I am having serious issues controlling the heat in my 2001 XJR. It, on its own changes from blowing heat to blowing cool to cold air. When I then turn it off it comes back to heated air. It doesn't seem to have a consistent theme it does it indiscriminately. At times it seems it is somehow related to the engine RMP's and then other times it just kind of changes on its own. I have replaced just about everything in the HVAC system, Heat Pump, exchanger, coil. I have even replaced the control panel. Has anyone out there ever had this problem?? If so how was it fixed ?? I am getting tired of having to start and stop the system. When you live where I do it is a real inconvenience. [

 

Have you tried looking at the cabin air temp sensor (motorised aspirator)?

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Run the diagnostic and see if you get any codes -- you can also run the self test. Do a search.

The CC's water pump must be working to get enough heat -- also the valve. Have you confirmed they are working?

When you do get heat -- how hot?

 

I have run the diagnostics several times and no codes come up which is the ultimate confusion. My machanic is taking the car for a couple of days next week and hopefully when the problem comes up he can put the portable computer on it and figure it out. Sometimes the heat is very hot sometimes it is just very warm and when the problem is evident it is cold. I can tell you we replaced the water pump, the coil, the condenser etc. etc. I thought about the cabin air temp sensor but have not replaced it. My mechanic says it is by the firewall and would require pulling the dash. He is reluctant to do all that labor if it is not obvious what the issue is.

 

Are you speaking of the engine water pump -- or the electric pump for the climate control... The XJR also has another one for the intercoolers.

Has the valve been inspected -- it has an unusual operation. Not sure what the "coil" is referring to and the condenser would not have anything to do with the heat.

If you get hot air occasionally -- then the matrix should be free flowing.

If the cooling system is working correctly and the engine is warm - it has to be the pump or the valve.

 

When it is faulty & blowing cold, what happens if you set the climate control to HI? If it then starts to blow hot, that would implicate the control, including the cabin temp sensor. If the air continues to blow cold, that would suggest the problem is elsewhere.

 

Your mechanic is not aware of the motorized aspirator that samples cabin air. In both of my cars it's located behind a little grill in the lower part of the dash panel above the hood release. It could be full of dust, etc. or not not running.

Also check that the thumbwheel for the face vent in the center air vent is in the middle position.

 

Does anyone know how to replace the cabin heat sensor behind the truck release without removing the dash??

 

Take out the coin tray; it's accessible from there without removing the dash by pushing the trunk button and surrounding grille forward by pressing up on the latch (you'll have to feel around for it; IIRC, on the bottom of the assembly) and gently pushing it into the passenger compartment.

The complete process is in the shop manual, electrical section.