1985 xj6 intermittent heat in winter no Ac in summer

Hello ;I have intermittent heat .fans are working "flaps"are opening and closing

If the flaps open and/or close at the same moment that you lose heating, it suggests the amplifier ā€˜thinksā€™ heating is no longer called for and is adjusting the system to cool the cabin.

If you lose heat while all other operating aspects remain the the same, it suggests lack of heat in the heater coreā€”no hot coolant is entering the heater core, or thereā€™s a blockage, or an air pocket. Failures of the heater valve a common so thatā€™s a good place to begin diagnosis.

Can you give a more detailed description of the problem? ā€œIntermittentā€ is very broad.

Cheers
DD

Thank you very much for your reply .The flaps are opening and closing every 5-10 minutes.When I have heat is doesnā€™t last very long.Seems that when blower motor in automatic slows down a little the heat goes on .It is very difficult to pinpoint, based on my observations this is all I can offer.I changed the heater valve mounted on the firewall ,and that seems to be working.Is it possible the thermostat is the problem?

Yes, possible.

If youā€™re driving along and the temp gauge shows normal operating temps, and then starts dropping down, it suggests the thermostat is opening too far (or for too long) and allowing the coolant temperature to drop to a point where the heating system can no longer provide warm air. I had this happen on a different model Jag. However, it isnā€™t a common scenario on the Series III cars.

Try this as a simple jumping-off point:

Disconnect the vacuum hose at the heater valve. This assures that the valve is always open. If your intermittent loss of heating is magically solved then we know thereā€™s a problem with the control side of the system.

Airflow from the outlets provides another clue.

When heating is called for most of the output will be from the foot well vents. When cooling is called for most the air will come from the center dashboard vents. In blend or ā€œstableā€ mode, the air will come mostly from the dashboard side vents and a bit from the foot well vents.

Turn your temp control to the max heat setting. If the airflow switches mostly to the foot well vents then we know the system understands the request for hot air and is trying to complyā€¦even if a fault else where prevents the air from actually being hot.

Cheers
DD

Hello Mr.Dwyer;
As you suggested ,I disconnected the vacuum hose from the heater control valve located on the firewall.Yes, the heat was very hot and constant for about two hours,I did not hear the ā€œflapsā€ at all during this time. Suddenly the heat was gone and very cold air replaced the heat.I reconnected vacuum hose hoping somehow it would make adjustments and emit the heat again. This was unsuccessful.I once again began hearing the flaps. I disconnected vacuum hose.I removed decorative center dash panel and stuffed the opening so the louver could not open or close. At this time ,I am getting warm not hot air coming from foot vents and defrost vents if I choose that setting. I can not completely express my gratitude for your assistance and knowledge relating to this problem.

Ok, good.

Two hours of hot air pretty much leaves out flow thru the heater core as a problem. Thatā€™s something!

Drat.

Coolant flow thru the heater core seems OK so what youā€™re feeling, i suspect, is refrigerated air.

The default mode of the system is to cool the air; all incoming air is first refrigerated and then heated only as needed/requested. This is accomplished by opening a flap to let the refrigerated air pass thru the heater core. It would appear that this was happening for about two hoursā€¦and then stopped happening. But why?

Iā€™ll have to do some mulling. While I mull others will hopefully chime in. Faulty amplifiers are common in these cars and can cause weird operating characteristics. Iā€™m not quite ready to condemn the amplifier just yet, though. That would be too easy.

Also, when max heat is called up on the control dial does the system send the air primarily to the foot well vents?

Cheers
DD

Mr. Dwyer;
When max heat is called up on control dial the air primarily goes to the center and side dash vents. That is why I temporalily blocked any air coming thru them.I do have mildly warm air coming out of the foot vents. Is Sherlock Jaguar required?Or as I believe you are him!
Wes

I have seen this kind unpredictable and illogical actions by the climate
control systems in my 3 Series III saloons at least 3 times. In all cases
a replacement A/C amplifier fixed the problem.

Paul

Wes,

There are two likely causes of your main problemā€¦although you may have others. You likely have either a faulty amplifier or a bad feedback potentiometer.

In my XJS during the winter i would have heat for 5-10 minutesā€¦then the system would go straight to full coldā€¦made the car unbearable to drive in the winter.

The feedback pot is a wire wound potentiometer and is part of the AC servo. As the servo changes positionā€¦it provides a signal to the amplifier to ā€˜reportā€™ the servos position. I have seen feedback pots fail in one of 2 waysā€¦either corroded ( which can be cleaned) or broken wiresā€¦( not easily fixed).

