Next job, a 2004 Express Van with a slipping tranny, only slips slightly in 2nd and 3rd under load. This vehicle is a heavily loaded work truck with well over 100,000 miles. My suspicions is there is really no root cause of failure. Most likely a hard work filled life caused slow degregation of clutches and then lack of fluid changes with clutch filled fluid sealed its fate. The fluid is black and nasty. When tranny was torn apart just general clutch wear was found. I completely overhauled the unit with all new seals, clutch plates and flushed converter and cooler good.

Tranny out and apart on bench

Inside of gear transmission oil pump, nasty black clutch filled fluid!

Gutless transmission!

Guts on bench!