Went to start my 2001 and the engine wouldn't start but the starter ran away. had to undo the batteries to get it to stop. Starter bad, relays....no clue

 

I would bet that the solenoid stuck. I've experienced that once before.

 

Yup, contacts need replaced. Once you have the starter off, easy job. Search on this site for contacts and instructions. Pulling the starter is the hardest part.

 

I carry a wrench that fits my battery lugs in the cab after my truck did this to quick unhook them if it ever happened again. I was in the boonies when they stuck on me. Removed cables to stop the meltdown. I whacked the starter solenoid with a wrench lightly and they released. I put the battery cables on again and luckily it worked. I removed the starter and rebuilt the fried contacts. Low battery voltage is bad for these contacts if you know Ohms Law. Look up Larry Bs or search EBay on the internet. They sell oversized contacts!

 

Low battery voltage is also a killer of relays. Not enough current to lock it in solid. The contacts flash from the load and may even tack weld themselves together.

 

Had this happen on a Ford once. Check the actual ignition switch down by the base of the steering column. Mine stuck on and no matter what I did with the key it wouldn't stop the starter from turning over. Scared the crap out of me.

 

Sometimes they let go and sometimes they need a good smack.