My 1997 C2500 GMC with a 5.7 and 4l80E transmission starts in 2nd and intermittently has overdrive. It shifts from 2nd to 3rd fine and never even tries to slip. There was no metallic "swirl" in the fluid, not burned, and no buildup of scum or paste in the pan or fuzzy stuff on the magnet.
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this sounds like the pressure valve is going bad on third gear. cheap fix try Lucus transmission additive to increase the pressure. To do it right go to a good transmission shop an have it serviced and flushed.
Sounds like your transmission is in limp mode. This is a fault mode in which the
transmission either loses electrical power or the PCM deactivates all of
the electronics. The transmission will default to max line pressure.
The driver will have Reverse and 3rd, with manual 2nd available by
putting the gearshift in 2 or 1 (doesn't matter). 1st, 4th, and TCC
lockup will not be available. It will not hurt the car to drive it like
this for short periods, but use manual 2nd to get the car moving, and be
aware that the trans will generate more heat than normal while
operating in limp-home mode due to increased torque converter slippage
during 2nd gear starts and no lockup in 3rd gear.
The other from what you described, trans shifts 2nd to 3rd by itself in D or OD and locks the converter : ShiftA solenoid failed. Easy fix in the pan. Or a wiring problem from PCM to trans.. or PCM.
Thank you for using fixya and have your Safari scanned for the DTC codes that would indicate what issues your transmission is having to put you in to limp mode.
Bad transmission, that's not a good answer. Bring it to the shop [so we can guess around], not a good answer either.
A good mecahanic will respond with something about the solenoids.
I have repaired the 4L80E on my tow truck, I have to do that almost every year, the last time it ran perfectly but when I push the pedal to the metal, it shifted from 3rd gear to 2nd and stays on 2nd gear until I turn the ignition off and start again.
The transmission was totally rebuilded so the answer "Time to rebuilt the transmission" is not acceptable.
I took it back to the shop and the mechanic replaced both solenoids A and B, that was the only thing that hasnt been replaced before, it was just a guess but works
So, I am not a mechanic but my sugestion is to replace the two speed sensors and solenoids before you take the tranny out of the car and be carefull who are you taking the transmission to because guessing is not always the best course of action
The two speed sensors are easy to replace, the solenoids are inside of the valve body but you may be able to replace them without removing the transmission from the truck