1985 xj6 intermittent heat in winter no Ac in summer

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Just first test the amp for servo function as described, Wes - just to make sure it’s one of the culprits. Even if there is little doubt…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)
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Good Evening to one and all;
Not so good news. I changed the AC amplifier and I still have the same problem as described in my earlier conversations.Removed small vacuum hose from heater control valve located on firewall /no change in behavior .put back hose no change in behavior. center vent does open and close sometimes when it is closed I have heat for awhile then it opens again and I am back to cold air ???

Someone mentioned a setting on AC amplifier .Small blue dial but I do not recall the purpose,or how to adjust setting. I assume it must have something to do with temperature.Am I correct?

Wes,

The small blue dial you mention on the amplifier is a trim potentiometer used to calibrate the amplifer with the rest of the sensing circuit.

Out of curiosity, did you do the sensing system test on the feedback potentiometer?

I cannot stress enough the inportance of a functioning feedback pot. The feedback pot gives a signal to the amplifier to control the direction of travel and balance the sensing system… The range is between 0 -2 k ohms. As the car warms…the temp sensors change resistance and the amplifier moves the servo to try to keep the system balanced… So for example… If the wire wound feedback pot and the servo is moving from 2k to decrease to say 1.5k and the resistance suddenly drops to 0 because of corrosion or a break in the wire winding. The amplifier will try to balance the circuit …which could suddenly go to full cold or vice versa.

Cheers

Gary

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Have actually verified if the servo turns consistently as described when you alter temp settings, Wes…?

The only proof of correct amplifier function is that the system works as it should - there is no other amp test than servo reaction. While unusual, you may have two faulty amplifiers…:slight_smile:

However, as Gary says; the fault may be in the servo - it controls all flaps and other functions mechanically, electrically and by vacuum valves. 'Hot/cold is basically done by resetting (by the servo) the heater flap for more or less heater core bypass.

System switching arbitrarily between hot and cold means that the servo is turning. But it may be heard to react with temp setting variations, though reacting incorrectly - in this case; normal minor amp adjustment signals while driving may cause the servo to overreact due to a feedback pot problem…

Crudely, the amp senses the difference between the in-car temp sensor and the temp control resistances and compare that to the servo position given by the feedback pot. If an error is detected, the amp produces a polarised signal to turn the servo until the error is ‘zero’ as fed back by the servo pot.

The pot wires are also accessible at the (disconnected) 9-pin connector at the amp; yellow and orange is the ‘end’ connectors of the variable pot - while green is the centre ‘pick-up’. Resistance between the yellow and orange is invariant when the servo is turned - resistance between the yellow and orange and orange and green vary inversely as the servo is turned. The variation should be smooth as Gary implies, no breaks - though ‘0’ is likely at the servo end travel…

The servo can be turned ‘manually’ by applying +/- voltage, reversed for motor direction, to red and purple But while there are ‘endstop’ switches to cut power to avoid ‘overturning’; some care should still be taken…

Frank
xj6 85 Sov Europe (UK/NZ)
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Good Morning Gentlemen;
Is the feedback pot an actual part that can be replaced.Your knowledge of this problem is impressive but way over my pay scale .I printed out all responses that I have received and more than likely I will have to go to jaguar service and hope they will both understand and remedy the problem .I do not know what else to do .Any further suggestions would be greatly appreciated
Thank you;
Wes

What is a pentiometer ?I have also noticed when I initially start the auto none of the fan blowers work except “defrost”. After awhile the fans work at all various levels .

Wes,

i have not successfully found a replacement feedback pot that meets all the necessary criteria. Your only options for a failed feedback pot is either a reconditioned servo, or going to a manual control bypassing the amplifer and the sensing system.

Testing is simple if you have a volt meter and 9 volt battery with clip leads. If you message me…we can setup a time that i can call and walk you thru it if you like.

Cheers

Gary

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Wes,

There is a hot water temp switch which will prevent the system from blowing “unwarmed” air on the occupants until the engine has warmed sufficently in heating mode. The switch is located on the hot water pipes on the left side of the heater box.

Cheers,

Gary

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The potentiometer is a variable resistor, Wes - in this case with a ‘centre’ lead…

If the AC system is in ‘heating’ mode, the fans will not work until coolant temps reaches some 41C…

The servo is turned to heating mode if the cabin temp is below the set temp - or if the fans do not work until the coolant is warmed up; the servo is in the heating mode…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)
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Good morning everyone from New York;
Another day of peculiar occurences. I did as suggested and set dial to auto.I then proceeded to turn heater control dial to 65degrees flap on center dash opened .I then immediately set dial to 85degrees the center dash flap closed .I did this about 15 times and each time it worked properly.I had hot heat for about 2 hours while driving.Suddenly the flap decided to open and I was back to cold air. The procedure performed the other day that was successful " flap opening and closing" based on temp dial does not perform as it did .The flap remains in open position and only closes when ignition is turned off.

