1990 Caprice 305 140,000 miles

The th700R4 transmission, has a history of shifting into high gear(overdrive especially)prematurely, this can be caused by the premature opening of shift valves 1to3, and that can be caused by weakened springs, allowing the fluid to rush past the valve and trigger the next shift stage, old fluid etc, but could also be caused by a detent cable which is out of adjustment. The detent cable is located on the driver's side, and runs from the throttle linkage down to the detent lever on the side of the transmission, this mechanism is responsible for holding the transmission in a perticular gear, based on throttle position, it can be adjusted by removing a small insert at the cable stop and then pushing the cable housing through the stop for a couple of notches, and then re-inserting the small plastic insert, slide the cable housing toward the transmission to delay the shift, hpoe this helps.