Apparently it is your solenoid or solenoid wiring. Forget about the electronics. I chased this type of problem for a year and exchanged all with new parts and wiring until I finally just routed a channel in my old solenoid and put it back in.
See my post earlier regarding the AW71.
You don't need to spend $40 plus for that plate when you can do it yourself for $0
Go for it unless you really want that third gear for constant driving (towing)
see my post for AW71 OD fix dated January 9th of this year in the 900 thread.
Forget about the replies the replies that are there, they are about me calling it a 4th gear instead of overdrive it is.
When needed your kick down will still give you third for passing.
Do it yourself and save money.
Good luck.
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EJO now a 1994 940T 192K; ex '65 123S; ex '75 245; and ex '81 242GT; also Chrysler 2002 T&C and 2006 MINI Cooper