I got 7 jags and my 1996 xj-12 likes it cold . my problem is when its cold it shifts fine and its fast but after about 2 minits it shifts down to first and the tranny light comes on . I went to 3 tranny shops including aamco and no shop has the right computer stuff to plug in to find the problem. one guy said its the trany thermostate senser. im afraid to leave it for them to experiment on . what a ya think. drive it on the cold days only jag.

 

You have a different transmission on the XJ12 like the supercharged X300 or the XJR . It's a GM 4L80E . There was a flaw they corrected in this transmission after a certain production # X identified by a stamp on the transmission .

TSB :

http://www.jagrepair.com/images/TSB/...Engagement.pdf

Manual :

https://www.dropbox.com/s/h7ckb9edafc7cgd/4L80E.pdf

There was something about accuracy servicing at the correct observed temperature with a inferred thermometer available at Harbor Freight and someone adds a few degrees to the temp reading .

 

thank you