Any ideas before I start to investigate further? even with heating off the vents at the windscreen base allow hot air through. the aircon doesn't work so possibly its still blowing air through?

 

The heater valve being stuck open is my first guess but there may be more to the problem.

What year is the car?

Convertible or coupe? (in some cases the climate control was different between the two)

Cheers
DD

 

I do not think I saw your model details. But, in principle, the XJS has an air blend heater and mixes cold air from one chamber with warm air from another to give the required temperatures.
So if the aircon is not working, that might well be a factor in confusing the system, which will be looking for a given low temperature in the cold chamber.
If the RHS knob is at off, then I think the flaps on the inlets to the system (which are part of the fan assembly each side of the dash) should automatically close. Though this feature may be activated only when the car is actually switched off, I cannot remember. See, when you actually switch off the car, if you can hear some flaps closing. I can on mine, but even after all these years I do not know if that happens when I switch off the aircon system with the knob!
I do not lightly disagree with Doug, but I have a feeling that even if the system is switched off (unless you are calling for full cold - the lowest temp setting on the LHS knob) the water valve remains open, and this should not in itself affect airflow into the cabin, which you are obviously getting somehow when the system is switched off.
Greg

 

Thanks guys, sorry should have said, it's a 1982 fhc

 

We're probably not in disagreement

In asking for more info I myself reply I was a bit too economic with words.

In some convertibles the system is never actually off, even with the control knob turned to 'off'. The system, at least some portions, remains powered up so the compressor will run and provide fuel cooling.

But we now know that isn't relevant, so onward we go !


Cheers
DD

 

Ok, good, from that we know A) it isn't a later convertible and B) you have the Delanire MkII climate control.

The system is a little tricky but my thought process begins with fundamentals.

Air coming thru the screen vents certainly suggest the defroster flaps are open. And the defrost mode defaults to full heat. (and, if in thinking about it, the heater valve is always open in all modes *unless* max cooling is called for). And, by design, the defrost mode bypasses the 'auto' aspect of the system.

Next question/test:

With the mode knob set to 'auto' slowly turn the temp knob thru it's range, allowing time for the system to respond. You should hear a slight whirring sound and hear the internal flaps operating. If you do then we know the system is at least fundamentally operational and is at least trying to comply with requests. No sound at all tells us the system is fundamentally non-operational....and that issue must be addressed before going any further.

My thinking is that if the system failed in while in defrost mode then there's nothing to bring it *out* of defrost mode.


Cheers
DD

 

Greg is correct the heater tap in the engine bay is on all the time unless the AC is on full cold. This causes hot water to flow through the heater matrix heating up the AC box with the AC off and hot air comes out the vents.

The foam surround on the air flaps may have disintegrated causing air to leak. Mine is like this and a nightmare to fix.

What Doug said is also a common problem and one cause is coolant leaking from the heater matrix onto the connectors.

 

Thanks fo all the very helpful info in this thread. I've learned several things I didn't know.

One more question. The heater valve is normally closed and opens under vacuum correct?

 

Other way 'round...

No vacuum = valve open
Vacuum applied = valve closed


In 'normal' operation the system keeps the valve open (no vacuum applied) at all times *unless* maximum cooling is dialed up on the temp control. When max cooling is requested, vacuum is applied to close the valve

Cheers
DD

 

Darn. That makes my summertime vac solenoid heater bypass switch a no go. I have no AC hooked up and don't want that Heater matrix hot at all.

 

Just run a vac hose from the heater valve to a vacuum source....tee-in-to an existing vacuum hose or whatever....so that vacuum is applied all the time.

Cheers
DD

 

I guess I could combine the two. Vac solenoid between manifold and water valve. I want it electrically switchable for chilly mornings.