Small Engines (Lawn Mowers, etc.): B and S 12.5 HP, starter solenoid, incorrect position

Question
Hi Eric, Standard old 12.5 HP engine (10 years), refitted to olld mower (blown 12.5). Everything works perfectly except I cannot turn it off from the key, nor do the safety cut outs work. What have I neglected, it all looks good, where should the cut out meet the block or otherwise, thanks for your advice,
Cheers
John Chester

Answer
Check out this link:

http://www4.briggsandstratton.com/miscpdfs/RNT/alternator_replacement.pdf

There should be a connection, usually near the throttle plate control on the engine, to connect the engine shut-off wire.  Usually this is a spade connector and is connected to the "M" terminal on the ignition key switch.  When you turn the key off the switch grounds the "M" pin which grounds the primary side of the ignition module stopping the spark from being generated by the ignition module.

There are a couple of primary designs for the safety interlocks.  The most common design prevents the engine from turning over with the key switch by interrupting the power for the starter solenoid.  If one of the switches is not in the correct position the engine will not start.  Usually this design is coupled with the blade, seat or reverse switches connected to the "M" terminal on the ignition switch.  When you get off the seat with the blades on the switch connected to ground and shorts the primary side of the ignition module.

The second design will allow the engine to turn over with the key switch but if any of the switches are in the incorrect position then the ignition system is shorted to ground.  This system is cheaper to manufacture but is not as user friendly as the first design with regard to the ability to start the mower even if the blade or seat switch is defective.

What brand of mower do you have?

Eric