I was so happy this evening…. I went to the mech that has finished reassembling my xj40 after putting back on the aircon fans after I succeded in replacing darlingtons. While driving home I tested the system and ok… fans work great at any speed, but the central vents does not produce any air. I have air only from the windscreen vents and lateral face vents. The temperature control works, but the central vents are dead it doesn’t matter how I set the controls.
I suppose that there is no vacuum so the windscreen vent is always open and the central vents are always closed. Now what is the best way to troubleshoot the problem?
Andrea …
It’s highly likely that the center vent vacuum actuator has come apart (blown up). When this happens the large internal spring in the actuator will hold the center vent permanently closed. Very common problem. Remember the system is designed so that the ONLY time the center vent opens is when air conditioning is selected.
Quick fix … remove the center vent wood trim (it just pops off) and grill. Now reach in and wedge
something in the vent door to hold it open, a 2" piece of heater hose works nicely.
Note … If you’re getting air out of the windscreen (demister) vents with air conditioning selected that’s another problem, that’s caused by one or both broken vent assemblies.
before repairing the fans (new darligtons) the system worked flawlessy… i hope is just my mechanic who forgot to reattach a vacuum tune… now i brought the car to the mech again 
Andreal …
If it turns out your problem is vacuum related here’s a diagram of the climate control vacuum system. Although it’s for an earlier XJ6 it’s essential the same as your '92 so this might help you understand how the system works.
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There are 5 vacuum operated actuators,
- one for each of the two dash top demisters
- one for the center vent
- one for each of the two fresh air flap doors in the fan assemblies
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The center vent actuator has the black vacuum line running to it.
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The ends of all the colored vacuum lines terminate in a number of small electrical-vacuum switches all located together underneath the evaporator box. These switches are electrically controlled by your climate control panel on the ski slope to turn the vacuum on and off to the 5 actuators and the heater water control valve (if your car has one).
My mech tolto me that the problem Is in the vacuum switch of the front vent he told me that he can ear the click but the vacuum switch does not open so no vacuum arrives to the front vent actuator. Is It possibile tò find this switch? Do you know which Is the part Number?
if i use this solution (blocking the center vent open do i damage someting? I can use this solution until i find a replacement fo vacuum switch ?!
No, just push it open. You can’t hurt anything, once you remove whatever you use to keep it open it will close again. The hose may not be connected too, that would sure stop it from working.
Andrea …
There is no “vacuum switch” at the center vent and no clicking going on, only a silent vacuum actuator. Basically this is what the actuator looks like (this isn’t a picture of the exact Jaguar actuator, but it’s close enough) …
In the diagram I previously sent the black vacuum line runs to this actuator. The blue arrow in the picture points to the lever that moved in and out to open and close the center vent door. The red arrow points to the failure point that is very common on these units. There is a very big spring inside the actuator that puts a lot of pressure on the plastic housing and if the housing splits it permanently holds the vent door closed.
Regrettably the only way to replace or repair (I epoxied mine back together) the actuator is to remove the top of the dashboard, a very big job.
Also when this actuator fails if you select air conditioning this will result in a vacuum leak which may degrade the operation of the other vacuum actuators in the system and possible affect your idle.
the mech told me that there is a valve electrically operated that let the vacuum to arrive at the actuator. The actuator works, because if he bypass the valve the depression opens the front vent actuator and the front vent goes open.
He told me that this valve is operated by the AC CPU and that after turning on AC the cpu send power to the valve to open and let the vacuum arrive to the front vent actuator, but as this valve in not working anymore no vacuum arrives to the actuator.
For me is like talking in arab language so i tried to understand the situation. So forgive me if i am not clear in the description .
By the way there is another problem , maybe more simple to solve. The little fan that is located under the dash on the right side of my LHD car that is supposed to send air to the temperature sensor (is very little, like a cpu fan) is very noisy. Anu suggestion to repair it (or replace with something similar) ? its whining continuously and i hate it.
I used to be a computer\ office equipment tech; frequently quitened noisy cpu type cooling fans by removing the stick on label over the central bearing, which is usually just a brass bush, and drip some very light oil into the bush. Nothing to lose.
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Andreal …
As I previously mentioned all the switches that turn on and off the vacuum to the various actuators are located behind and underneath the evaporator/heater matrix housing, rather difficult to get to. In operation they’re very quiet and don’t click like an electrical relay.
If I remember correctly if you remove the kick panels under the dash you may be able to access the black vacuum line and by using a hand held vacuum pump apply vacuum to the actuator to test it.
Remember the center vent is held closed by the large spring in the actuator (vacuum off) and only opens when vacuum is applied,
The way your mechanic explained to you how the vacuum system works is correct But how does he know that the vacuum switch isn’t working (rather uncommon) and the actuator isn’t the problem (very common) ? You said, “if he bypass the valve the depression opens the front vent actuator and the front vent goes open.” How did he bypass the valve and get the center vent to open?
