Hi i have my 2001 gmc sierra that i have just swapped the transmission from a 4l60 to a 4l80 and i am facing a problem with the 4 gear i will provide a video it start reving back-and- forth and when i start the car my speedometer mph reads at 25mph on park (car not moving) its not only on the speedometer i see the vcm scanner too does the same reads at 25mph on park and i feel that the 2nd to 3th shift does the same

i have change the throttle position sensor, speed sensor, and tried the ict billit adapter and change it and did the rewiring to the harness still nothing

 

I think you have the input and output speed sensor pluggs swapped . If so when you put it in gear brake aplied goes to 0 ?

 

i have plugged the one that we wire to pin 23 and 22 to the one in the back should i put it in front and try ?

and yes if it is on p the mph reachs 25 and if i put it in d or r its drops to a 0

 

Yes, the plug that is added goes to the front sensor.

 

thanks soo much it worked i have switched the plug as you said and the car is driving good

 

thanks bro i did as you said and it has fixed my problem

 

That wasFranksCustomTransfor the win.

 

Over the years I cannot count how many times I have gotten the tech support call on this and not just for swaps, Somehow it happens alot with even stock set up with install. Which kinda baffles me. When it really is a pain is when people wont just swap them. Particulary with older ALD pre OBD2 systems and they ussually dont have the stuff to see codes or data and dont beleiave me. I have actually argued and said JUST SWAP THEM! Then they finally do it and are like AH LOL .
Its always so fun when someone calls for advice then argues THAT CANT BE IT I KNOW I DID IT RIGHT.
Like I have a guy now , NOT A CUSTOMER called with a 4L60E get got froom someone else. He has 1st and 4th OBD1 and his speedo is of cause he changed from 323 to 373 gears. I explained while yes he would need to correct for the speedo in tune it should still shift all gears and IMO there is an issue with shift sol A 1-2 so change it first then proceed to speedo for which he will either have to send out PCM or get TUNERCAT or something, He has EEHACK so he can see that the shifts are being commanded. 1-2-3-4 . What does he do ? tries to fix speedo first now wont shift at all (probably will but 4th would be way up there) . I told him one thing at a time address solenoid first but many well just wont listen.
But glad OP all fixed up and did listen