Finally figured out why my A500 OD transmission was going into overdrive all the time in all ranges beyond about 10 mph and burned up the OD clutch so quickly and badly. I had modified the tranny to use a non-lockup converter and pump and added other goodies for durability. But the OD kept coming on uncommanded. Here is why... The OD piston retainer (a pot metal housing you see when you remove the overdrive unit from the transmission case) has a tiny pressed-in orfice and filter that popped out of the retainer on the tranny case side. This orfice is just 'staked' to retain it in the case-side of the piston retainer housing and mine was not staked too good from the factory. Without the orifice, the OD piston gets too much pressure from the governor oil circuit and engages the clutch prematurely. Easy fix, but you have to completely gut the transmission to remove the OD piston retainer. There are 6 external bolts plus a snap ring inside the case to remove to get the OD retainer free of the case. Might as well also inspect the sprag clutch while you are digging that far down. You can buy the orfice from the dealer (and also the OD and retainer gasgets) and re-insert it and stake it. This fix was overlooked by a "professional shop" I took the transmission to first...took me about 2 hours to do this myself...Lesson learned again..Always do things yourself if you want it right.
Mr. Bill