ok i have a 1990 nissan 300zx and when i put the key in the ignition it blows the battery fuse right away and i cant start my car

its an electrical issue where the ignition is overloading the fuse....so even if you keep changing the fuse it's going to keep blowing it.

You have a short some place in the circuit. This could be caused a wire that over time lost its protective coating and its now hitting meatal. So every time you put the key in it shorts out . Finding the bad spot can be sort of a problem becuase it could be almost any place along the circuit.

lightGear (3,799) 06/08/2007 its an electrical issue where the ignition is overloading the fuse....so even if you keep changing the fuse it's going to keep blowing it. I think he figured that out after he blew his 10th fuse

Find the online service manual, it has the wireing diagrams for the ignition circuit, get yourself a multi-meter (prefer analog for checking continuity), and start eliminating sections of the circuit by checking the Positive to Ground (most likely where the short is going to), If you find somewhere that the iginition circuit is connecting with ground, you found your section, now find the wire, good luck. Read up a lot on the causes at places like 300zxclub.com and twinturbo.net.