Remember, that the defrost vent flap has to be totally closed for the center vent flap to open. The rest state of the defrost flap is open with no vacuum. If it never closes, engine running, with the controls set to anywhere other than defrost, you need to start figuring out why it isn’t closing. Until the defrost vent closes, the center vent cannot open.
Heres the late model parts diagram for the defrost and center vent. The two item #6 actuators are the vacuum actuators for the defrost and center vent.
Here’s the workshop manual instructions to R&R the two actuators.
Both are on the right side of the car. Enjoy laying on your back under the dash!
Jon
Thanks for the diagrams. Now let me see if I can actually find them.
So where do the (vacuum) actuators actually get their vacuum from? In other words, where do those hoses lead to in the engine compartment? 
From the inlet manifold, through a non-return valve
As JimD said, the vacuum comes from the engine. From there it goes to a vacuum reservoir, and from there it goes to four solenoids. One solenoid controls the defroster flap and another the center vent flap, one controls the heater water valve, and one controls the recirculation/fresh air flaps in the two blower assembies. The solenoids are controlled by the Delanair MkIII’s computer.
To trouble shoot the system, it helps to have a hand vacuum pump to actuate whatever actuator is giving issues. If the actuator works when vacuum is applied, you trace back the vacuum line to the solenoid. Then you need to determine if there is a vacuum line issue, a solenoid issue, or if the solenoid is not getting a signal from the computer.
Jon
2 Likes
I have a pump vacuum tester. I have teated the the line coming off the intake manifold an it pulls a vacuum. Ive also examined the check valve leading to the vacuum canister and its orientated to open on a vacuum from the manifold.
On the passenger side there is a valve block with a black line and and off-white line on one side with matching lines on the other side. The “Incoming” black line leads back to the engine compartment and the solenoid that opens the water valve to the heater. I remember there being a line off of the vacuum canister and I believe its the off white line in the valve block.
getting to the other two solenoids is going to challenging with the air bag and dash still in.
I kept thinking the latest XJS, such as 94 -96 models, had the Mark IV. I had the Mark IV training manual in my documents. Guess it was because I had a 2000 XJ8 VDP for a while. Thanks for getting me straight
So i have functional vacuum at the valve block accessible from the passenger foot well and to the actuator in the engine compartment water valve. If i have to access the two actuators from that side the airbag needs to be removed. Still trying to figure out how to do that safely without removing the dash.
I don’t believe that it can be done safely without removing the dash. The cover is bolted in from behind, so there may be a way to get to the bolts holding them in from behind, working in the blind, but I think that the airbag could go off at anytime.
Dash was removed, Passenger airbag removed so I could gain access. Found access to the vacuum valve block by removing the driver side floor vent.
Got delayed working on this cause i had to replace my truck’s radiator, some hoses and the overflow coolant tank. damn plastic radiator top cased some issues.
Quick non-Jag-related a.c. question … My friend in CT has a late model Suzuki Forester … She called me in a panic last night b/c she said on her road trip to MA the day before her a.c. started acting up. 
She said when you first turn it on it will blow cold, but after a few minutes it just blows ambient-temp air. Same thing whenever she repeats the process. It’s obviously not freezing up (i.e. blocked drain), b/c the ambient temps at the time were in the upper 70s only, per her. I wouldn’t think it would be a refrigerant leak, else why would it start blowing cold again for any length of time? Maybe the receiver-dryer is going/has gone bad ? If I can give her a confident answer this time (other than, “geez, I only know about Jags” - my standard answer when I don’t have one, btw 
), it might make points with her, if you know what I mean. 
Have tech check clutch on AC compressor
When our Subaru did same the clutch gap needed adjusting.
See an AC tech that knows Subaru.
oops, meant “Suburu” … thanks WC … 
Back at work on the vacuum system. Looks like its a failed vacuum module for the defroster. but have not confirmed I have vacuum at the hose yet. Have to adapt my vacuum tester input first. I cannot open the vent with the vacuum tester though.
The worst part of this job so far is re-installing the cover to the column. Broke the left side plastic attachment point on the top. Repaired it with a small drill bit glued into the tiny hole in the center and some shrink wrap on the outside for good measure.
Are any of the vacuum actuated functions working?
There is a vacuum manifold underneath the dash that sometimes fails.
Let me know if you need one.
I had the entire dash off to get to this thing. I believe the problem may be in the vacuum canister. The only valve block i found was on the driver side low. Does that sound right?
Do you mean the vacuum manifold block, or a ‘blockage’?
If you need that or a vacuum canister I have them.
Steve