1990 300zx starter problems, Wont crank?

Im having a problem with my starter on my 1990 300zx. It sounds like there lil power going to the starter. I took the starter to auto shop and they tested it & it passed the test. I changed the starter relay and still nothing. I battery I put in my z is from my nissan altima 2004 which works great so im pretty sure that battery has enough power to give the starter. Does anyone have any clue what it may be??