Hi, In my manual, it has a picture of a circled key?? If it is this one, it refers to the automatic gearbox oil temperature...and will come on when an excessive temp is reached, maybe when towing or being worked hard etc... the manual says put the lever into 3- 2 or 1 gears to extiguish and or select low four wheel drive if it continues,stop, and let gearbox cool..if it is happening in normal operation (with no load) possible slippage in top gear. presume you have a four speed zf auto box?
have an techinician to have a look, may only be lack of fluid. At worst i will be a "band" worn out, these can be adjusted, but a fairly technical job, and a recon box maybe your only option, I have a 1986 auto efi your auo box fluid dipstick is at the rear of engine bay on the left, as you look at it, with a red "button Top"
Hope this helps Gavin