suspect an internal tranny failure
many possibilities : Defective input housing assembly. 3-4 apply ring stuck in applied position.
Forward clutch not releasing.
Turbine shaft seals missing or damaged. Manual valve link disconnected.
Defective valve body assembly. 2-3 shift valve stuck.
Manual linkage out of adjustment.
Spacer plate and gaskets incorrect, damaged or not positioned properly.
Defective reverse input clutch assembly.
Clutch plate worn.
Housing and drum assembly cracked at weld.
Clutch plate or return spring assembly retaining ring out of groove.
Piston deformed or dished.
Seals damaged or missing.
Retainer and ball assembly not sealing.
Restricted apply passage.
Defective low-reverse clutch.
Clutch plates worn or retaining ring not positioned properly.
Porosity in piston. Seals damaged. Return spring assembly retaining ring not positioned properly. Case porosity. Case cover plate not tightened properly or gasket missing or damaged.