Volvo: 1985 240 DL overdrive delay, volvo 240, manual transmission

Question
I own a 1985 Volvo 240 series DL with manual transmission.  The car is experiencing a delayed engaging of the overdrive.  When the overdrive button is pressed, I can hear the relay click and sometimes it engages immediately, however, more recently it has been engaging up to 15 minutes after the overdrive button is pushed.  I noticed that if I am travleing down the freeway in 4th gear and press the button, I will have to reach a high rpm before the overdrive will engage, sometimes it engages sooner than later.  It is in no way consistent with the time it takes for the overdrive to engage...any help would be greatly appreciated!

Answer
Andrew,
The pump inside the overdrive is what creates the pressure to engages the OD. If the fluid level is too low it take a long time to make the needed pressure. It could also be the OD is getting weak. Have the fluid cheked and changed, see if that will help. While testing and changing the fluid see that the wires going to the OD solenoid are in good condition.