The amplifier contains two relaysā€¦which after 30-40 years failā€¦ In some casesā€¦the relay contacts fail and will drive the servo to its limitsā€¦drive the car a while and hit some bumps and the system may return to a " normal" operation.

I recommend testing the feedback pot first. If it does funtion correctly and or drops signalā€¦the amplifer gets bad data and can cause unexpected results. You can find instructions on how to test the sensing system at www.jag-aire.com website. Go to the amplifier product page and download the manual. It contains the directions on how to test the system.

Finally, as Doug questionedā€¦if you do not get heat from the footwell ventsā€¦you may have a vacuum issues as these lower heat flaps are controllex by vacuum.

Let us know the results.

Cheers

Gary

**
That the centre vents open when ā€˜heat is called upā€™ implies a control problem, Wes - almost certainly the AC amplifierā€¦

And you do not ā€˜call up heatā€™; the system is automatic. The AC am senses the difference in the cabin temp and compares it with the temp set by the driver. If there is a difference - the amplifier 'orders the servo to turn. The servo controls the flaps, heater valve and fans mechanically, electrically and with vacuum, and sets them as appropriate to heat or coolā€¦

To rapidly reach the set cabin temp, the AC dictates max cool or heat as appropriate. When the set temp is reached, the system goes into ā€˜blendā€™ - the air is around the set temp, and the fans drop to low speed. In ā€˜full coldā€™, the centre vents are open - all other vents, except side vents (manually controlled), are closed. In ā€˜full heatā€™, the centre vents are closed - all other vents, except defrosts are open. In ā€˜blendā€™ vent openings are open to the extent required to maintain set tempsā€¦

If the AC amp fails, the heating/cooling is either random - or does not to react to changes in temp control settings.

To check AC amp function; turn the function control to ā€˜autoā€™ and alternately set the temp control to ā€˜65ā€™ then ā€˜85ā€™ repeatedly while listening to the 'buzz ā€™ of the servo turning. It must faithfully buzz for some 30 seconds in each extreme position each and every timeā€¦

If the servo does not react, or react intermittently; it is defective and must be changed. Further faultfinging with a faulty AC amp is pointless - the amp must function properly before searching for other faultsā€¦

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

I second that.
The AC Amplifier is known to cause problems Wes, I had the same erratic behavior and it was exactly thatā€¦

As suggested, check you vacuum lines, either you have a leak or they are disconnected.
Most of them are accessible behind the right side lower panel of the center console.
The easiest way would be if you had a vacuum source, connect each hose to the vacuum and try to listen or see if flaps are moving.

Aristides

Hello
I would like to thank each and everyone of you for the valuable information you provided to me ,concerning my problem.It seems you all agree that the ac amplifier is the villainā€¦I have one final question. Where is the ac amplifier located?
Thank you all so very much
Regards;
Wes Wickers

The A/C amplifier is located inside the center dash beneath and forward
(towards the engine) of the radio. It is very difficult to access and
remove. If your A/C amplifier is original will see it as a small brown
rectangular hard plastic box when you remove the heater vent and vinyl
cover near the driverā€™s right knee if you have left hand drive car. It
has a rainbow of colored wires going to itā€™s white plastic plug. I removed
the first one that failed on me and then got smarter and just left the next
bad ones in place as it is very difficult to remove.

Mr. Novak;
Thank you for taking the time to answer my question.It is greatly appreciated
Regards;
Wes Wickers

**
Just first test the amp for servo function as described, Wes - just to make sure itā€™s one of the culprits. Even if there is little doubtā€¦:slight_smile:

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

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 systemā€¦ The range is between 0 -2 k ohms. As the car warmsā€¦the temp sensors change resistance and the amplifier moves the servo to try to keep the system balancedā€¦ So for exampleā€¦ 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 ā€¦which could suddenly go to full cold or vice versa.

Cheers

Gary

**
Have actually verified if the servo turns consistently as described when you alter temp settings, Wesā€¦?

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 amplifiersā€¦: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 problemā€¦

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 ā€˜zeroā€™ 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 ā€˜endā€™ connectors of the variable pot - while green is the centre ā€˜pick-upā€™. 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 ā€˜0ā€™ is likely at the servo end travelā€¦

The servo can be turned ā€˜manuallyā€™ by applying +/- voltage, reversed for motor direction, to red and purple But while there are ā€˜endstopā€™ switches to cut power to avoid ā€˜overturningā€™; some care should still be takenā€¦

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

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