Volvo Repair: transmission, volvo 740, poor car

Question
Dear Chris,

I have a 1991 Volvo 740 sedan.  My question is this:  the arrow that is indicating that the car is running in too low of a gear is always on now.  Per the manual, you are supposed to be able to push the button on the side of the gear shifter and it will go off.  It is also apparent from the sound of the engine that, indeed, the poor car is always running in 3rd gear and should be relieved at higher speeds by going into 4th, but it doesn't.  I am a bit concerned that this is causing some undue strain on the transmission.

Now, it didn't used to do this.  It was just a few months ago that we had to pull this really heavy U-haul trailer and in order to do that, the car should be in a lower gear.  However, we are not any longer doing the trailer thing, but I can't get the car to think that way, if you know what I mean.  

Also, about the same time that we were with the trailer,  we were in a snow storm and the car had a really bad time and wouldn't hardly go at all.  At that time, we had to take it in to get  something fixed, but not sure what.  

Concurrent with that, we also had to disconnect the battery in order to park the car in a certain location and now the radio won't work (evidently needs some code entered in to get it working again, which I don't have and don't know how to get).  

So that is the scene with the car, I don't know if the datums above are related, but gave them to you just in case.  

A few months ago, the car just stopped dead in the street and it seems something was wrong with the transmission at that time.  The mechanic had a diffficult time to figure it out and put in a new fuel something or other but that wasn't really the problem according to some other place we took it to.  

And before that, the same thing happened, maybe a year before.   (This is before I owned the car)

Answer
wow, informative, and well written. i am lazy so i wont have sentance structure or even paragraphing (well not properly at least) as to the overdrive button. simple fix is to replace the over drive relay. check the fuse first. if the fuse is blown. the wires might be shorted underneath the car but 8 of 10 times the fault is inside the relay.  As to the radio code. that MUST be retrieved from your local dealer, and depending on which radioi model you have, will determine how to re enter the proper code. As to just stopping, the only 2 small things i can suggest is apossibly an engine speed sensor, or possibly a fuel pump relay.