In my 1988 XJ40! It goes from blowing cold to feeling like
the heater is turned on and vice versa! I’ve replaced the
dryer, expansion valve, evacuated(converted to R-134A long
before me), fan temp switch, fan clutch.–
clarkecars
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In reply to a message from clarkecars sent Sat 27 Aug 2011:
Is this happening in ‘Manual’ mode or ‘Auto’ mode - or both?–
The original message included these comments:
In my 1988 XJ40! It goes from blowing cold to feeling like
the heater is turned on and vice versa! I’ve replaced the
dryer, expansion valve, evacuated(converted to R-134A long
before me), fan temp switch, fan clutch.
–
Bryan N, '91 Sovereign 4.0 L, RHD
Cambridgeshire, United Kingdom
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In reply to a message from Bryan N sent Sun 28 Aug 2011:
It does this in both modes.–
clarkecars
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In reply to a message from clarkecars sent Sun 28 Aug 2011:
If it switches rapidly between full heat and full cold at
random when, for example, the controls are set to ‘full
cold’, then it sounds as if the blend flaps in the climate
control unit are not under control. Since it does that in
‘manual’ mode (which means that it is not subject to
variation based on input from the cabin temperature sensor)
I suspect that the problem is something to do with the A/C
ECU - perhaps a bad connection in one of the three 15-way
white connectors on the ECU on the right side of the climate
control unit above the tranny tunnel under the dash.
If the change between full hot and full cold is only gradual
You are happy that the A/C system is fully charged? If it is
low on gas, the compressor clutch will disengage to protect
the compressor.–
The original message included these comments:
It does this in both modes.
–
Bryan N, '91 Sovereign 4.0 L, RHD
Cambridgeshire, United Kingdom
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In reply to a message from Bryan N sent Sun 28 Aug 2011:
Whoops!
Obviously that should read ‘‘from cold to hot’’ 
Sorry for any confusion…–
The original message included these comments:
the next time it switches from hot to cold, check if the
compressor clutch is still engaged. If not, then the answer
–
Bryan N, '91 Sovereign 4.0 L, RHD
Cambridgeshire, United Kingdom
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In reply to a message from Bryan N sent Sun 28 Aug 2011:
Thanks for the replies. It is fully charged as it was just
serviced and a leak down test was done. No leaks, holds a
full charge. Can a very small amount of moisture in the line
or a bad new drier cause this? Doesn’t seem logical to me.
The center a/c registers are typical of this model, low
frontal pressure and blow through the defrost vents. Before
now it would blow cool on the highway but not in town. Now
it either blows cold or hot on its on.–
clarkecars
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In reply to a message from clarkecars sent Sun 28 Aug 2011:
PS: sometimes the control panel doesn’t illuminate and the
controls do not respond to touch and the fan doesn’t come
on. It will remain off and then come on by it self or
sometimes it seems to return if I tap the panel with my finger.–
clarkecars
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In reply to a message from clarkecars sent Sun 28 Aug 2011:
Mr Clarkecars…
OK my warm friend, it appears to me that you have several issues
going on here.
As Bryan mentioned, first thing you need to determine is if you
have an ‘‘air cooling’’ problem (something that keeps the evaporator
from maintaining the correct temperature)…
Or an ‘‘air distribution’’ problem (something that’s keeping the cold
air from being directed from the cold evaporator out of the proper
vents)
If you can sucker punch your a/c control panel and it will
occasionally start working …uh…I think we’ve found one of the
problems.
You mentioned that the low air flow out of the center vent
is ‘‘typical’’ of this model…Oh I beg to differ. Your two demister
vents are broken, (they should be held close by vacuum with the a/c
on) and a great majority of the center vent air is wrongly (even
criminally some may say) being diverted through those open vents.
That is the reason you have inadequate center vent air…now I am
assuming both your a/c blowers are working.
To repair or replace those two rascal vents the dash top has to
come off. Now you could jusr put duct tape over the top of your
dash, but if you do that why not just buy an old pickup truck and a
mean dog.–
Groove
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In reply to a message from clarkecars sent Sun 28 Aug 2011:
OK, you probably have two or more problems. Let’s sort the
easy one first - the intermittent A/C control panel
functioning.
That is most likely due to a failure of the micro-switch at
the back of the fan speed control rotary switch on the
panel. I will PM you the fix for that.
That will almost certainly be the problem unless the radio
stops working coincident with the panel lights going off, in
which case you have to suspect the A/C & Radio relay - the
one in the green base on the component panel under the
passenger side dash (visible when the knee bolster on that
side is removed)
When you have resolved that, report back on any continuing
symptoms of malfunction and we can progress from there - but
in the meantime, when the A/C panel is functioning, check
the A/C compressor clutch is still engaged when the air flow
‘gets hot’ as I previously suggested.–
The original message included these comments:
PS: sometimes the control panel doesn’t illuminate and the
controls do not respond to touch and the fan doesn’t come
on. It will remain off and then come on by it self or
sometimes it seems to return if I tap the panel with my finger.
