Not changing into top gear with automatic transmission

[teach]

Hi
I have an 1989 Isuzu Bighorn 2.8 Turbo Automatic.
Since I got it 7 years ago, every now and then, a little light "O/D off" flashes. SOmetimes for a few minutes and sometimes for a day or two. My mechanic told me to ignore it.
In the last couple of days, I have noticed the truck will not change into top gear on the motorway. It gets to a hundy okay, but the RPMs are up at 30.
When I pressed the power or Normal drive buttons. Nothing happened.
I checked the Transmission and it was a little low, so I put in a litre. The O/D off light is now on. and when I press the Power drive button, Power drive the light comes on. It still does not seem to be changing onto top gear.
Is this an indication of maybe a sensor failure somewhere or is at about to blow up? From reading this forum and others, I suspect that there may be a temperature sensor not working correctly or am I barking up the wrong tree.

Should I do a complete change of transmission oil?

Any suggestions would be greatly appreciated. I have a mechanically mined mate who is quite capable of changing sensors etc, but does not want to stuff around if he is not sure what is wrong.

[mrsuzu]

Did you get it fixed?

[geeves]

the original poster last logged in in 2012

[mrsuzu]

Thanks Geeves, i'm new here and didn't realise that.

[geeves]

Often threads never see a conclusion. Every just thinks a lack of responce means it must be fixed.
In this case I had a look at the original poster and see hes long gone so must be ok. I dont think you can see the log in status of other members so you couldnt know but each post is dated

[mrsuzu]

Thanks for the Info Geeves.