Volvo Repair: 1991 Volvo 740 Overdrive, fuse panel, volvo 740

Question
Regis,

I recently purchased a 1991 740 GL (no turbo)with 140K miles. It runs quite well for the most part, but I am having a problem with the overdrive.
Sometimes the car disengages the overdrive when I do not push the selector on the gear shift, disabling 4th gear. This is especially problematic when I need to get on the interstate as I cannot go over about 45 mph without running the engine too high.
The problem seems more commonplace when there is water involved (if it is raining outside or if I drive through a puddle).
The car usually kicks back into overdrive after a few minutes or if I turn the car off and come back later.
Is it possible to disable this feature so the car is permanently in overdrive (I live in the midwest, so I do not deal with much change in elevation or tow anything behind the car). If not,do you think this is something I can try to fix myself? I have done a little work on the car, but nothing too involved.
Thanks,
Adam  

Answer
you could wire the overdrive solenoid with some type of ignition power source there by bypassing the relay and having overdrive all the time providing the problem is before the solenoid.if it is in the solenoid or in the transmission  then this will not help.the most common problem is in the relay the relay is easy to replace.

the overdrive relay is located on the fuse panel behind the ashtray.remove the ashtray and then remove trim around lighter socket then you will see 2 screws remove the screws and the storage area will come out the relay is white and about 1x1x3 inch.it will be towards the passenger side of the panel.