Erratic AC temperature on XJ12, 1992

Doug, I have read the procedures for proper bleeding of the coolant system (raising the car little bit, and all the rest), even your own post on that but I have two questions - doesn’t by doing that procedure also heater core gets bleeding? Same question about flushing the system - do I have to bleed and flush heater core separately? If so - how do I do it?

I just hope no access to heater core is needed for that - I would not dare to do it, even wouldn’t allow mechanic do it… for me it is mission impossible, watched video on you tube…:slight_smile:

I will try all you asked when car is back, for sure.

Thanks Doug, all suggestions are appreciated.

Cheers,

Cene

Yes, I see - will check all the posts on bleeding again, thanks!

Cene

Perhaps the best way to flush the heater core in situ is to disconnect both hoses, one from the RH coolant manifold on the engine and one from the pipeline to the radiator outlet, hook up a garden hose and flush water through it backwards, opposite of the direction the coolant normally flows through it. You should be able to tell if you get decent flow, whether or not any junk comes out. If it seems to flow OK, you’re good. If you barely get a trickle, you’d probably need to get the heater core out and either have it professionally cleaned or just replace it.

I dunno what video you watched, but R&R’ing the heater core is not particularly difficult on a '92. It’s really not too difficult on an earlier car either if you’re willing to shortcut the job and cut the supply pipes and put the new core in with short hoses. If you’re not willing to do that, R&R of the heater core on the Delanair MkII is a bear. Basically, the factory suspended that heater core in mid-air and then built the car around it.

Great, thank you for that - will sure try it!

Cheers,

Cene

you mean leave the old core where it is and put new on one the system? Where?

Cene

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Indeed it does, Cene - getting air out is done by circulating coolant…

Flushing the heater core to get rid of ‘obstructions’ inside, as described by Kirbert - which won’t hurt, but may not help much either. After flushing you air the system as described…

Frank
xj6 85 Sov Europe (UK/NZ)
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Thanks, Frank!

Got my Jag back yesterday, and did again the proper bleeding later at home. Beside at least 4 points where leaks were, there was also loose temperature sensor (not in the cabin)… first drive home was OK, the temperature was nice warm and never got colder by itself…so I guess and hope that was it… will see if that is the case in next few days.

Thank you guys for all your help and suggestions. Now I will open new topic on cruise control…:grinning:

Cheers,

Cene

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Certainly good news, Cene!

Frank
xj6 85 Sov Europe (UK/NZ)
**

Guys, now some much warmer days are here… I can see kind of pattern… and I again suspect that the cause is in in-cabin sensor (?). When we have sunny day with warmer temperature, it seems AC is very sensitive - too sensitive, really. It blows cold - I have to turn the knob to say 75 - 80 degrees in order to stop that cold blowing… eventually, after few minutes it gets warmer or even quite nice warm. Turning back the knob… just a little, towards colder, turns AC full power cold - too and more cold that should be in that position… When there is no sun and colder, it seems AC works fine, warm enough.

Any idea?

The system is somewhat slow reacting to the temp control, Cene…

Working perfectly; set to, say, ‘75’ and cabin temp above this - the system will go to ‘full cold’, fans at high speed. When the cabin temps reach set temp; the fan speeds drop to ‘low’ - and the servo resets to a ‘blend’ position. As programmed into the AC amplifier based on ambient air temp. If the cabin temp drifts off the set temp; the fan speeds will change to ‘high’ but without noticeable vent temp changes.

The same sequence applies if cabin temps are below set temp; the system goes to ‘full heat’ - but in this case; fans will nor run until coolant temp reaches 41C. After that; heat is applied until set temp is reached, fans at ‘high’ - again dropping to ‘low’ at set temp. And so forth…

If you operate the temp control at any time; the AC amp will send the system into ‘full cold’ or ‘full heat’ as appropriate - directly changing vent temps accordingly.

So the ‘appropriate’ use of the system is to set the temp to ‘75’ (or whatever) - and leave it there. It will indeed blow (too) ‘cold’ (or ‘hot’) until set temp is reached - this is how the system is designed to work.

Since you manipulate the temp setting; your symptoms description does not in itself imply a system/component fault. You have to let the system do its thing, observe the outcome - then consider.

It’s certainly possible that something is malfunctioning, but drivers’ perception of ‘too cold’/‘too hot’ is unreliable - you need a thermometer to get impartial verification for proper remedial actions. So…?

Frank
xj6 85 Sov Europe (UK/NZ)
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I see, Frank. It might be the case, indeed. If the system is as you wrote - by going to desired temperature full power until reached - then in these days when the temperature outside is not yet so high but the sun is already more intense then, say month or two ago…is is quite possible that the cabin of the car is warmer then one would think - especially when you are outside for a while.

I agree that the actual temperature and what one ”feels” can be quite different. And if the Jag’s AC system works as you wrote; it does not go to desired temp gradually but only ”on full power” - then I guess it is quite possible that my feeling is too cold for first few minutes.

I guess I will indeed check that with thermometer.

Thanks, Frank!

Cene

Hi Cene,

keep also in mind that Jaguar’s interior heating process is different from about any other make.
Jaguar chose to always heat the incoming air, but then cool it back down again with the AC when lower interior temperatures are selected / required.

Other way 'round, Patrick.

Incoming air is first refrigerated and then heated as necessary.

Cheers
DD

Yeah Doug, right you are… my bad :wink:

Verzonden vanaf mijn Galaxy

-------- Oorspronkelijk bericht --------

Thanks, guys… two days ago driving my Jag… engine temperature started to rise, I saw smoke going from the bonnet… the plastic T with temperature sensor between hose (see on the photo above the transmission fluid dipstick) cracked… and the coolant was everywhere…

So I guess that sensor even before finally cracked, was not sending correct info… is that possible?


Cheers,

Cene

that one, see.

:slight_smile:download

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Sure, Cene - but it is more likely that the loss of coolant caused the temp to rise. And that the temp gauge’s sensor did exactly what it was supposed to do…?

Frank
xj6 85 Sov Europe (UK/NZ)
**

Sure, the coolant loss caused the temperature rise when sensor broke. But I was wondering if the T sensor, before finally broke, was cracked a little bit but still hanged on - but sending wrong info and cause strange behaviour of AC?

Waiting for new one to come and will see,
Cheers, Cene

Ah, forgot to ask - that T sensor measures the temp for water going to heater core, right? That one does not sending info to temp gauge? Or does it?