any idea on what is wrong with AC system on my XJ12, year 1992. It somehow does not hold the desired temperture and is usually too cold. In summer when outside temperature is high, I have to rotate the knob sometimes almost to the end of red position to stop freezing cold coming out of the vents. In winter the situation is slightly better – but still after few minutes after blowing nice warm it starts to get colder and I have to «trim« it again by turning AC knob clockwise… it just does not stay around desired temp… Situation is slightly better when knob is pushed out to manual, at least such extremes are not seen. Any idea what is wrong? It all started after Jag was not driven for about a year and half… Thanks, Cene
whereabout are you located? - Northern Sweden;-) There’s at least a double-digit forum members who envy you for your problem, I bet!
Obviously, your heating and cooling work and “only” the control is at miss. I’d first do the simple things and check the connectors of the temp sensor underneath the crash roll (black dash board cover). Others will know more about the AC amplifier …
75 XJ6L 4.2 auto (UK spec)
Thanks, Jochen! It is Slovenia, slightly warmer then Sweden… I have already checked the connections of dashboard sensor…I thought I should replace it, just to cancel out that (I first thought on that, too) but I’ve read that dash sensor is rarely faulty…
Then you need both heating and cooling:-)
Indeed, faulty temp sensors are not among the top ten thread topics here. Anyway, if you happen to find another good one you might just try it out. Even freeing up some corroded contacts might cut the cake …
But yes, very likely you’ll end up in A/C hell … didn’t the late SIII cars benefit from later models’ Delanaire versions? Others know a lot more - I don’t even have climate control on my own car.
75 XJ6L 4.2 auto (UK spec)
I am not sure exactly what is wrong with you climate control system, but I thought that I should point out that your car has the Delanair MK III system which is different in many way than the Delanair MK II system that was installed in the Series III XJ6s and earlier XJ-Ss. The Delanair MK III was installed in the later XJ12s and XJ-S starting around 1988/1989. If you get advice make sure that they know that you have a Delanair MK III system, which has many improvements over the failure prone MK II design.
Yes, many improvements – but unfortunately the Mk III is also failure-prone, just in different ways. The control flaps are operated by a pair of servos, both the same item and referred to here as the “upper servo” and “lower servo”. These things fail with alarming regularity, leaving the control flaps stuck wherever they happened to end up, and dialling different temperatures makes no difference any more.
Unfortunately, both servos are pretty expensive, and the upper one is a serious bear to R&R due to its location. In another thread, we are discussing ideas for replacing that upper servo with something generic actuating through a link so it doesn’t have to be located up in that tight space.
Thanks, guys for your advise. I know that Mk III supposed to be less problematic, but still. I would like to at least somehow structure less possible and more possible causes in order to start with later… See, there are maybe only 4 or 5 such Jags in my country beside mine…and the last thing I would do is drive the car to Jaguar Service … that some 23 years old mechanic with laptop and less knowledge then myself (and I have slightly more then zero) will look into the problem.
I hope it is only the sensor in the dashboard… I have noticed the problem is more obvious these days…it is still quite cold outside but with stronger sun then month ot two ago…so I was thinking if sensor in the dashboard is sending wrong data because the dashboard gets more warmth.
Would that possible, at all?
Can fault on heater valve be possible cause for that? Just trying to find more desirable problem…
Checking the heater valve for condition and proper operation is definitely worthwhile in my opinion. I have tested many heater valves in our Jaguars in situ with a small hand operated vacuum pump. But a more thorough test can be done by removing the heater valve and testing it with a vacuum pump and visually to make sure the valve is really opening and closing properly. I also recommend that you make sure that there are no air pockets in your coolant system. Trapped air in the coolant system can result in strange happenings.
Thanks, Paul for that. I just wasn’t sure whether that valve could also be the cause. Two things to start with…valve ans sensor.
Does that mean that it always drifts down to ‘colder’, Cene - never warmer?
The Del III that you likely have still use the same principle for temp control as the earlier system - it alters flap positions to divert more or less air through the heater core. Symptoms, resetting the temp increase heating temporarily, imply that the ‘heater’ flap resets to more air through the heater - then drifts to the ‘colder’, more heater bypass. Basically, the heater core runs always hot - temp is regulated by heater bypass.
