I have an 04 Navigator, I just purchased it friday. It has 63000 miles on it. Of course I did not discover the issue until driving home. The issue is when running at any speed over 35 mph and a load is put on the tranny, it begins to slip in and out of gear, especially noticible going up hills.
Under accelleration from stop, no issues, shifts thru all gears just fine. Also i can accellerate up the hill and it is fine as well. It only occurrs while driving at constant speed, typically in the 1k-2k rpm range.
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I added a slick 50 high mileage tranny txt, against the manuals reccomendation, and it seemed to help the problem, but it has not gone completely away.
I plan to get a transmission fluid flush done at valvoline in the next few days.
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I have searched online w/o much luck, any advice would be appreciated.
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second issue:

back up sensors seem to not be working correctly, I tried to remove them one at a time, but they seem to be placed in the most difficult spot to reach. Is there any way to pull them out w/o taking off the bumper cover?
Or a way to determine which one may be bad?

Thanks again

 

Might sound odd, but are you sure it's the transmission slipping and not a miss / studder from the engine?

Have the trans flushed & see if it helps solve the issue. I have seen something about the torque-lock out crapping out and doing this, but I know from experience also that if there's a busted injector, it'll studder and feel like it's slipping (for your sake, I hope this is the issue if a trans flush doesn't fix the problem)