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I have a 1996 Celebration AJ16 4.0 which has one of the laziest heaters I have ever come across. Starting from cold, I wait till the gauge shows "NORMAL" then when turning the climate control and face vent to warm, receive cold air from the facial vents, despite setting the controls to HOT.
This continues for on average 20 minutes driving and only then does warm air begin to flow. The foot vents on this setting appear to deliver correctly until I have cause to reduce the thermostat to cold, when not only does the facial vent continue to deliver warm air when set back to cold but so does the foot/rear vent. Now to correct this, if I turn on the A/C for a moment, this seems to correct everything and both the facial vents and foot vents produce cold air. However should I then have cause to increase the TEMP (Foot & Facial) it takes an equally long time before any warm air is delivered, particularly to the face vents. As for the DEMISTER settings, I have yet to feel anything warmer than an arctic breeze, so no hope clearing a frosty screen. I hope this makes sense to someone.

nelson1952,

Welcome to the Bulletin Board! There are others more knowledgeable than me on the aircon side, but 'll offer you my thoughts for what they're worth.

It feels to me that because you eventually get the temp you want from the place that you want, then the problem wouldn't be the various temp sensors and solenoid-driven distribution flaps. From what you've said, it feels like a slow-reacting or partially-seized heater valve or else a gunged-up heater matrix. The heater valve can often be prone to failure, so I might start investigating there.

The demist problem might be something different. As you probably know, the demist function on the late cars forces a high fan speed and a forced distribution to the windscreen vents. Is it that it never blows warm air or that it takes a long time to get warm as per the other problems? If it never blows warm air to the screen but can eventually blow warm air to the face and particularly lower vents, then it might be the screen distribution flaps not operating properly.

I hope that's of some little help.

Good luck

Paul

Sounds as if one or both servo motors (rhs of unit top and bottom) are slow to operate. Could be the motor, or the feedback pots (lhs of unit.)

It is possible to diagnose the unit with a high input resistance volt meter, but you really need the book as there are over 50 tests, each with their own individual voltage check.

There's a copy of this on the JDHT's XJS supplement CD JHM1123, buried in the Service Information section.

My thanks to all. I now believe I have located the main source of the problem following discussions with Clarkes Jaguar in East Sussex. The problem appears to be caused by non reacting flaps within the facial air vents, these are supposed to close when the heating is on and the A/C off and mine do not thus preventing warm air from reaching the screen vents.