E38 with t42 speedo stuck at 158

[truckfan2]

I have a 2010 LS3 out of a 2010 camaro ss with a 15 pin 4L65 trans and a psi conversion harness all nestled into an 05 shortbox 2wd truck. I switched the ecm to an 08 tahoe e38 with an 09 silverado t42 tcm. Truck runs great and the trans is working but it is maxed out on line pressure and the tcc wont lockup due to the speedo in scanner always reading 158. Can someone please look at my tune and see if you can see anything that would fix this?Dave Lewis new ecm w new 09 tcm3 throttle changed to rpm speed changes.hpt

[kevin87turbot]

Just get you a T42 TCM that matches your 08 Tahoe E38 ECM and you'll be all set. Don't go with the 2009-up TCM's.

[truckfan2]

Just get you a T42 TCM that matches your 08 Tahoe E38 ECM and you'll be all set. Don't go with the 2009-up TCM's.

Is that the final answer? The t42 from the tahoe was damaged and the salvage yard only had one from an 09 silverado. No way to reflash it to a different OS or anything?

[blindsquirrel]

I have not found any T42 that won't interchange with any other T42. There's about 20 different service numbers and they all interchange. '08 Tahoe and '09 Silverado both have the same list of compatible numbers.

There was a comm protocol change (in the software, not hardware) in '09, if you have an early ECM you have to use the early TCM OS. Late ECM must use a late TCM OS.

[truckfan2]

When I initially set up the tahoe ecm I just unlocked the vats and such then did the write entire on both which would not have changed anything in the t42. Then I threw in the old camaro cal to get the fueling and spark stuff correct. How do I change the OS of the T42 now? Sorry for the newbie questions as I have not done hardly any mix and match stuff as I usually have everything together.

[blindsquirrel]

Several ways to do it... you can license a file from the repository and write-entire the trans only, you can get the current TCM reflashed by somebody with GM tools using your current '08 Tahoe VIN, or you can buy a used TCM pre-flashed with a stock calibration for your Tahoe VIN (this is probably the cheapest option).

[truckfan2]

Thanks man, luckily I had a file for an 07 silverado that had a compatible OS for the T42 so I think im good to go.