After doing a LS1 swap in my 1964 Buick Skylark, transmission will not shift at full throttle. 1999 Z28 5.7,4L60E with just a little over 40,000 miles. Rear tires are 28" tall with 3:55 gears. I had the wiring harness done and computer flashed. Everything else seems to work just fine. Transmission will shift, overdrive works and tourque converter locks in under normal driving. From dead stop, full throttle and will not shift out of 1st gear untill I let off throttle, hits rev limiter. Full throttle at 70 mph, downshifts to 2nd but will not shift, hits rev limiter. If I let off the throttle, then it will shift.
Any ideas why this is happening?
Computer problem?
Transmission Problem?
Thanks, Billmilner

 

Sounds like a garbage tune if we are to assume the trans is healthy

 

Yes its in the tune. Sounds like wot shift speeds aren't set right. But first you would have to read a scanner and see if the vss output is matched up with actual vehicle speed. Its probably not with all the changes, but you have to get the ecm programmed to match actual vehicle speed so it knows when to shift before changing anything else.

 

A 1999 LS1 will have come with the "411" PCM. Under WOT conditions (95%+ throttle) it will require that BOTH the specified WOT Shift RPM and WOT Shift Speed be reached before it commands the upshift. Many people assume it will shift when the WOT Shift RPM is reached and that is not correct. If the WOT Shift Speed is only a few MPH too high you will hit the rev limiter.
Personally I program the WOT Shift Speed at 20% less than it could be; that way I can tune my WOT shifts using only the WOT Shift RPM value and ignore the MPH value. Therefore, if you tell your tuner to drop the WOT Shift Speed by e.g. 20% that will likely solve the problem. As 2knight said, first make sure the PCM's speed output is close to the actual speed.

 

Yes, ^ THIS, just to reiterate. This is a common mistake made by novice tuners and those unfamiliar this OS and the 4L60e. Ask me how I know.