However, if the water valve fails closed, there is no reheating, so even if the flap is in the ‘hot’ position the air remains cold - as delivered from the evaporator. The water valve is controlled by vacuum, but in the Del II a vacuum leak would open the heater valve - and I don’t know if the Del III has the same feature…
Supposedly, the AC amp sets flaps by inputs from sensors, like the in-car temp sensor and temp control - and only alter the settings based on these readings. Ie, the flap setting is only altered when the set cabin temp is reached. Crudely, the amp sets the system to full cold/heat as appropriate, then alter settings when the resistance in the two sensors match up - to deliver blended air to maintain set cabin temp.
Whether the servos can actually ‘leak down’, or only resets by amp signals, I don’t know. But both the temp control and the in-car temp sensor must work correctly - ie have the correct resistances. The in-car sensor varies resistance with cabin temp, and like the Del II, likely has a ‘vacuum’ hose connected to the fan housing - to draw cabin air through the sensor for correct reading of actual air temp rather than its surroundings. That the system responds to in-car sensor heating is an iffy test; it will make the system go ‘cold’ - which is the problem in the first place…
All this may of course relate to an AC amplifier fault - faulting to ‘cold’ commands. But equally; a flap ‘leak-down’ is a possibility - if such a situation is possible with servos as opposed to the Del II wider use of vacuum…
xj6 85 Sov Europe (UK/NZ)
indeed - I never have problem with too warm air… it is warm enough for a while and then it goes colder… and I have to turn the knob more to the red… but that almost never happen when outside is really cold and no sun. In such circumstances usually the temperature remains quite OK…
Different story in summer… it gets colder and colder - so I have to turn the knob even more clockwise… to stop the force of blowing, at least. I even think that the temperature of cold air remains the same, and by turning the knob more to the right only reduce the power of blowing…
So - what would you suggest to start with? In cabin sensor and heater valve?
I have repaired a couple misbehaving Del III systems by cleaning the connections. There are several small wiring connectors (tiny wires, tiny contact pins) that are prone to corrosion. If you remove the console cheek panels and under-dash trim panels you’ll see them. A couple are up into the dash rather high and difficult to reach, though. There are also a couple grounds to clean on the vertical dashboard bracing.
If you want the Delanair Mk III testing manual give a shout and send me your email address
Hi Doug, you advised me years ago with wipers park issue… and of course it was the stick, and the stick is again and again with third piece in last 15 years…
Thank you for that info - will try that for sure. I would be grateful for manual - email@example.com
When ‘really cold’, Cene - the compressor disengages as the temp in the evaporator is constantly below +2C. This does not really explain the ‘drifting temp’ issue - the heater flap still require to be suitably positioned to reheat the cold ambient air.
Generally, I’m suspicious of driver’s perception of temps. I strongly advice the use of thermometers to actually measure temps in the cabin - preferably near the cabin sensor inlet. This will give objective measures - which is what the system is working with…
This is ‘interesting’…
The first step of temp control is to vary fan speeds; fans run at high speed until set temp is reached - then drops to low. And as the temps drifts off set temp; fan speed goes high again - repeated as required. This is the logic of the Del II, and I ‘think’ this is also the case for Del III?
These fan speed variations was a general indication that the system was working as intended - and the longer it remained on low speed the better. It was also used, in conjunction with a thermometer, to verify coincidence between set temp and cabin temp. By gradually turning the temp control until the fan speeds drop; at that stage the cabin temp should coincide with the temp control setting.
I don’t ‘think’ the heater valve is the problem - but it is very easily checked. In principle (Del II), the hear valve should be open, easily verified by disconnecting and clamping its vacuum hose. With the heater valve open, both heater coolant hoses should be hot with the engine warmed up and running - the ‘out’ hose of course slightly cooler than the ‘in’ hose.
One possibility is a clogging up heater core, or some other restrictions in the heater feed circulation. In such situations there may be initial heat, but as the lack of circulation gradually drops core temp - there is no/limited heating effect. It doesn’t quite gel with the symptoms, but is worth a look.
Using an infrared thermometer on the two heater hoses will likely provide some clue, one way or another?
