I have a 1997 Chevy Tiltmaster with Isuzu motor (4 cylinder diesel) automatic. (Old Lowes delivery truck). The truck will shift through all four gears, but I have to have rpm's maxed, then let off pedal to shift. The tranny won't shift up normally. Took it to mechanic and did voltage checks on sensors that were normal, but found voltage between trans. computer and transmission to be wrong. Suggested a new computer, bought used computer for $400, still not shifting correctly. Also, changed sensor near transmission.

The transmission downshifts, but can not accelerate through gears as stated above

Mechanic contacted Chevy dealer, but they were not much help. Any ideas on what could be wrong?