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Dear Forumer,
Nice to have a new member here again! 245s are nicecars. Practical and fun. Diesels are very good for towing and camping etc fun. Share some pictures of the rare survivor.

The m46 transmission overdive button issue is very familiar for those with the standard tranny. I would place my bets on some bad wiring in or around the shifter. Do a gentle inspection of the area and try to see if you can figure out where the wires chafed and may be old or worn. Sounds like an easy fix. Stay in touch how it goes.
The button itself can get `tired` but I think it is the two wires that run into the shifter body.. track those wires back towards the relay area and try to see if there is continuity.

It could also be a faulty 4th gear switch on the transmission, or badly worn wiring that leads to that area through the chassis, out and into the tranny...but let`s wait with that. I mentioned it only because my car had all of those issues, relatively often, repeatedly, and intermittently combined (also on more than one vehicle). Start with the most obvious and track the wires first.
When the 5th gear `jumps out`, try this: hold the shifter in 4th position and with the button engaged into 5th gear and while doing this, try pushing the shifter a little bit towards the ground AND towards the right side of the car. This may help the 5th stay on. When issues developed in my wiring, I noticed that larger bumps or road imperfections would turn my 5th off ... that`s how I knew my wiring was old.... when the 5th gear popped out I often would drive the car holding the shifter and pushing to the right to keep it in 5th. See if that works. That would indicate that nothing major is wrong with your overdrive internally.
I would diagnose it in this order:
1. see if holding it to the right keeps it in 5th
2. track wires, maybe remove the button and connecting the wires manually to see if the section withIN the shifter was the reason
3. go deeper if nothing helps and trace all wires that lead towards the tranny
4. approach the 5th gear switch and maybe just maybe install a new one if nothing else helps. (it isnt the button you press! it is maybe called 4th gear switch, actually, and it is located on the tranny)
But again, wires in the area love to get worn and I bet they are worn by now. I am amost sure it will be a wiring issue.
Check to see if the little button works better in keeping the car in 5th when the shifter top is removed and its wires hang outside of the shifter and the button connected tightly.


The higher pitch sound may be related to a slightly loose alternator belt maybe, especially if the weather is wet where you are experiencing this problem. The power steering belt can also do this, both happened to me earlier, and multiple times. Another easy fix. I would start at the power steering belt and tighten it. Quick and easy..then see if it goes away. Tightening the alternator bet is trickier but if you wanna do it safely and properly, just use the search function of the forum. Multiple threads detail the process.
Or, maybe you can describe further what you mean by `engine hot` and also by meaning `high pitch sound`? What kind of a sound it is and is it always related to RPM, etc. or is it similar to when a turbo whistles or when an exhaust pressure leaks away somewhere? We will have a much better idea about how to help, if you describe even better what *exactly* you hear!

About the speed gauge,
Hopefully it is not a faulty aging connection somewhere in the cluster. The speedo info comes from the alternator, I think, if i recall correctly about the diesels.... maybe check (or clean) the connection there as well.