Don't understand why my fix worked, but it did

4.0 OHV engine.  Began as an intermittent no start (crank but no fire).  
Finally would not start at all.  Started with starting fluid.  Would start, but
stumble, then smooth out.  Would start a couple of times after that, then no
start again.  Hooked up fuel pressure gauge, turned to on position and
pressure raised to 55 lbs.  No start.  The tach would not move at all in
cranking mode so I suspected crank position sensor.  Before climbing under
the truck I decided to swap out the horn relay with the fuel pump relay.  It
started right away without starting fluid like normal.  Did so about ten times
with no problem.  I am stumped as to why I was getting 60 lbs pressure with
old relay, but no start?  Then swapping out relay, it starts fine.  I also thought
the tach should bounce a little to indicate rpms while cranking if there is a no
start condition.  NOTE:  The fuel pump and horn relay, and two others are all
5 position relays, but I don't know of the b pin is used at all.  Could the crank
position sensor also be bad but not affecting start?