Friend,
I recently had a trans with the exact same symptom. It functioned normally for a high mileage trans. Suddenly the trans began to slip in all ranges selected. The fluid level was actually high. Fluid was not burnt. Small pieces of yellowed nylon were found in the pan. When the valve body was removed I noticed the nylon oil deflector at the front of the trans was broken and the pieces scattered about in the valve body. The pieces had blocked the function of the primary valve. The part was cheap but the labor a little intensive.

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