2000 Silverado Z71 5.3 1500 Sudden loss of 3/4 gears with no previous slippage. This is a used transmission with 70,000 Mi. with a new pump and torque converter. All the clutches appeared to be in good shape less than 700 Mi. ago. I find it hard to believe that the clutches could be worn out. It has been driven normaly with out being loaded or towing a trailer I repaired a tow/haul switch wire that was broken and had been grounding on the steering column. The question is what are the other possibilities?

 

My Trailblazer did that and it was a bad valve body. Could also be a shift solenoid.