After finally finishing my XJ12, it was time for a test drive.

The air conditioning worked like a charm, but after switching to hot air and back, only hot air is coming out of the ventilation openings.
The air conditioning lines are could though and the system is full.

What can cause this behaviour? Vacuum leak?

 

is the compressor switching on and off?

a bad heater valve?

a blown compressor fuse?

 

What series is you xj12?
In my series 1 the heater core has two pipes that stick out through the firewall. I'd guess the thing that opens the heater core got stuck open? Not sure how the air con fits though, mine was a separate under dash thing.

 

Dutch-Cat's signature.

 

I agree that the heater valve is a good place to start.

Have a helper turn the temp selector knob from max heat to max cool while you observe the operation of the valve. It has a lever arm that visibly moves....although it might not be easy to see on a V12 car.

If no movement then the valve may have physically stuck. Or, the vacuum control isn't working. Remove the vacuum hose and put your thumb over the end. With the controls set to max cooling you should feel vacuum. Do you?

Also, does the airflow change as you operate the temp control? When set to max cooling most of the air should come out of the center dashboard vent. If set to max heating most of the air should come out of the foot well vents. If the airflow changes like this then the system understands , and is trying to comply, with requests for temp change.

At some point it might be necessary to know of your have the Delanair Mk II or Delanair Mk III system. The Mk III is easily identified by the temp control knob having a "Pull for manual control" feature.

Cheers
DD

 

I agree with the suggestions above but remember that, in general, on Delair systems the hot water is flowing and the freon is pumping almost all the time. The modulation is achieved by the pneumatic controlled flaps behind the dash moving. After you have checked the above items, remove the right hand cheek panel. That is the little panel the floor vent is fastened to. Remove the 2 screws that hold the floor vent grill and the panel will wiggle out. Takes about 30 seconds to get it out. Note this is on the passenger side of a lefthand drive car. Now, start and warm up the car, get down there with a flash light and turn the selector know to "auto" and then move the temp knob from max cooling to max heat. After several seconds you should see the push rods moving and hear the flaps opening and closing. If nothing is moving you have a problem with the amplifier, the servo, or the vacuum system.

 

Right.

The system is designed to refrigerate all incoming air.... and then heat it as needed. The heater valve should be open at all times except when maximum cooling is called for.



Which would be the visual confirmation of what I was suggesting in discussing the differences in airflow thru the vents. Watching it all work is rather fascinating

I'll add, tho, that if this car has the Mk III system, there's much less to see....as all those push rods and bellcranks are eliminated, as is the servo and the amplifier. The Mk III system was installed on V12 cars mid-1986. Diagnosis and repair is quite a bit different than the Mk II system

Cheers
DD

 

BTW, if you haven't already, be sure to look at the excellent document Jose posted it the 'sticky threads' section entitled XJ6 series III - Air Conditioning. It is the complete, comprehensive manual you need. In it you will see all about the push rods, flaps, etc. It is the one down below the section with chapter numbers.

 

geez I had forgotten about that doc. !! Thanks for reminding me.

 

Ok guys,

The airco system is a Mark II, because there is no "Pull for manual control" action.

thanks for the feedback. Now I have something to work with.
I suspect I jammed the heater controls with a coax cable for the navigation antenna of my radio....to be continued...

 

Yeah, at times, we create our own woes.


Carl

 

So, "what did I do between the time it worked and the time it did not?


Sort of related to finding lost articles. Where was I when I last had it. For me, my keys, glasses, hearing aid and lately my cell phone!!!!
Oh me....


Carl

 

Elas.... no cable jamming the controls...

No whir from the servo too...
I pulled the controls down manually for now and fixated them with a tie-wrap...
Any thoughts?
All fuses in the fuse box are intact....

 

If, when you turn the knob from full cool to full heat and no movement or sound from the servo I would pull the cheek panel from the other side and check the amplifier (or amp) It is the size and shape of a pack of cigarettes and has a multi pin connector coming out of it. Check the connector (like any other Jaguar electrical connection). Gary Crosby at Jag-aire is a true expert and can talk you through it. He manufacturers and sells replacement amplifiers.

 

When my amp went bad the system would work sporadically. Turn it to cool and maybe 5 minutes or 5 hours later it would start to work. The amp is actually a type of early computer. It takes signals from the input knobs and sensors, decides what to do and sends a current to turn the servo that actually moves the levers and turns things on and off. Think pre-computer age. No digital processing. All analog.

 

Thanks, will check the amplifier and the connector. What you describe is exactly what happened, before the servo completely stopped working...