How incredibly exotic! I read the entire board, get here, and someone finally has the exact problem I've got, and they've made everybody so upset, shucks!

What I've done with my "Won't go into overdrive for first 15 minutes" 1990 765 automatic with 318,000 miles on it:
1. resoldered od relay. No effect. Resoldered again, just to be sure (removed solder and replaced).
2. dumped ATF and replaced. Nicer shift, but still no 4th gear.
3. thought, "Maybe it's just a little sticky," so when it started working, pushed and repushed the OD cancel switch on the shifter repeatedly. (Also tried this when it wasn't working to see if I could encourage it to work.) No change.

I see the part about looking at the solenoid on the right side of the tranny. Gonna try that next. To whomever mentioned it, thanks!

FWIW, Sears sells two different batteries for this type of car, both the same size. I bought the more expensive one (with more cranking amps, etc.)

BTW, I've applied for an account. My email is Louis@Hogan.com.