4l80e not shifting properly

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wkobe88822

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Ok i swapped 4l80e into my truck from a truck with a bad frame that sat for about 3 years and was fine prior to sitting. changed the filter and fluid dropping the pan not a complete flush and fill. when i get on it to downshift sometimes it doesnt want to shift back to OD. is there anything i can do for this short of a rebuild? any additives? 98 k2500
 

jasons99hoe

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fluid level is super important. im not saying yours isnt right but i see alot of people do it wrong. i have seen on ls1 tech that the 98 pcm wont run the 4l80e without a stand alone controller. either that or you may need to have the pcm flashed so it controls it because it still thinks you have a 4l60e. look into those areas.
 

df2x4

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i have seen on ls1 tech that the 98 pcm wont run the 4l80e without a stand alone controller.

It will, just needs to be tuned accordingly.

I'm no help other than that, sorry! Interested in what you find out, I'm trying my best to mentally prepare for any kind of potential issues once I finish with my 4L60E to 4L80E swap.
 

wkobe88822

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fluid level is super important. im not saying yours isnt right but i see alot of people do it wrong. i have seen on ls1 tech that the 98 pcm wont run the 4l80e without a stand alone controller. either that or you may need to have the pcm flashed so it controls it because it still thinks you have a 4l60e. look into those areas.
noboth trucks had 4l80e i will assume that pcm isnt the problem the donor was a 96 into a 98 but everything was a direct fit just the cooler line came in and out in different spots
 

magimerlin

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Check the plug connection as well.. actually unplug it and look at the transmission side of the plug. Is is wet? If so it could be messing with the shifting. The plug should have no fluid in it at all.. I had it happen on my 94 c3500. Had to get a new transmission side harness as mine was leaking through the pins..

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jasons99hoe

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noboth trucks had 4l80e i will assume that pcm isnt the problem the donor was a 96 into a 98 but everything was a direct fit just the cooler line came in and out in different spots
well if it was a direct fit for everything you may have issue with the new trans from sitting for so long? just a guess. you will figure it out
 

wkobe88822

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Check the plug connection as well.. actually unplug it and look at the transmission side of the plug. Is is wet? If so it could be messing with the shifting. The plug should have no fluid in it at all.. I had it happen on my 94 c3500. Had to get a new transmission side harness as mine was leaking through the pins..

Sent from my SAMSUNG-SM-G891A using Tapatalk
how hard is it to change that harness?
 

magimerlin

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how hard is it to change that harness?
Not hard at all... gotta drop the pan.. use a large socket(forget what size) to undo the clips from the harness from outside. Push connector into trans and disconnect at solenoids... replace is just the reverse.

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Supercharged111

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I put a new harness in mine as well since it too was leaking. Is it not going into OD or not locking?
 

wkobe88822

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I put a new harness in mine as well since it too was leaking. Is it not going into OD or not locking?
Not going back in to od after I get on it to force a downshift. But it may have been a fluid level issue as one of you guys suggested. I thought it was good but it was low. Was getting false reading on dipstick from adding fluid . So today I added about 2 quarts. I still need to test drive it though will let you all know after the the test drive
 
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