I'm pretty sure #1 speed is 540 rpm and #4 is 1000 rpm. The other two are in between somewhere, not for sure what though.
According to my manual, rated voltage of the glow plugs is 10.5V and rated current is 6.5A. This is for the Bolens equivalent so I ASSUME it's the same for the Iseki model since it's the same Isuzu motor. To test, disconnect the glow plug connector and measure the resistance between the glow plug terminal and body. It's normal if the measured value is 1.8 ohm. If it's zero, it's shorted and needs replaced. Good luck.
the other Bill