Volvo: Overdrive occassionally does not engage, electrical relay, 1992 volvo 240

Question
Jason,
I recently (within the last month)purchased a 1992 Volvo, 240 series with 180K.  This car has no hardcopy service records.  It has automatic transmission that seemed to shift OK before purchase.  But within a day or so, I notices it did not want to shift into overdrive consistently.  Sometimes an rpm change would do it but for the most part it shifted when it was ready.  After about a week or two, I drove it about one hour across town without a shift problem but after a five hour visit, it did not want to shift to overdrive until we had driven about thirty minutes back toward home.  From that point it shifted OK for two weeks or so.  This past weekend we went to the beach and the Volvo sat at home for three days.  It didn't want to shift on Monday morning but did much better on Tuesday.  So what are the likely issues:
is it transmission sludge or an electrical relay or slenoid?

thanks,
Rod

Answer
This is a common problem and most likely the electrical relay that is behind the glove compartment and responsible for activating the solonoid on the overdrive unit.  If when the overdrive kicks in finally, if it kicks out on its own at a later point, your overdrive unit might be going bad, but if when it shifts in it stays there, then the relay is probably the culprit.