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4L80E Overdrive Issues HELP!!!

MTPKTS

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Knoxville, TN
Thread starter #1
Hello All!!! New time subscriber but a long time reader. I have been all over this site trying to find some direction with my transmission problem but I feel like I am striking out. Any Help would be appreciated.
1997 GMC 2500 4 WD 6.5 TD 4L80E.
The truck drives fine when cold and shifts through all gears as it should Once the vehicle warms up (15-20) and under normal driving all is fine until I attempt to go up a slight grade or stomp on the accelerator and the truck down shifts from OD to 3, usually around 50 mph. When it down shifts is slips and revs real high, like I pushed the clutch in. If I manual shift the trans into 3 it drives without missing a beat. I have no warning lights on and this started out of the blue with no gradual signs. Any help with this is greatly appreciated.
 

WarWagon

Well it hits on 7 of 8...
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#8
OBD1 1995 and older uses a paperclip. OBDII requires a scanner or other trick I don't recall offhand.
 

THEFERMANATOR

FRANKENBURBAN
Staff member
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Staff #9
You have to have a code reader, and you need to pull the codes. Sounds to me like your losing fluid pressure. When the trans is cold, the fluid is thick, but when hot it thins. Thinner fluid is harder to build pressure, and if theres a leak, it will show when the fluid is thin.

Have you serviced this transmission or had it serviced since 06? If so, do you know what fluid was used? If dexron 6 was used, it has taken the seals out in several transmissions as it is a very dry low viscosity fluid. GM superceded dexron 3 to dexron 6 because they changed on the assembly line when dex 3's patent ran out. Dex vi has killed many transmissions, but it normally takes 6 months to ayear to happen.
 
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