–
Bryan N, '91 Sovereign 4.0 L, RHD
Cambridgeshire, United Kingdom
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In reply to a message from Grooveman sent Sun 28 Aug 2011:
Does this mean the evaporator is eliminated as the problem?–
clarkecars
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In reply to a message from Bryan N sent Sun 28 Aug 2011:
Thanks Bryan. About the fan/radio relay; the radio was
professionally modernized 3 years ago with a built in
satellite/USB port unit. The radio works fine.
I will check the compressor during cooling and heat. Where
does one obtain new blend doors?–
clarkecars
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In reply to a message from clarkecars sent Sun 28 Aug 2011:
Mr Cars…
‘‘Does this mean the evaporator is eliminated as the problem’’…
I would be inclined to say (drum roll plese) YES, you can eliminate
the integrity of your evaporator as a source of your problem. If a
leak down test (by this I assume you mean a deep vacuum was put on
the system and held for 30 minutes) was properly performed then
your evaporator should be OK.
Running a vacuum check on an car’s A/C system can be decieving. If
you have a pin hole leak in a hose the vacuum will tend to pull it
together and seal the leak. Then under pressure (vs. vacuum) the
hose may leak.
Likewise an evaporator goes through an amazing number of thermal
cyles and can develope microscopic leaks which tend to hide
themselves when it warms up again…and you perform the vacuum
check.
The good news, most older evaporators (made before around yr 2000)
are made of rather stouter stuff and aren’t prone to this malady.–
Groove
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In reply to a message from Grooveman sent Sun 28 Aug 2011:
Well Groove I am pleased by that news. 1988 Evaporators are
scarce (unless used and that seems pointless) not to mention
the hassle and labor of pulling my dash apart to reach it.
Thank you.–
clarkecars
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In reply to a message from clarkecars sent Sun 28 Aug 2011:
One does not ‘‘obtain new blend doors’’. They are built in to
(i.e are part of) the climate control unit (CCU) over the
transmission tunnel under the dash, so the entire centre
console and dash has to be removed to get at them by
replacing the CCU. Sometimes they ‘stick’ in a certain (hot
or cold) position, but they can be released. However, that
is not your problem, so don’t worry about that (or anything
else) until you have fixed the control panel.
I will PM you some pictures of what the blend flaps do.–
The original message included these comments:
I will check the compressor during cooling and heat. Where
does one obtain new blend doors?
–
Bryan N, '91 Sovereign 4.0 L, RHD
Cambridgeshire, United Kingdom
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In reply to a message from clarkecars sent Sun 28 Aug 2011:
If the A/C system is pressureized and hold the charge, the problem
lies elsewhere. The ‘blend flaps’ are actually long cylinders with
holes across the width of the cylinder. They are driven by a small
gearmotor from the right side that can be accessed by removing the
entire glove box. Each cylinder drives a potentiometer on the left
side that tells the CPU where the holes are. It sounds like the
cylinders are rotating continuously or are stopping randomly, which
suggests that the potentiometers aren’t connected electrically.
I’d check that first. Drop the knee bolster on the left side and
examine the connections. While under there, turn the system on and
see whether the blend cylinders both move to control panel
settings. That will tell you whether both gearmotors are
functioning.
The 88s have a water valve that closes by vacuum. There are 4
solenoid valves under the climate control system, two can be
accessed from each side of the car under the knee bolsters. The
demister flaps and water valve are controlled by the two on the
left side, so you can check those out as well.
Finally, you may want to pitch the plastic water valve altogether
and install a manual ball valve to turn the water off during hot
weather. That’s what I did rather than hunt down a valve that
works.–
The original message included these comments:
The center a/c registers are typical of this model, low
frontal pressure and blow through the defrost vents. Before
now it would blow cool on the highway but not in town. Now
it either blows cold or hot on its on.
–
Pete Peterson 70E(193K) 88XJ40s(253K & 242K) 94XJ40 (122K)
Severna Park, Maryland, United States
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In reply to a message from Jaguarpete sent Mon 29 Aug 2011:
Mr. Cars…
Pete told you that ‘‘If the A/C system is pressureized and holds the
charge, the problem lies elsewhere’’. In your case he is (as usual)
spot on because if your system is cooling, stops cooling for a
period of time and then starts again it hasn’t lost it’s ABILITY to
cool…just it’s desire.
By the way, the trouble with freon leaks is that they can be very
slow and not show up for months. This is especially the case with
R-12 systems that have been converted to R-134a. First 134a is
composed of smaller molecules that work their way through the a/c
hoses that were designed for r-12 (unless you did a complete
conversion and changed everything). Second R-134a systems operate
at higher pressures that try to push those Mini-ME molecules out.–
Groove
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In reply to a message from Grooveman sent Mon 29 Aug 2011:
Agreed, but that leak would be over a long period of time
not a few days or even a few weeks. It would likely lose a
pound or so from one summer to another. Unless Jaguar hoses
are unique in compound. I have converted numerous R12 cars
in the past with great results, though my SAAB 900s ( 84-92)
would leak down over a year about 1 lb.
With the current situation on my XJ40 I get hot air, but see
A/C condensation pools under the car after it is parked.–
clarkecars
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