is hooked up to the kill wire it will not start. Could that right coil be bad somehow - even though it runs fine without the kill wire? It occurs to me now that I should have mentioned this; these new coils are not OEM from Kohler but Chinese parts from Amazon. For what it's worth; I'm thinking the problem may be with the coil now based on the above test and Bert's statement :
"Always pull the kill wires off the coils because as little as 9V dc at 0,01Ma will fry most f the control circuits in the trigger chip."
which, I'll be honest, I don't fully understand but I get it enough to know that I've been screwing around with this thing for days and maybe I could have inadvertently done some damage to the coil... Thanks for sticking with me here!

Hall effect trigger 101.
Back in the early 60's when transistors were invented a bright spark worked out you could use them to replace the points on an engine.
What happens is when the magnet passes by the coils it generates a voltage.
However this starts at 0 V rises to the maxium then goes down to the same Voltage negative then back to 0 .
If you measure the inital rising voltage you can set the electronic switch ( transistor ) to ground the coil when the voltage is at or near its peak which mechanically is the same as closing the points to allow the voltage in the coil to discharge through the spark plug .
You can do all sorts of fancy things as well but basically that is it .
At first the timing chip was a stand alone item which made ignition coils very cheap.
Then some bright spark ( probably with an MBA ) worked out you could encase the chip within the coil thus a magneto coil became a magneto module and become unique to your engine thus by adding the chip which costs around 5 ¢ you add $ 40 to the end price .
The voltage measured by the trigger chip which is connected to the kill wire is in the micro volt range so shoving battery voltage down it is the equivalent to plugging a 110 V device into a 220 V outlet.
Poof and the magic smoke escapes .
Also because of the chip, the coils ( modules ) are polarity sensitive so if fitted upside down will not work at best and be destroyed at worse .
Because this voltage is so low on 2 cylinder engines that have a common kill wire the first magneto coil can send a signal to the second which causes a missfire so a diode was put in the wire that goes between them .
latter this was added to the chips

Actually the module on the newer Briggs coils is just a circuit board with a few surface mounted transistors, resistors, capacitors, and inductors with one main D13003 transistor.

StarTech said:

Actually the module on the newer Briggs coils is just a circuit board with a few surface mounted transistors, resistors, capacitors, and inductors with one main D13003 transistor.

Click to expand...

I would have thought a PCB would have been a lot more expensive than a chip.
The people who invented this , Atom Industries were one of my customers.
IT was funny talking to them, Hungarian immigrants post WWII.
They would happily tell me that the chip cost less than 50 ¢ the cardboard sheet was $ 2.00 .
They sold them for $ 10 & they retailed between $ 15 & $ 20 .
I had bought hundreds of them because we fit them to BSA Bantams ( Harley Pups in the USA ) before picking up atom as a client .

Yes I was expecting a chip myself but that wasn't what I found when cut one open as I had a bad one here. I would know what the individual surface items were but most were destroy as they stuck to the epoxy except for the main transistor. Even when I found the transistor was expecting it to be a SCR or Triac but I ran the numbers it came back as a NPN transistor. They could have put the steering on the PCB but they didn't. I reckon they wanted to sell more coils.

The windings are rather robust, good size enamel wire. The step up transformer has a turns ratio of about 1 to 18. Due to how the kill terminal is connected it is nearly impossible to test a coil by just using a good ohms meter as you would only testing the transformer and not the electronic trigger. This because the kill terminal connects directly to the ungrounded side of the primary winding. The main transistor during operation grounds the primary winding via resistors when it is triggered to collapse the primary winding magnetic field which introduces a current and voltage in the secondary winding.

Well, I understood enough of what you said to order a(nother) new coil. I'm betting that'll be the end of it on this one but I'll sure follow up to let you know. Thanks!

It's been a minute but with the state of the world.... ah anyway, I finally put (another) new coil in and the symptom was unchanged. Now I'm thinking it may be an incompatibility of the Chinese coil? I don't know but I had to move on with this one so I guess I'll never know. Thanks again for the input and stay safe out there.