LT4 8L90 issue
 

My friend Kelly and I both have LT4 8l90 crate packages. His is in a 72 Blazer, my in a 66 C10. Every since he started his he has had a check engine light. Codes are to do with the transmission solenoids. 7 codes in total. The Blazer runs and drives without any issues. He has had techs look at it and delete the codes but they always return. I bought HPtuners for my setup to change the trans shift points. Worked very well. He has talked to GM phone tech and they think it is software.
Kelly called me and I looked at the diagnostics and tried to clear with no success. I downloaded his tune and saved then uploaded my original TCU program to his. No change. He drove for a couple days as these codes are permanent and take a few cycles to clear. I just uploaded my ecu and tcu program to his successfully then immediately checked the diagnostics. The codes remain. Our next plan will to be replace his TCU, but if anyone has any ideas let me know. Attachment 2419612

Re: LT4 8L90 issue
 

Looks like this was the wrong place to ask this. It is fixed.

Re: LT4 8L90 issue
 

I've heard of these communication issue/s on other forums but no firsthand experience thus keeping quiet. What was the fix?

Re: LT4 8L90 issue
 

No communication issue. I have HPtuners. In the end I went into the diagnostic list and unchecked the 7 codes listed at issue, loaded, end of story.

Re: LT4 8L90 issue
 

Did you have HP tuners unlock the TCM?
I sent my ECU and TCM off to Speartech when I had the harness made. They told me that If I did not unlock the TCM that it would throw codes so I sent that as well, and they had HPT unlock it.
I'm using a factory ecm and tcu with a pulled LT4/10L90 combo from a wrecked 2018 ZL1

Re: LT4 8L90 issue
 

So this customers LT4/8L90 is a crate pkg. To my knowledge it is unlocked from factory. My LT4/8L90 is the same but did not have this issue.

Re: LT4 8L90 issue
 

Ok. Makes sense. He never got it sorted?

Re: LT4 8L90 issue
 

I did sort it. I stated this in my post 04-18-2025