why won't it downshift
 

Hi, i am new to these forums and hope you guys can help me out. I have a 99' F-150, with a 3.55 rear end and a 5.4 engine. A couple of weeks ago while towing a trailer up a hill I noticed that it did not want to downshift like normal. Now it will downshift but it seems to bog down before it does. It used to be shifting all the time and would stay right in the 'meat' of the powerband, but now it seems to harldy ever shift. It has 40,000 miles on it and I want to change the fluid soon. Will this help? What could be wrong? Thanks for any help.

You're overdue for a fluid change, especially if you've been towing. It's hard to say how hot the fluid has gotten if you don't have a trans temp gauge.

Do you have the 4R100 (E-code) or the 4R70W (U-Code) transmission???

it is a 'U' code

It does sound to me like the fluid simply needs to be replaced.

These are thorough instructions for changing the fluid in the 4R70W transmission and torque converter, if you're not familiar with the procedure.

I'd also suggest tapping the pan for a drain plug so it is a simple task next time.

thanks for the advise ajrango~
well i finally changed my fluid. i dropped the pan, drained the converter, changed the filter, and cleaned the magnet and pan all out. the pan still had the dipstick plug in it so i know that this is the first time the fluid has been changed. is it just me or does any one else have to jack the tranny up to get the pan to drop past the crossmember? seems like poor engineering to me. anyways, put about 100 miles on and it does seem to shift better but not sure if it is 100% yet.

Your tires appear larger then stock. Did you notice this after installing the larger tires?? My truck had some problems in the shift pattern until I corrected the speedo which effects the shifting.
I agree the first place to start is with a fluid change.

I bought it 1 year ago with the larger tires already on it and it has always shifted fine, until recently. Now that I have changed the fluid it seems to be at about 90% of how it used to be. Also I have tried every dealership in our area to get my speedo reprogramed, but they all refuse to, saying that it is "illegal" for them to do so.