Good afternoon dear Jaguarists,

Maybe someone can help me to track a problem - hot air is only coming from Foot vents but nothing only Cold air is coming via Central Vent and Demist areas - any ideas?
As well i've found a vacuum hose plugged with what it seems to be a fuel filter - hose is leading towards Firewall of the car. - Any ideas?
1994 Jag XJS V12 Roofless

Apreciate for your help,

 

The filter is for the distributor vent system. It's normally tied-off to the diagonal brace seen in your photo. Here's a pic from the parts catalog showing the hoses:

https://parts.jaguarlandroverclassic.../brand/jaguar/

Cheers
DD

 

Your statement isn't quite clear to me

Cheers
DD

 

Dear Doug,

Thank you for your answers. Hope my clarification will help -
Car is running, it's up to temperature. I'm changing climate temperature to max both in Temp regulator and in Central Vent regulator - assuming that hot air will enter car from all directions.
What i get is - Hot air is flowing only to my legs (Via bottom vents) but central vent keeps blowing only COLD air.
Also if i press DEMIST button - only COLD air is flowing to the window;
Basically - only my legs are warm and face is always getting cold air

Hope now it's clear.

 

Those symptoms would indicate that the air distribution flaps within the HVAC unit that mix the air-conditioned cold air with the warmed air, and also that direct the air to the various outlets, are not functioning as they should. On Defrost, you should get hot air blasting to the screen, and if this is not happening, but there is warm air available, it must be the flaps inside the unit not working. They are, I think on the MK III Delanair, vacuum operated flaps like the Mark II has, so it is just possible the vacuum source for the unit has come adrift.
It is situated as a vac take-off on the end of the A bank inlet manifold, and you should find a vac tube running into a cover on the opposite side to (but the same relative position as) the brake servo/ABS/pedal box top in the engine bay. If you unbolt this cover you will find a round vacuum reservoir under it. Once you have identified all this, you should find a non-return valve in the vac line between the manifold and the vac reservoir, check that this valve is not stuck shut, and check that the reservoir holds vac. If, having run the engine and the aircon unit, you switch off the motor and immediately disconnect the vac line from the reservoir, you will hear a strong hiss if the vac reservoir has been holding vacuum.
Then let us know your results.

 

Dear Greg, thank you for your advice.
Are you speaking about this black cover behind ABS Pump?

 

On the MkIII system, only the defroster and vents are vacuum operated. The air temperature blend doors are driven by servo motors.
This sounds to me like either the upper flap servo motor or feedback potentiometer are faulty.

 

yoopas,

Also remember that whilst the demist vents are designed to flow air as per the temperature setting, the central face-level vents are NOT designed to flow hot air on the late system that you have. The temperature slider at top centre of the controls is merely designed to give a variable slight differential of temperature compared to the footwell vents when the main temperature is set to cold. When the main temperature is set to hot, the central face-level vents are designed to close and not flow air at all.

Hope that helps

Paul

 

Dear Ptjs1,

So basically you are saying that it's either blowing external temperature air or being shut off and Cold/Hot settings on it means nothin?

 

yoopas,

Not quite. If you set the aircon to its coldest setting, (irrespective of external temp), and slide the face-level switch to the "blue" end, the face-level vents will blow cold as per the aircon temp setting, and the same as the footwell vents. Whereas if you slide the switch to the "red" end, the face-level vents will blow "slightly less cold". If you set the aircon to Hot, the face-level vents should then shut off, irrespective of where is the slider switch.

Paul

 

Yoopas
Yes, the cover in your photo has the reservoir under it. If you find the vac tube going to it, and disconnect the one way valve, as you disconnect it you should be able to hear the hiss if the vac reservoir is OK.

 

Good info and needed. Been wondering about that doggone switch...lever...slider...