I have a 2001 ram 1500. my son left lights on and battery drained. tried to jump start and starter wont engage.

Check all the electrical connections . grounds and to starter. One of them are bad or the new starter crapped out on you. Remember just because it is new doesn't mean it is good. I have got new items that were junk out of the box.

unless you have the key fob also,you need a new ignition switch,with key fob and key with a chip in it,the switch turns on the fuel pump when the chip is recognized,take it to someone that knows what they are doing,you CANNOT hotwire it.

2001 dodge ram 1500 key fob Bing

get a jump, take it to Auto Zone or other parts store, have them run a free diagnostic on the battery, and the alternator. My bet is that it's one of the two that needs replacement.

Weak connection at battery or weak ground or just a bad battery.
check all connections and clean terminals. could try to jump start if it still fails. FYI i have seen many faulty NEW starters out of the box

I had the same problem...I changed the battery ..took it to a mechanic...started fine all day for him ...He still said he changed 2 relay switches and charged me 300...He just said they were in the engine ..I think it had alot to do with the battery...

any clicking? Any lights? Try cleaning your battery posts?

Replace the starter, the solenoid cant engage the starter because it is locked up, hence the extreme voltage drop with the fan when you try to start it with the fan on.

First thing is to check is the ignition fuse/relay under the hood for a blown fuse or bad relay. If everything is ok then set parking brake with truck in park/netural and have someone to turn the ignition over to start and at the same time use a test light to check for fire at the starter solenoid. If no fire then the ignition switch or netural switch on transmission is causing the problem. If you do have power to the starter solenoid then the problem would be either the solenoid or the starter drive locked up on the flywheel. Be sure to remove the negative wire from the battery if you need to remove the starter. Hope this will help you.

Does your key have a chip on it? If it does its probably the problem

run another test on the starter...they are about 20/80 bad/good right out of the box!