My apologies. Was a long day in the heat and too close to bedtime...

To test, ground the case and supply 12v to the terminal, the spring will "wind", the points will open and the clock will run for a bit (about a minute or two) and then repeat the cycle.
Sometimes the points burn, stick or the clockwork gets dusty and stops.
But they are reparable and/or you can get an electronic kit to replace the clockwork.