I have a 1990 XJS convertible. I have 2 issues 1) the center vent has been stuck only opening appr 25% I had the complete unit out of the car and thought I had fixed it. Come to find out that the door in the center will only open max of 25%. Can't sort out the reason.
2) The control section under the center panel has a number of vac switches that seem to port less vacuum that needed to actuate the functions controlled by those switches. There are 4 colored plastic tubes which are plugged into the various switches. I am afraid I have lost the connection order and need a chart of them to get it right.

 

The linkages for the defroster flap and the center vent flap are interconnected. If the defroster vent isn't closed, the center vent can't open all the way. That is likely your issue.

 

This should tell you all you need to know about the vacuum connections.

 

That's a great document. Unfortunately, SKJagTech seems to have gone away.
Do you know if a similar document for the later (as in a 1996 MY) XJS is available somewhere?

 

Nevermind ....
I found one.

 

Hi Pete,
would you mind sharing that document? I am also looking for it for my 96 XJS.
Thanks!

 

Actually, page 8 of the doc that Jagboi64 posted has the updates for 1994 on. Here's the Vac Diagram and the schematic.

 

This rings a bell with me. I had a center flap problem and it was due to this interconnection. It was so many years ago, though, that I can't remember the details. I think I disconnected the center flap, cycled the system in-and-out of defrost mode, and then reconnected the enter flap. Something like that. I do remember being happy that it was an easy fix!

Cheers
DD

 

Check the defroster flap actuator. If it isn't operating, the defroster vent fails open, which means the center vent flap won't open .

 

I'm in the same situation where you are right now. If you are without your center console - i strongly suggest you to check you AC Evap since it was born to be broken.
Heater matrix is also relatively cheap fix if you are there..