The vacuum shift units use the diaphragm to preload the shift direction, and I don't think they have any springs, but it's been a long time since I've been into one of them. The electric motor runs for a short time and winds up the shift spring, which snaps the shifter into place once the gears are unloaded, either HI or LOW. Without engine vacuum holding the shift collar in place, that one can find a "false neutral" between the high and low positions, particularly while rolling along in neutral. If the engine had been running recently, the reserve tank would have probably had enough vacuum to keep the shifter in one position of the other.
Jerry
