Delanaire MkIII - 1994 Model Year - Defrost question

[AndyF]

Hello all

I need some help diagnosing a couple of issues with the HVAC system on my 1994 6.0 V12 coupe.

Following some investigations last winter into an unstable cabin temperature and replacement of the compressor (with clean and recharge) in the summer, I have spent the last couple of days checking the system over, as I am still not happy with it.

My key questions are these:

1) Should hot air come out of knee vents when full screen defrost is selected? (LH dial fully cold, RH dial full to screen, engine off / warm, ignition on II) / What could be the root cause of the knee/lower vents not closing when full defrost is selected?
2) Where does the black foam come from, which gets blown out of the centre facia panel and is this a problem?

Some background:

Last winter I noticed that the cabin air temperature wasn't stable. I did some initial investigations and found that the cooling system needed a thorough purge and that the atmospheric catchment tank in the NS wheel arch was empty. This was all cleaned and reinstated. During the summer the compressor seized spectacularly and shreaded the AC drive belt. While the car was with Dave Marks, the compressor was replaced and the system was fully cleaned out, recharged and the belt replaced.

Investigation:

Since driving it again early in December to the Lake District, I was still not entirely happy with the HVAC performance. Therefore I set about some investigations.

I used the following guide from KWE as a starting point: https://www.kwecars.com/jaguar-tech-cen ... 04c84-d524

There were a few key differences to this guide that I spotted. Firstly, the AC fans in my facelift 1994MY car are now on 20A fuses, rather than 25A as listed in the KWE text (confirmed correct in handbook). If you are working in a low ambient temp (it was about 5 deg C while I was doing this!), with both doors open for access, you may need to use the "manual" switch to get the system to switch to fully cold, which should move the hot water valve up and down. Note this works with both warm engine ignition on II AND with engine running. One thing I don't understand is where does vacuum comes from when the engine is off to actuate the valve?

So I've checked the condition of the condenser, the general underbonnet plumbing, the belt tension, the responses of the main and aux fan, confirmed that the radiator sealing foams are in place. It has a new compressor, the hot water valve has been degreased and relubricated and confirmed functioning, recirculation function appears to work, as does the manual function on the control panel.

When I listened in the driver's footwell for the movement of the air blend flap motors, I could definitely hear 1 running (but I cannot be certain that I could hear 2 motors running). The motor sounds were certainly quieter after a few cycles and this shows value of full HVAC operation cycle each time it is driven.

The ROM and the KWE instructions say that "when defrost is selected the fans operate at maximum speed, front screen vents open fully, lower flaps close fully and the maximum output is directed to the windscreen (there may be a delay of up to 30 seconds from selection to execution of this function"... but this was clearly not happening in my car when the controls were set as described. Warm air was directed to the screen (ok - ish intensity, but hardly a hurricane), and hot air still continued to pour out of the lower / knee vents???!!!

All suggestions and ideas welcome!

AF

[AndyF]

Some images to go with the words in the previous post

Foam inside centre facia vent

Driver's side fuse board - 20A fuse

Passenger's side fuse board - 20A fuse

Cont...

[AndyF]

Some further images

Hot water valve (1)

Hot water valve (2)

New compressor and belt

Cont...

[AndyF]

Final images

Clean and functioning condenser

I have half a plan to remove the plenum cover and stick an endoscope inside the HVAC air box, to get a view on how much debris has accumulated in there. I'll post images of these if I get a chance.

[AndyF]

Hello all,

Some progress on my investigations into the HVAC system issues on my car.

From some further reading, I now know that it is a Delanair MkIII (no "e" !!!) system and the following forum post has provided links to some excellent information:

https://www.jaguarforums.com/forum/xjs- ... on-204028/

In particular - 2 documents on the overall operation of the system and an update for the 1994MY system.

From this I now know that the system will operate with the engine off, and still maintain vacuum control, via the use of the vacuum reservoir for the system.

Secondly, the following 2 diagrams give a good overview of the workings of the system

And finally, from additional testing today, I am certain that the lower blend flap is stuck on my car - which is the cause of the warm air coming out of the knee vents, even when full screen defrost is selected.

Cont....

[AndyF]

And also, in a round about way, I have found a plausible reason that the cabin temperature is not stable. As the lower blend flap is stuck it will not control the temperature of the air going into the passenger footwell.

This is important, as this is the location of the in-car air temperature sensor and aspirator - see below:

Therefore, in all conditions in automatic mode other than on fully cold (when the hot water valve is shut and the system gets no heat from the cooling circuit), the knee vents will have warm air coming out of them. As a result, the in-car temperature sensor will sense this, and keep pushing the cabin temperature down, in an effort to compensate for it!

I tried the "tapping the motor" trick to free off the stuck air blend flap, but without success. The short term solution is to simply run the AC system in "manual" mode, which will prevent the system from trying to adjust the temp continuously.

Case closed... for now!

AF

[PaulGover]

One of the standard aircon problems is its desire to enter "recycling" at times of maximum load - i.e. if it's too hot outside, the system adjusts the flaps to suck air from the footwell, chill it, and blow the results at your face. However, if your aircon system is (as is all too commonly the case) broken (typically all the refrigerant has leaked away), this means the cabin temperature continues to climb! The hack to cure the situation is to disconnect and plug the vacuum line feeding the servo attached to the lower flaps. That prevents recycling, so the otherwise inoperative aircon at least now blows air from the outside world in your face, rather than the already hot cabin air!

Perhaps your system was once in this state, and therefore the lower servo is no longer connected to the vacuum system. Failing that, the full workshop manual contains a fault-finding procedure that should enable you to identify what's wrong.

[AndyF]

Hooray - fixed it!

Some further tinkering this morning and found out how to get better access to the HVAC system.

Removal of the knee rolls on both sides of the car gave more room. Then the lower IP cheek pieces were removed via unscrewing the two knee duct fixings per side.

This gave a much better view of the HVAC system - LH side shown below. The key item here being the red potentiometer for the lower blend air flap. I removed the same parts on the driver's side and then managed to get a screwdriver end into the stepper motor on the blend flap. While cycling the HVAC controls from mid 9 o'clock fan position to 12 o'clock fan position, I tapped at the blend flap motor. Suddenly the sound of the air coming out of the ducts began to change, as more was diverted to the screen. The knee vent flow slowed to a trickle while the screen defrost vents roared..... a few cycles later and the knee vents were fully sealing / closing and all was working fine!

A quick reassembly and retest - to check all was still working - and job done!

Stay safe and enjoy the Christmas break.

AF

[davidr]

Well done Andy - isn't it amazing what a small "percussive adjustment" can achieve?!!
That looks like a useful magazine article to me!