Small Engines (Lawn Mowers, etc.): will not start or turn off with key, mtd mower, thanks norman

Question
I have a MTD mower mod.#134L66oF352, S/N 2F234C20203 I believe to be a 1994. Most of the time the mower will not start. I ran a jumper from the spade lug of the solenoid to the + post of the battery and the starter cranks the engine. This leads me to believe there most be a problem with an interlock. My problem is how to check them. The seat one seems ok. the clutch/brake one has 4 lugs. I disconnected the wires and checked the two on one side with a meter and had no connection, the other two showed a connection. When I pressed the plunger it showed the reverse of when I did not press the plunger. When it does start it runs great. The engine does not shut off with the key. I separated the yellow wire that connects to the black wire that goes to the coil. I hooked one side of my meter to ground and the other side to the black wire the black wire was grounded, not sure if it should be. I hooked one side of my meter to the yellow wire while I sat on the seat. With key off it shower a connection to ground, with key on it showed an open circuit. This is as far as I have gone and need help to continue.
Thanks, Norman

Answer
First you need 12.6 volts DC in the battery to start the engine cold.  When you turn the key your start circuit is from the battery + to the top of the solenoid.  Current then goes through all safety switches and interlocks and back to the small terminals on the second.  The current then goes from the solenoid to the starter to ground.

If 12V goes to ground anywhere along that circuit the tractor will not start. Follow the 12V switch to switch connector to connector. When you put your meter on each side of the bad switch or connector I've will go to ground instead of the next link in the chain.  

The yellow wire is the kill wire on most systems. When the key is turned to off there should be current to ground through the yellow wire.  And vise versa.

The clutch switch is a double throw double pole switch.  That means that it is two switches in one and both have to work.  I suspect the key switch or the PTO switch.

Good Luck