alarm sounds constant at 1/2 volume 1996 searayder 120xp

by shawn
(Wilmington N.C.)

When i turn the key on i get a 4 second alarm and fuel gauge pegs full, then drops.

when I start the boat the alarm starts to groan as the boat fires then keeps on going when it starts at a 1/2 or lower tone.
I only have a temp heat alarm and low oil in tank alarm both are unhooked to test and when I ground the wire I get the normal full alarm.
I think it has something to do with the black box under the dash it has a tachometer wire a hot wire a ground and a ignition wire to it, its about 2.5 inches square and black epoxy, i think its for the alarm check at start, WHAT IS IT?