I had a 1985 Nissan Pickup with 100k mi and the starter wouldn’t go. I checked out the electrical input and it was ok. Having little money I figured it was in motor and I should diagnose to find out exactly what went wrong?hoping it was fixable. I took the starter out and put it on the ground. I used jumper cables and when connected to the starter solenoid it turned just fine. So I put back in the truck and it wouldn’t work. I took it out and the same thing happened?it worked with the jumper to the solenoid. After 3 times in and out I broke down and took the motor apart. I found that the brushes had worn so that the would just make contact when the starter was on the ground. In the car it was in a position 180 deg from what it was on the ground. I bought new brushes and was ready to install them but decided it would be simpler if the bracket would just allow the existing brushed to get a little closer to the commutator. I used a rat tail file file to deepen the groove where the brush pigtail sticks out. It worked great until the transmission died 50k mi later.
The solenoid contacts may be bad. If they are worn badly you may be better off just replacing the starter if it is the original unit. You can replace the contacts though.
I always check the solenoid and make sure all connections are clean and secure.Even the Ground.Starters are usually made of stronger stuff.