LS Swap , Having shifting problems

Aug 4, 2013
33
0
0
Alabama
#1
So last October i bought a perfectly running and shifting 01 2500HD out of Kentucky to swap the 6.0 and 4l80e out of and put into my 2wd 97 model which was originally a 350 Vortec with a 4l60e. I finished the build about 2 months ago and it runs great , i didn't feel like i had the Know how to rework the harness so i just bought a LS Swap harness from Street & Performance over in Arkansas and used for it. I never did wire up the speedo because i was so excited just to drive the truck lol Everything works and runs great apart from the fact that it has a shifting and speed problem. When you start off just under normal accel. in DRIVE it shifts into second relatively quickly by like 11 mph and into 3rd by 16mph or so and 4th by 25mph and does not go into lockup ever at highway speeds or cruising. Also when it does shift into second you can feel it ever so slightly shifting back and fourth from 1st back to 2nd , 1st back to 2nd etc. until you increase 2 or 3 mph and then it stops and then eventually will shift into 3rd. Also when you manually shift it by starting off in 1st , once you get up to a certain speed it will automatically override you and try to go into second and when it does this it is really violent and shifts really hard because it starts that back and fourth thing again and it wont stop until you pull it up into 2nd ... but whats weird is you can drive in 2nd and 3rd as long as you want to and it wont override you or do the back and fourth thing in any other gear change. So i get my tuner to tune the my engine to run better for the cam, stall converter etc. and he notices right off of the bat when i crank it up that my Speed sitting still and idling says we were running 25-27 mph flickering between 25 ,26 , and 27 . I revved it up to 3 grand and the speed jumped to 70 something mph , when the rpm's came back down i revved it to 6 grand 1 time and the speed shot all the way to 130 something mph... ... ... ... my tuner said he had seen this before on a guy that did a 6.0 4l80e swap into a 95 model c1500 ; except for the speed being hooked to the rpm ; and it turned out that the company that guy got his harness from ( which wasn't the same as me) had wired his harness for a 4l60e and when he rewired it for an 80e if fixed all the problems. When we drove it to tune it we noticed at WOT it wouldn't shift gears at all unless you let off the gas for a split second and then stabbed it again. After we tuned it and i got back home i crawled under my truck just to check stuff out and noticed tranny fluid leaking out of the ISS sensor , i pulled it out and the O-Ring and all looked fine and i didn't know if it was supposed to have tranny fluid on it so i put a new O-Ring on it and wiped the hole out and wiped the Sensor off and put it back in and it hasn't leaked any more . But as i have read recently online , i heard that when tranny fluid gets on that sensor, that it can get inside and mess it up and cause problems. So do i need to just put a brand new ISS on and see if that helps? ... can anyone tell me if they've had this problem with the shifting and rpm's being hooked to the speed or at least seen it and what fixed it ... i don't know what to do ... ... Any and all advice would help , i am lost on what to do haha
 
Feb 26, 2011
138
1
18
Michigan
#2
Re: LS Swap , Having shifting problems

Sounds like the OSS is more the culprit. Since it drives the shifting and speedo. I had a bad OSS on an Allison (external harness is nearly identical to a 4L80) that was driving fine when pulled. So I had to replace it.

Did it throw any codes?
 
Feb 26, 2011
138
1
18
Michigan
#4
Re: LS Swap , Having shifting problems

I would try the OSS, as was explained to me. They can go bad and still get pulses but actually be broke in the interior and causing improper operation. But yet will not throw a code as the computer only looks at if it is or is not getting feedback from the speed sensors. So maybe to be safe replace both. And try that. Its not a really expensive replace and then you also know they are new
 
Aug 4, 2013
33
0
0
Alabama
#5
Re: LS Swap , Having shifting problems

Ok will do thanks man , Hopefully it will solve this unbeknown problem ... Fingers crossed that itll help . I will update the post to confirm if it fixed the problem or not for others that might be having the same issue
 
Aug 4, 2013
33
0
0
Alabama
#6
Re: LS Swap , Having shifting problems

Update : I changed out the OSS first and that fixed the weird shifting problem but it still wouldn't go into Lockup/Overdrive so i went out and bought another speed sensor and changed out the ISS as well and nothing changed , still shifted fantastically and by itself but still no lockup.I thought at first maybe the 3000 Stall Converter i bought didnt come with a lockup clutch but as i went back and looked at what i bought it states "Unit has lockup Clutch". So not real sure why its not locking up yet but but everything else is A-OK and thank you 454CowGirl for helping me with this delimma. I was so worried id have to pull the harness back off and send it back to get rewired for a 4l60E.