How much to fix 1990 Volvo 740GL that won't go into overdrive?

IPD USA offers a few do-it-yourself things to save you some $$. The actual solenoid is a bit costly ($150), but a few things to try may be the OD relay, OD switch, or wiring. if you don't want to ever fiddle with it again, they also offer an OD solenoid bypass plate ($40). you wouldn't be able to shift out of OD, but how often do you really do that.

Try FCP Groton