As for the air collection fan you can just unplug it and run the climate control system in manual which bypasses all the sensors and generally provides better cooling that way.
thanks Grooveman for your patience and interest in the question.
When i brought the car to the mech it was for the darligton fan problem.
i had only max speed. He dismantled the fans and i replaced the darligtons
then he reassebled the system.
Before the operation, the actuator of the central vents worked flawlessly when i turned ac on
after few second air arrived at central vents.
After the repair of the fans, i noticed that the central vents did not work anymore.
i tolt it to the mech and he controlled the hoses of vacum and told me that if he apply depression to the central vent the vent opens but if he reconnects the hose to the valve it does not work. He told me that probably the valve is locked close and does not open.
I really don’t’ know if what he says is the truth, but his description seems to be plausible.
My idea is to open manually the vent and leave it open. Iw i want to stop the air i close manually them with the cursors. This until i find a new valve to test the system (and maybe a better mechanic).
Andrea …
OK I think it all makes sense now. My fault for not reading your original post more carefully.
Since your center vent was operating properly right up until your mechanic repaired your two fan assemblies I think you’ll find that while digging around in there the black vacuum hose accidentally got disconnected some where. Very easy to do and overlook.
The center vent vacuum hose is actually made up of long sections of plastic tubing (like skinny drinking straws) joined together by short rubber hose connectors (straight and 90degree curves). These connections are merely pushed in together and not fastened by any type of clamps.
I don’t remember which side of the center console the hose runs down so I would start by removing the
passenger kick panel and if you can’t find it try the driver’s side. I’m betting that’s your problem. It makes a lot more sense than having one of your vacuum switches fail at the EXACT time your mechanic was digging around in there. I don’t believe in coincidences 
If you do find that the black vacuum hose hasn’t come apart somewhere there is also a chance that the yellow hose SUPPLING vacuum TO THE switch has accidentally been disconnected.
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But just in case my mechanic is right (he told me he checked all the hooses and they are correctly connected, do you know the spare code of the vacuum switch… just in case…
Andrea …
Here’s a link to the climate control vacuum solenoids (switches) …
As you can see there is a LOT going on under there. By selecting each item you can find the part number. Items #1 and 2 are the electrical vacuum solenoids (switches) you are asking about. There are four; one operates both demister flaps, one operates both fresh air flaps on the blower units, one operates the center vent flap, and the fourth operates the water control valve to the heater.
You’ll also see restrictors listed in the diagram. The purpose of these is to slowly apply or remove the vacuum so the vent doors don’t slam open or shut, which would produce a noise that would be very un-Jaguarlike.
Andrea …
If your problem does turn out to be a bad vacuum solenoid switch (which I still doubt because of the timing of the failure) I couldn’t find any new “Jaguar” solenoids anywhere listed for sale. I saw 1 or 2 used ones for up to $100. So I starting looking online and there are many, many solenoid for other cars that are very similar.
or you could order this one from Jaguar …
Oh heck they don’t have it in stock 
Or you could purchase this one (has the correct 90 degree nipple orientation) and do a modification on the electrical connector …
Still too expensive ? How about this one …
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I opened the front vents…the actuator looks good, no evident issue. Now i Will leave the front vent flaps open with some object but before digging for a new solenoid i want to check the vacuum hoose. The Better way, i suppose, Is to disconnect the black tube before the solenoid and After and put the tubes together to see if the flap opens, but i have been told that the solenoid Is buried under the radio panel
Andreal …
First thing I would do if I wanted to fix your problem would be to located the black vacuum line. I know you can get to it with the knee bolsters off, but as you mentioned you may be able to access it by removing the ski slope and radio … It’s been a few years since I’ve been in there so I don’t remember for sure if that’s correct.
Now that you have access to the black line use a hand held pump and draw a vacuum on the line. Either the center vent will open (which means something else is wrong downstream) or the pump won’t be able to pull a vacuum (which means the actuator is broken and the center vent won’t open).
If you can’t pull a vacuum … wham bam … end of trouble shooting, the actuator is broken.
You can remove the center vent grill and you still won’t be able to tell if the actuator is broken because of the way it sits behind the plastic piece it’s attached to. It only takes a small split in the housing … really.
By the way no comment on the research I did for you …
Your research was great, except for the original unobtanium part, I have bookmarked all the items for future use, you did a great job and I appreciate all your efforts to help. Yesterday I spent a lot of time in the cold pubblic garage where I have the jag to fix the temp sensor fan. I managed in making it silent again. I found the black tube and in the next days I will make more troubleshooting, but I had to deal with the rear window stuck open (new issue) and I came home very sad and frustrated.
Andrea …
Although my wife often tells me I act like I’m 9, I do very proudly have the vocabulary of a 10 year old but “Unobtainium”?, I had to look that one up ! (But hey, maybe now I’ve moved up to 11 
).