Hello,

An 85 XJS, will run in 1st and 2nd, no high gear? Maybe a servo?

Thanks,

ncng

 

I would be checking the vacuum line that runs from the RH inlet manifold to the transmission, if this is leaking or come adrift it will give you this sympton.

 

Could be a couple of things...or more, but the easiest to check are the vacuum modulator and it's vacuum line that comes from the right side manifold, rear. The modulator is on the left side of the trans. I had a similar problem and tried to swap it with a common GM one. I could never get it to shift as smoothly as the Jag on and had to pay the piper for a new one...around $60 or so.
The other is the governor. It is located on the top right side of the trans and accessed through a removable plate in the floor under the carpet. It is near the dash end of the tunnel. You can pull up the carpet to get to it. The governor on the GM TH400 has a small plastic gear that is known to strip. Not very hard to replace and a cheap part. I had a couple extras and have them on ebay. It is centrifugal and sometimes will stick. A lot of info and pics on the Internet about these two problems since the trans is a GM TH400 with a special cast bell housing to fit the V12.