If the voltage, measured AT THE BATTERY with a digital volt meter, is less
than 10.4 volts with the starter turning the engine the ECU will not work
correctly and it won't start.  The starter will, however, still spin the
engine over pretty good at this voltage.  I've forgotten if the ECU will
fail to produce spark or fail to activate injectors at this voltage
because it was about 18+ years ago this happened to me.

A simple way to check is to try a booster battery to make sure there is
enough voltage.  If it starts, you probably need a new battery although it
could be something wrong with your charging system.

I've been thinking that maybe the tachometer will not work if the ECU is
below this voltage threshold.  Check to see if the tach needle indicates
the appropriate cranking speed while cranking.  If it is correct, you
don't have any new information but if it doesn't indicate correctly you
know that the ECU isn't working.  I don't remember what the tach did when
my '86 had this problem.