[rotorwrench]

These old transmissions have a freewheeling sprag unit that allows it to coast anytime its not locked out. The shift function can not happen if the power from the battery is disconnected from the relay. It is still locked in but it just won't shift. When locked out, the electrics are still hot but the function is physically blocked by the shift rail. If it's an older unit that still has a rail lock out switch then the solenoid can not be switch on when the transmission is locked out. I personally thought it was a bad idea to eliminate the rail lock out switch. This kept the solenoid from going hot when the governor switches on to ground. That solenoid uses up a lot of amps when it can't shift. It's not a problem if a person doesn't lock out the overdrive very often but if the local has a lot of steep hills, it's necessary to lock out more often.