so my 90 dodge d250 diesel is having problems starting... things ive done. new ignition switch, new alternator, new voltage regulator, new starter relay, new serpentine belt... but still when i turn the key it tries to turn over, but it buzzes and clicks ans makes weird noises but sometimes it turns over and keeps turning over and if i hold the key to start for awhile itll eventually fire. but usually it just kills the battery. i tries to test the started by jumper cable method but it just sparks and doesnt turn

It sounds like either you have a bad battery(have it load tested) or you have a bad ground.I personally would go through and check and clean the main grounds and it will probably solve your problems.

Change Battery Cables. Check all others ground.

Running the Starter for long time will heat up the cable and weaken them.

Likely have damage battery too.... Starter Battery hate being drain to dead condition. see my cave, battery.

Test "new" starter at parts store... maybe bad out of box.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)