6L80/8L90 swap, now stuck in 4th gear

[Dark_LTX]

I recently completed an L83/6L80 swap to an L86/8L90 swap in a 2014 Silverado. The donor vehicle was a 2017 Silverado. Only problem I'm having is when I shift into drive, trans stays in 4th gear all the time. Also getting DTC's P0700 and P2534/P2535. I read somewhere that someone with a similar problem used a relay to control ignition wiring with ECM/TCM. Anyone familiar with this or maybe point me in the right direction? E92 & T87A

[Dark_LTX]

Got it figured out. Wired the ignition wires from ECM and TCM to a relay. I had a P0700 and P2534. ECM an TCM were turning on/off at different times.

[yrkz06]

Thanks for following up on the fix

[HoedropperLtx]

Anyway you could get into contact with me? I'm having the same problem after doing the swap

[Dark_LTX]

Do you still need help?

[Nguyen5.3]

Hi there, I'm about to do the same swap for my 2014 sierra. The l86/8l90 came from a 2018 sierra denali with t87a that came with it. Did you have to switch the ecm OS over to 8spd to get it to start the truck? I'm just trying to find out more info before I put the transmission in. Any help would be appreciated. Thanks

[slow5.3]

Hi Dark_LTX i have the same issue going on with my 2014 silverado but originally it had 4.3 LV3 6L80 and swapped out with a complete pull out 2018 5.3/8spd but i have the same issue it wont go out of forth gear and basically same codes that you have. I would really appreciate it if you can help me figure this out and share your solution to this problem. Ive been working on it for three weeks already and i cannot figure it out.