Checking the in-car sensor is straight forward, sort of; measuring resistances against specs on the disconnected sensor. It is temp variable, of course, but the actual specs for proper AC amp reaction is a bit iffy. However, with a verified cabin temp (thermometer), lying somewhere between the min and max on the temp control; moving the temp control as describe above - the fan speeds should react, if the cabin sensor is functional.
But such a test requires some care and interpretation - and it does not exclude an AC amp fault making the wrong decisions. Certainly, if the sensor is shorted; the amp reacts as if cabin temp is too high - max cooling. However, it doesn’t fully explain the symptoms described either…
You could get a (hopefully working) replacement sensor - to learn what to look for. But it is essential that a thermometer is used for objective temp measurements.
The system is very intricate, it is only the perfectly working one that has a clear cut outcome - a stable cabin temp as set…
But the problem is worth while solving - and a one-eyed approach is too narrow minded;. Doug’s points are well taken, as are others’. And, of course, dissecting the symptoms in detail while musing is well worth the effort and time taken. ‘Measure twice - cut once’ is a valid approach…
xj6 85 Sov Europe (UK/NZ)
Frank, I see - thank you very much for your input. You are quite right about temp perception - especially when extreme temperatures are outside and inside the car. I will again read all that I have got from you, guys and also Mk3 manual that Doug sent me - and probably wait for some warmer weather when AC system is more “active”.
Thanks again, Frank.
Perhaps more pertinent than you think, Frank.
As you know, to work properly the system depends on blending heated air into refrigerated air so, with a complaint of “too cold”, a fault with the heater core is worth considering. Clogged cores are somewhat common and, on top of that, it can be difficult bleeding air out of the heater core (and cooling system in general) on the V12 cars. Air pockets in the system can cause odd symptoms, as Paul mentioned a few postings back
On recommissioning for the road was the coolant very low?
no - after year or even two I did not start the engine first but towed the car to the mechanic to change all fluids and check the engine and clean it before starting (Jag was on restoration, sandblasting, respray, etc) because I did not want to risk any troubles…
Could the cause of my problem with AC be just air pockets in the system? Because only few days ago I solved all the problems of coolant leaking - leaking was not huge but was on several different places (hoses, water pump, expansion tank). The car is still at my mechanic for few more small issues, but as he informed me there is no coolant leaking anymore…he installed new hoses, new water pump and new expansion tank.
I will get it tomorrow, so is there a possibility that my problem will disappear? I mean - can just air pockets cause all above problems?
On both my V12 cars I’ve had heater problems caused by air pockets. The cooling system on the V1s can be difficult to bleed and the heater core specifically can be a problem. Ask your mechanic if he bled the system at all. If he isn’t familiar with V12s he might not know it’s even necessary.
Many have added a bled port for the heater core in the form of a flushing “tee” in the upper heater hose. What I do is open this first and add coolant till it flows out strongly then close it off. Then proceed to bleed the rest of the system which sometimes takes a couple of tries.
In my own experiences an air pocket in the core will result in low outlet temperature and/or inconsistent outlet temps. Hot/cold/warm rather randomly.
Might not hurt to ask him to flush the heater core. Both my V12 Jags have had clogged cores (but none of my 6 cylinder Jags, oddly). Afterwards, of course, the system must be bled again.
When you turn the temp control to full cooling does the system respond by opening the center dashboard vent? And, if max heat is selected, does the system send most of the air to the footwell outlets? If so, then the system seemingly understands the requests and is trying to comply. That’s a good sign although it isn’t proof positive that the entire control side of the system is working properly.
What happens if you select “Defrost”? Doing so should default to max heat and max fan speed and, of course, send almost all the air to the windshield. If you get plenty of hot air in defrost mode then it suggests your heater core isn’t clogged or air locked
In any case, I’d personally make sure the heater core isn’t a problem and work from there.
In addition to Doug’s remarks, Cene - it’s very important to run the engine with the heater heating. This ensures that air in the heater core is flushed out. The xk engine self-flush air out of the system - but the V12 requires some extra precautions during filling, as Doug says…
In infrared thermometer is a good investment - and will likely clarify eventual heater core circulation problem, whatever the cause…
xj6 85 Sov Europe (UK/NZ)