Can't help you much but....

Try doing a search of the message board here.
I have seen questions before about the E40D
trans AND on another site I have seen numerouse TSB's involing the same transmission.

hope it helps a little

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96 F150 XL,special
Air/PS/PB/AM-FM-Cass/
Light Package/300 I6/
5 SP Man/...
Nothing fancy just my little baby

 

I just got this truck a month ago..it is a 96 F-150 XL 5.0l south texas edition. I notice a slip in the trans between 40 and 45....getting annoying, can anyone tell me what it might be???

 

You may just be experiencing the torque converter locking/unlocking. I have a '95 and it will "search" quite a bit in OD. I think this is somewhat based on rear end ratios, the higher ratio will tend to unlock sooner. Just a thought.
Try locking out OD and see if you still get the slip. My truck will unlock the converter with anything other than very slight gas pedal increases. Since adding a couple of mods, it stays locked more often due to better respone at part throttle.

 

I tried it with Overdrive off and it still does it to some point. Not as bad but it is still there. I can honestly say i don't know what it is.

 

If turning off the OD helped, you may still be getting converter lock/unlock.
Do you have a tall rear end ratio and/or large tires? Does it do it under very light throttle applications, or only heavier. My converter will unlock in 3rd at 40-45 if I go from very slight to say 1/4 or maybe 1/3 gas pedal travel. With the tall (3.31) rear end the converter works quite a bit.


If not "standard" tranny function, you might want to consider tranny service. I've heard in some cases that a tranny drain and fill with a new screen will clear up minor problems.

 

I am not sure what rear end i have, but it only does it when it is slight pressure to the gas pedal...and when it goes up hill it does it even more.