2006 Silverado lq4/4l70 set up. Had it built for the torque of a cammed 6.0. Nearly everything upgraded. A few days ago I took it too the local shop. Accelerating from a dead stop you could feel a gear engage and then the engine would bog until shifting up. The mechanic figured it was not commanding 1st gear and starting off in second. The next day we hooked up the snap on scantron and it behaved as normal other than 1-2 shift was extremely soft compared to what it was before due to the aggressive shift kit and tune. Now, however it will start in first gesr no problem BUT it revs to the sky in 1st and randomly shifts to second when it wants too. And the shift is still quite soft. No throttle input seems to affect it. All other gears work as normal. Any ideas or has anybody had similar issues? Thanks in advance

 

any dtc? does it do that at all temp cold ... hot. have you tried shifting manually.

 

cammed 6.0...has it been tuned (i'm assuming it has)
if so...post the tune file here in HPT format and I'll take a look at it.
Torque management, shift points, pressures, etc

Ask the builder what he did for a 1-2 accum setup and what he drilled sep. plate hole sizes too.

Also...do a pressure test. Check line pressure in all gear ranges at idle and at 1800 rpm footbraked. Be safe obviously...don't send your truck through a garage wall trying to brake stall the thing.