Strange 4l60e issue

Hi

I have a 2004 ls swapped Colorado with a rebuilt 4l60e using a t42 TCM and current performance wiring. The issue is as follows.

The truck will drive fine on side streets, shifts completely normal. After cruising on the highway for a few miles (usually 6-7) if you let off the throttle or hit the brakes and reapply throttle the truck will drop out of OD and into 3rd. It will then not shift back into OD until you hit 74mph. Then when you exit the highway it will downshift normally but will wind out the 1-2 and 2-3 shifts, not shifting into 3rd until 40mph and will not go into OD. Then occasionally when you come to a stop it will go back to acting completely normal. Also turning the truck off and back on will stop it.

Neither the shop that built the truck or current performance has been able to figure it out. We tried different tunes and 2 different TCM's with no luck.

Here are two data logs I took. First is on the highway near the end where the command gear changes from 4 to 3. Second is after coming off the highway where it winds out the shifts
Attachment 102341
Attachment 102342

Here is the current tune on the truck
Attachment 102343

Current DTCs ?

Pending DTCs ?

The wiring harness inside the transmission oil pan, and / or solenoids, are often the issue with 4L60/65/70 transmissions.

The LS conversion has been programmed as a 2006 Trailblazer - using an E40 ECM and T42 TCM.

I took a very quick look at the tune - specifically the T42 portion.

Attempts have been made to disable the ISS in the tune - which tells me that you are using an older 4L60 (likely a 1999 to 2005) without an ISS. This setup rarely works 100% correctly - there are typically some tradeoffs and shortcomings.

Take a look at two tables:
- TCM 15000
- TCM 15002

See your 74 MPH 3rd to 4th shift speed ?

The TCM seems to be using the Hot Mode shift speeds - sometimes conversions will do this for whatever reason - hopefully your transmission fluid is not actually too hot.

Copy the TCM 15000 table values to the TCM 15002 table - and go for a test drive.

You may have a few other issues ...

No trouble codes and trans temps are usually 170-180 when it goes into this mode. I will take a look try to copy the values...

Well I can?t believe it but i think you fixed it. I copied the tables and it hasn?t done it the last 3 days. I just did a 30 mile drive on the highway trying to get it to do it and it worked perfectly. Thank you, really appreciate it!

You are very welcome.

I have supported a lot of LSx conversions over the years. From time to time, one will act like yours was - not sure why some conversions do this - but homogenizing the shift speed tables always seems to create a seamless driving experience.

Yeah it was definitely strange, everybody said they have never seen anything like it before. Was gonna bring it to a different shop but definitely glad i posted here first