Transmission Failure......coolant sensor????

i am having this message come up on my Display
"Coolant sensor Bad" also the transmission is having a wierd old time, it is very sluggish in 1st gear, sometimes will not pull away in 1st, only in second.
Sometimes it will not go into third gear, it also sometimes will rev for no reason, the overdrive light kept coming on so we have disconnected that, but still it is all happening.
The oil has been checked by the mechanic he says that it is OK, yesterday I had the tyres changed as they were low in tread, afterwards it revved once, and is reluctant to change into third gear but for the 12 km or so home it was OK, the coolant sensor problem is still coming up in the display, can ou help me at all, we have sold the car and the guy is coming to pick it up on Monday, cannot belive this has happened now.
Is it anything to do with the coolant sensor
Hope you can help we are desperate.

Thanks Rob

Also anyone know where the coolant sensor.

what year, milage???

hi Thanks for getting back to me, it is a 1993 and has done 196,000 kms

it could be a faulty speed sensor, this would make the trans go nuts like that, does it also seem to lack power compared to the rpm its turning, what size engine do you have?

The engine size is 3960cc, it does seem to lack power sometimes yes, where would I find the speed sensor, is it easy to find, could I get it myself from a automart, and could I fir it myself, I am a girl and not great with things but hate paying the mechanics, why did I buy a compecated car????
Thanks for your help, I really appreciate it.
It is not the coolant sensor then, is that a problem???

i dont' think the coolant temp sensor would have such a drastic effect on the trans, doesn't seem to be related to me. The vss, speed sensor is located on the transfer case or the tail stock of the transmission. Easy enough to replace, I think I would start there, if you have no luck, I guess you would have to take it in.

Thank you very much for your help, I will have a look at that and let you know.
Thanks again