The key switch is a three position switch with two inputs into the ECM. When the key switch is moved to the ON position, a system voltage input is provided to the ECM. The presence of this voltage input will cause the ECM to transition from the "sleep mode" to the operational mode. When this mode change occurs, the ECM will initialize and then become operational.
When the key switch is moved to the OFF, the system voltage is no longer present at the ECM contact. The absence of the system voltage signals the ECM to transition from the operational mode to the sleep mode. The ECM will perform some inter power down procedures and then enter the sleep mode.
The transition period from the sleep mode to the operational mode happens in a short time. When the key switch is moved to the start position, a system voltage input will be provided to the ECM start contact. When the ECM detects this voltage and the machine system start status is correct, the ECM will initiate the engine cranking sequence.
The fuel level sensor is an active analog sensor. The sensor determines the amount of fuel in the fuel tank and reports that information to the Machine ECM with an analog voltage signal.