Small Engines (Lawn Mowers, etc.): derby mower wont turn off, steering levers, fuse one

Question
I have a zero turn Derby (Ferris) Stallion, TW 2354. Kohler engine is a Command 17-25 vertical shaft: CV23S. Refused to turn off. I shut off the gas line to stop it, explored and found a fuse had melted in the fuse holder  (4 fuses total). Eventually replaced that one fuse holder, put in new key starter/ignition. Still no success.

Nothing within the interlock safety system
works right now. PTO will not shut off if I take weight off of seat, seat will not trigger shut off, nor will  brake or steering levers out of neutral shut mower down as they are designed to do.

This is hardly my area of expertise, but I don't mind getting involved with simple repairs in order to save a few bucks.

Now I am wondering-again seeking simple solutions-if perhaps I installed the adjunct fuse holder incorrectly. Don't know: everything else works. I really don't know what 3 of the 4 fuses go to. I do know which fuse goes with the starter.
A gentleman at the parts store suggested I pull out each fuse one at a time to see if I could shut down that way. Nothing happened. Now he has suggested that I test each wire attached to the fuse box, to see if a wire is staying hot after I have shut down. I will take that on next.
Meanwhile, any suggestions would be greatly appreciated.

Answer
The ignition system is not receiving a ground to shut-off the mower.

There is a dis-connected or broken wire that should be connected to the engine.  I am not familar with Derby mowers but there are two ways you could find the problem.

1) You could remove the top engine cover to expose / gain access to the ignition module(s).  Follow the wire(s) from the module to the engine block.  This will be a small wire connect to each module.  Follow this wire to the engine block.  You should see another wire connected to it.

See page 34 of the Engine service manual:

http://www.kohlerengines.com/onlinecatalog/pdf/sm_24_690_07.pdf

2)  Trace from the ignition switch. I need to know what switch you have.

I'm betting the wire that is supposed to turn the engine off is dis-connected or broken.  

You can attach pictures of the switch to help me provide advice.

Since you said none of the interlocks are working then this points to the ground wire being the issue as this is the common point for all the safety switches.

Do you have a multi-meter?  Do you have a schematic/wiring diagram for the mower?

Eric