Hi~
I had the exact same problem on my 1993 940. I removed the ashtray, and all the relays are right behind it.
The overdrive relay is on the right hand side. It is white. I pulled mine out, cleaned the terminals, and re-installed. I no longer have the problem, but just to be safe, I ordered another on ebay.. it was about 25.00..
There is one other relay on there right now, so check it out! The part number is 3523804
Here is the picture from the item I bought..
GOOD LUCK!