I have occassionally had the overdrive go out but a relay replacement cured the problem. This time I replaced the relay but the car still won't shift into overdrive.

Any suggestions on what else I can try?

Hi LDinNC, did you ever get your problem solved? I've got a '90 740 with a similar problem. I've found out that there is an overdrive sensor on the tranny that may have malfunctioned. Now trying to figure out how to change it without taking out the tranny.

t3dennis,
Did you look at the link I gave you for your "ecu" question? You didn't give what type of tranny, but that link covers a wealth of info. Scroll down to trannys.