WHAT FLUID DOES THE TRAN TAKE FOR THE 03

this is covered in the service manual or
google
03 mercury mountaineer 4.6l firing order, right bank 2nd cylinder rear of #1
right means driver seated perspectives.

https://www.fixya.com/cars/t8536955-firing_order_2003_mountaineer

Yes thier is several fuses. Depends on for what. 1 fuse is for the solenoids and 1 is for the reverse lamps. 1 is for PCM neutral safety switch

Your speed sensor might be hanging up.when you changed the trans oil and filter did your old oil smell burnt, and make sure you put the right fluid init.If your oil is burnt looks like you have a big job ahead of you.

tran is going bad.onese you slip the tran. the frition plate

It could be a bad tourqe converter. If it wasn't drained it still has metal shavings and old fluid in it. I would recomend draining it and replacing the fluid/filter.

U may have over filled or used the wrong fluid.

I know is not what you would like to hear around the holidays, but.... a lot of times when the overdrive flashes, there is a mechanical problem, most likely in your case lockup in the torque converter. i would say it would be a good idea to get it to a garage asap, to save yourself from any other damage that will cause you to do a complete rebuild.

use a ruler or any stick where you can see the level of the fluid.

The simplest way is to take your own trans fluid to a shop with a flush system available...otherwise you have to drop the trans pan....make sure you use the factory fluid

If same tires as before & air pressure set correctly, or as usual, a humming when swerving, or turning one way or the other or both, it's usually a pretty good indicator.