Ford 4R100 Transmission

I forgot to mention that if I'm under a strong acceleration it shifts really good, Its just under a slower or moderate that it does this.

Did you ever find out the cause? I am having the same issue- same make/model as well.

Mine was just a small axle leak at rear gear and top up was all it needed..... very common problem 02-05 Explorer and Aviator.... mechanics love to jump to conclusions pointing to tranny prematurely!

Clearly this is an old question& I'm sure his problem has been fixed. For others who are having this issue, it's most likely not the rear end being low on fluid. Most likely it's the transmission, and more specifically one of the, or sometimes both servo bore pistons. Over time the metal rod on the piston wears out the brass/bronze bushing that it rides in, and due to the wear, it'll allow pressure blow bye and acts like a funky shift. Some people get shakes or shutters. Others get drawn out shifts or reving/flaring shifts. The reason the transmission will shift great while under power or acceleration is because you're building up extra pressure in the trans fluid& the extra pressure helps push that piston into position better/faster, thusly engaging the next gear. Sooo..... Now for a solution. You have a few options depending on how bad your trans actually is. If you catch the wear early enough you can throw a qt of Lucas Trans fluid in the transmission& allow it to work it's magic. You won't notice an immediate fix but it'll take a few hours of driving to work itself into your existing trans fluid. If youre only adding the one bottle (and it may not be a qt, it may only be a half qt if I remember right) then you won't need to drain any trans fluid before adding it. Unfortunately these 5r55s transmissions have no dipstick to add fluid thru. They only have a stupid fill plug on the trans pan. So you need to either be quick with the squirt bottle& have or make the nipple attachment to get fluid in the trans. Second option according to some is to run 2 bottles of Lucas fluid and take away one qt of trans fluid..... Now as stupid as that may seem to many, i guess youre battling time& money. If your trans is having that many issues& the pistons/bushings are worn that much but not broken (some cups break away from the piston shaft itself), then maybe spending 50$ on changing the trans fluid, adding the Lucas& changing the trans filter is worth the chance of it getting a few more driving months or possible longer, instead of having your trans rebuilt& servo bushings drilled/reamed& new bushings installed. Which can get pricey quickly not including the labor. So the next option is swapping out the two pistons with the trans still in the vehicle. I've done this& while it's a knuckle busting job with a great amount of profanity, it's definitely possible& there are some how to videos on YouTube to show you what to do. Now you won't be able to put new bushings in this way, but you can at least replace the piston with a newer plastic/nylon sleeved piston which the company claims they'll help. I didn't do that part. I just pulled the pistons to clean them& check my bushings for wear. Lastly, you can rebuild the trans, buy a rebuilt/reman trans, or a new one. This obviously is the most costly option ranging from 1500 (if you have friends) and probably up to 3or 4k depending on what other work is also needed. The 5r55s trans is known to have a few issues. So make sure you do your research on rebuilt or reman trans and find a good transmission shop thats most likely well experienced in the 5r55s. They've came in many vehicles over the last 15 years so many of techs have played with them. Hope that helps.

Great read appreciate info !!