not to step on toes but the reed switch closes when put under load closing the 12 volt sircuit and idling the machine up. When the 12 volts drops out or is grounded the machine idles down.
If your machine won't idle down make sure the solenoid is clean, no oil on the plunger, then take the white wire (far left) on the board loose from the board and scratch it to clean metal to ground it out, if it idles down, either your board is faulty or you are not getting a good ground at the mounting spacers.
Another problem that causes the SA200's to not idle down is a faulty oil pressure switch. But with this fault, your pilot light won't come on and you will start having to jump your machine. If you encounter this problem, to check it put a jumper wire across the 2 terminals on the oil pressure switch, if the light comes on and your machine idles down, walla. Take the jumper off and replace the switch. It takes approx 8 psi to satisy the switch for operation, so you can get one at an auto parts store if a Lincoln dealer isn't close.