Wes,

The center flap is vacuum controlled via a vacuum switch located in the Servo. Based upon your description of symptoms…you either have a bad/intermittent amplifer or a bad feedback pot. Depending on ambient temperature, you may remain in full heat mode for a period of time until the in-car temperature rises enough to require action from the amplifier to rotate the servo to adjust flaps or change fan speed. The amplifer either gets a bad signal from the feedback pot causing it to go to full cold or the amplifier has a failed internal circuit.

I have had used amplifers that appeared to be bad…dropped on the floor and then worked for a short period of time. Used amplifiers are a risk…they are still 30-40 years old and do not last forever.

Cheers

Gary

Hello Gary;
Thank you once again. Can this “feedback pot be purchased? Where is it located?If nothing works could I just prevent 'flap” from opening ,so I could have constant heat? Does this make any sense to you?

Regards;
Wes

Wes,

The feedback pot is attached to the end of the servo camshaft. There is not currently a direct replacement. You would need to obtain a used servo with a good feedback pot. While it looks easy…I dont recommend moving a fb pot from one servo to another as they are " clocked" and putting a fb pot in the wrong position can strip the servo gears ( dont ask how i know).

The other option is to install a manul control to bypass the entire sensing system.

Jagaur AC sensing system parts including refurbished servos are available at www.Jag-Aire.com

Wes…please dont just throw money buying parts at this problem. The feedback pot test is quite easy to perform and reveals alot about the condition of your system. You can pick up a cheap harbor freight multi meter/ volt meter and i can talk you thru the testing in less than 15 minutes. At that point you will definitely now where your problem lies and then you can make an informed decision about which route you want to take to get control of your system.

You can message me by clicking on the user icon at the top of the post then click message. I would be happy to talk you through the testing.

Cheers

Gary

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Hello Gary ;That Jaguar website is wonderful I looked at the manual control that fits into ashtray model# MC-17 it appears that will remedy the problem of heat .I temporarily prevented the flap from opening .I have hot heat coming from foot vents and defrost and rear passenger vents. It wants to open as soon as I lower the temp from 85 but it is sealed shut. At least I have heat until such time I look into manual control. I will wait until summer and look into the air conditioning problem.
Thank You very much for all you have done for me. It is greatly appreciated.
Regards
Wes

Wes,
Really sorry to hear that the replacement A/C Amplifier didn’t solve your problem…
Was this Amp new from JagAir or a second hand used unit?
Before you get any deeper I think you must be certain that your A/C Amp is working ok.
I had the exact same symptoms… changed the Amp twice until I found a used one that worked…

As Gary suggests the feedback pot would be the next on the list…
An other possibility could be the Temp sensor (located in a whole above the glove compartment on the underside of the fascia) even though it’s not known to fail…

Best,
Aristides

Aristides;
Thank you for your suggestions. I have printed all information I have received from various helpful persons ,yours included. I am doing what I had stated in my last post.At least, for now, I have heat for the upcoming winter in New York
Regards
Wes

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This of course the standard symptom of an AC amplifier fault, Wes…

…but it is somewhat crude - adding that with two amps showing the same; Gary’s point is certainly relevant. However, it is also relevant that during driving, when the temperature is reached, the fan speeds drops to ‘low’ - and the centre vents are partially open, blowing ‘heated’ air.

An alternative is some problem with the in-car temp sensor. The normal reaction of this system is full heat until the temp sensor signals that the set temp is reached. The servo then resets to ‘blend’ position, and the fans’ speed drops to low - and the main regulation to maintain the set temp is to change the fan speeds. If the system does not behave like this; something is wrong - but it is important that the overall system function is assessed this way; it ay clarify the problem…

The feed-back pot is certainly under suspicion, and should be checked out with procedures described earlier…

The in-car temp sensor is connected by a vacuum hose to the blower casing - cabin air is drawn through the sensor to keep it at cabin air temps. If the vacuum connection is disconnected, or the sensor is heated up one way or another; the AC amp will order the system to ‘cold’. One test of the sensor is to apply heat, cigarette lighter, at the sensor aperture - triggering the system to full cold. Cooling the sensor is a bit more difficult, one would expect the constant cooling of the system is full cold should have some effect…

Taking the car to a professional is viable, but few have the knowledge to sort of this particular problem quickly. It’s advisable that you do whatever you can yourself first - at limited cost…

Frank
xj6 85 Sov Europe (UK/NZ)
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Wes,
An other possible temporary fix:
When the system is on full heat just disconnect the AC Amp and the servo will remain in that seting forever and ever !!

Best,
Aristides

Maybe this could interest you Wes:

Also, in case you haven’t downloaded already, Kirby’s Book XJS Help has a very detailed description and troubleshooting info for the Delanair MKII system.