"Bob Gardner" wrote in message
news:_tkec.120289$K91.333719@attbi_s02...
It's common sense, when you think about it. With both halves of the switch
ON, the alternator field windings are connected across the battery,
creating
a drain in addition to that drawn by the starter

It's worse than that, too. Not only is the battery being used to energize
the alternator field circuit, but having that circuit energized creates drag
on the engine, making the starter work harder.

Pete