How do you replace the alternator on a 1990 Nissan pick up?

The first thing you will need is about 4 arms and a whole lot of patience. Before you begin, check the location of the voltage regulator socket on the replacement and the original. Some replacements may have the plug socket on the 'other' side of the alternator - as mine did. I had to really stretch the harness to make it fit! I was lucky that I could get it together as I have 'just' enough extra wire to reach. Pull up on ramps to gain access under the front bumper. Remove, what we affectionately call, the "tool tray" (the brush guard under the engine to gain access to the belts. Loosen the alternator pivot and tuning screws. Remove the belt from the alternator. (Note: If you need to replace the belt, you will have to remove all of the belts because it is naturally the one in the back.) Remove the pivot screws (fore and aft) however you can get to them. Once the alternator is free from its mounting bracket, (this is where you can use the extra arms), remove the electrical leads from the back of the case. You may need to remove the wire harness from the clips just inside the left wheelwell (look from the top under the hood) to gain some extra working room. It isn't easy pulling the alternator through everything you have to get past, but with a little patience and persuation, it will fall into your hands (or on your face, if you're not careful). Reverse the process to install the new alternator. I recommend tightening the front pivot screw before trying to find the rear one. Hope that helps!