4l60e won't shift out of 1st.
Trying to help out a friend with a transmission issue. 4l60e won't shift out of 1st until no back of gas, then it shifts fine through all the gears. If you keep on it, it won't shift until around 4 grand, then slams into the next gear. No codes on the scanner, voltage on tips is good, has a new tps converter, neutral safety switch, and it has done this on the old tranny, that we thought was shot, and a brand new rebuilt tranny (2 tranny shops claimed the old one was toast) It will also not manually shift into 2nd. It is for a 98 gmc jimmy, not an lt1, but I was hoping you guys could help. Fluid was flushed and replaced with new trans, including cleaning out of the trans cooler.
any suggestions would be greatly appreciated.