Volvo Repair: Car downshifts to second--and stays there, dashboard panel, gearshift knob

Question
I have a 1989 740 GL.  There is a little button on the left side of the gear shift knob for the auto transmission--when that button is depressed, the car downshifts car to second gear, presumably for use when descending long hills. My problem: that downshift engages at unpredictable times, w/o me depressing the button on the gearshift knob.  The little amber downward-pointing arrow appears on my dashboard panel, signalling that the down-shift feature has been engaged.  Is the transmission in trouble, a relay failing--or what?  Thanks.

Answer
Jerry,  remove the ashtray assembly and remove the panel that covers the cig lighter, remove the 2 screws and remove that panel.  On the right you will see a tall white relay.  Thats the overdrive relay.  Replace that relay and the problem is solved.  Roger