If the overdrive's off light is on that means it's off. If it is off then the overdrive is on.
Possibly, but it's more likely to be a wheel speed sensor.
You will have to drop the oil pan to access
The lowest speed at which an engine operates smoothly when it is not carrying any type of load. In the case of aircraft engines, it is the engine RPM when the throttle or power control lever is fully closed. In cold weather conditions, it may be necessary to keep the engine RPM of a reciprocating engine higher than idle to keep the engine running.