Chevrolet Repair Questions? Ask a Mechanic for Answers ASAP
You most likely have a chip of metal or some other debris in the valve body. I'll give you the factory check list for a no 3-4 upshift so you know what could cause this.
Oil Pump Assembly (4)
Valve Body Assembly (60)
2-4 Servo Assembly (13-28)
Input Housing Assembly (621)
Refer to No 2-3 Shift or 2-3 Shift Slips, Rough or Hunting
2-4 Band Assembly (602)
Worn or misassembled
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