I did a number of improvements on my 2001 S-Type with the 5R55N transmission.

When I bought it it took up to 4 seconds to engage forward, and reverse engaged rather forcefully. Also, I had to move the shifter to 4 to get it to engage - it would subsequently operate with all 5 gears when I moved the shifter back to 5.

The transmission now shifts perfectly - I did the following fixes and upgrades myself:

Changed pan filter and added a Magnefine in-line transmission filter (recommended and also supplied by Ford) in the cooling loop.

Changed transmission fluid a few times to get the old fluid out - each pan drop only removes about 40% of the fluid.

Had the dealership reflash the Powertrain Control Module (PCM - aka computer) with the latest software - which fixes transmission shift scheduling (and also some erroneous flagging of the engine's variable valve timing.) See TSBs 307-03 / 307-07

Installed the SK shift kit, cleaned the valve body internals, and replaced the shift solenoid block.

Finally, adjusted the shifter cable to remove the slack that had developed over 80K miles.

It now engages forward and reverse firmly in less than 0.5 seconds, but not abruptly/with a jolt. Total cost (doing it myself, apart from the reflash) was approximately $345 - way cheaper than a rebuild

$100 for the reflash
$20 for the transmission filter
$15 for the Magnefine in-line filter
$40 for 20 quarts of Mercon V fluid (got Valvoline and Castrol for cheap)
$100 for the new solenoid block
$70 for the shift kit

Of course, this won't help if the 5R55N has already suffered internal damage, but fixing the mechanical/hydraulic/electronic control system early on will hopefully prevent (delay...) the ultimate need to a tear-down/rebuild.

 

Very good value and outcome!

 

Forgot to mention that I also added a number of shift scheduling springs in the valve body - one of those came with the TransGo SK 5R55W/S/N shift kit - the others I ordered separately - Sonax manufactures those springs that are known to break or weaken in the 5R55N transmission. Since I already had the valvebody out for the installation of the shift kit, replacing those 5 additional springs took about 10 minutes.

 

Awesome and good to know!!

 

I did all of those things on my first Jag, with the exception of the shift kit. It did make quite a difference although mine didn't start out nearly as bad as yours.

Curious what fluid did youfinally leave in there? I think I used Valvoline full synthetic Mercon V(which I don't think is produced any longer).

 

I also finished it off with the synthetic Valvoline Mercon V - I am typically not the biggest Valvoline user, but got these for $2 a quart a few years ago.

 

Is there a part number for the fluid that we should be using? I am hoping to have mine flushed soon.