My Jaguar usually sits out the winter, but as we haven't had any snow yet here, we took the Cat out for a drive tonight to visit some friends about 45 mi away. The temperature is just about at freezing. (32F, 0C)

Something doesn't seem to be right in the heater, maybe several somethings. But two issues are noticeable:
1) After a cold start, as the engine begins to heat up, there is at first a blast of hot air out of the heater. Either through the dash vents or onto the floor, depending on the selector switches. After about 3-4 minutes, the air flow becomes COLD, for about 3-4 minutes. Then hot again, then cold, and so on. As the 3-minute temperature oscillations continue, the hot limit becomes less hot, and the cold limit less cold. So after about 45 minutes, the temperature of the air coming out of the vents oscillates between "luke warm" and "luke cool." It never really settles down to a steady temperature. And to have any comfort in the cabin, the temperature has to be dialed up to well over 80 degrees - though it never gets that warm inside, maybe only mid 60s. If the temperature is set to (say) 72, the interior temp oscillates about some cooler value, feeling more like mid-50s. I have no idea what could be causing this. Bad cabin temp sensor? Heater booster pump? Partly clogged heater core? Evil spirits?
2) With the air flow button for "panel" pressed, air comes out the dash vents (of course). However, when the air flow button for "floor" is then pressed, the left-hand dash vent does not shut completely off, and a certain amount of air comes out. The center and right dash vents shut off completely under this circumstance. A bad flap valve at the left of the dash? How could I troubleshoot this?

Thx!

OH - more re item 1): During one of the temperature oscillations, with the air selector in the "panel" mode, I noticed that warm air was coming out the left center dash vent, while cold air was simultaneously coming out the right center dash vent!

It sounds like evil spirits and bad carma in your past life Now seriously - the first issue you're describing is most likely to be the temperature sensor inside the car. I would check this one as a first step. If the core was clogged you wouldn't be able to get any hot at all... the changing temperature of airflow is either some sensor or "air bubble" inside the cooling system.

But yet... there is another scenario:
Did you noticed relation between your RPM and the temperature of the air coming from the heaters? If so - maybe the problem is with the water pump which could has a broken propeller and when you rev up the engine, the pump runs fast enough to rotate the coolant and warm the heaters. BUT! If you experience lost of heat when the car is moving fast with low RPM or without touching the acceleration (for example going downhill)- than I would recommend you check the thermostat.

The temperature oscillations occurred while driving between 45 - 55 mph. The oscillations were regularly timed, and did not seem to correspond to any speed or rpm. As a former engineer, this smells to me like an unintentional "feedback loop." Only question is, what is causing the oscillation? And why does it not get "really hot" in the car, no matter how much the heater temperature is cranked up?

I suppose, when the weather is warmer, I can buy some replacement parts - control console, for example, and see what happens. I had been wondering whether it seemed plausible for some flaw in the booster pump or the valve to cause this. Doesn't seem like it, though. Maybe it just doesn't get very cold in the UK, where the cars were designed! The system does seem to do rather a better job when the outside temperature is above 45F or so.

I'll have to see if I can find out where the inside temperature sensor is. It ought to be hanging down from the roof, in the middle of the cabin!

I hope this will help you... Check the attached image.

P.S> I just realized I had similar issue like yours - heaters with changing air temperature and it was the heater pump which was clogged and because of this the propeller inside occasionally stopped and started to spin again. It happens because the pump is working with magnetic connection. It's very easy to check it - just lay hand on the pump and feel if it's working.

Hmmmmm....... can't tell from that diagram, just where the interior temp sensor is. There is listed an "aspirator motor." That sounds like something that sucks air past the temperature sensor. I wonder if THAT could have failed??

You might check the ambient air temp sensor also.

Hi Boomer
I found the Diagnoistics for the Climate Control Module in the JTIS21 manual.

Apparently some of the Climate Control component faults flow into the Instrument Cluster module and it is send (and stored) into the Climate Control Module as special climate control fault codes.

These special fault codes are accessed by using a combination of climate control buttons (AUTO & recirclation buttons press at the same time) then turning the ignition switch to the "on" position.

I added a post in my Heating & Air Conditioning Systems Info thread that gives all the information on the procedures to test, view and clear these special fault codes.

https://www.jaguarforums.com/forum/x...nfo-faq-66454/

Boomer, also open the Climate Control Components PDF and look at the processes on how the components work together (how information from one component tell the other to open duct, etc). . You might be able to solve your issue by looking at all from pages 6 to 15. Looks like page 9 lists the inputs and outputs with conditions tested & acted upon.

http://www.mediafire.com/?8vge5xa3hs2ptl3

Jim Lombardi

Jim-

Thanks for digging up these details. I'll check my unit out whenever the temperature climbs up out of the basement! I'll have to come down your way and pay you a visit in the Spring sometime.

Not sure but it sounds like every time the thermostat opens it allows hot coolant to enter the heater coil. This leads me to believe that the coolant / heater pump may not be working. Check your fuses for it. If they are good I would see if coolant is passing the heater pump. Now this may be difficult and I will try to explain. The pump motor can run without turning the pump, this is because the motor and pump are only connected by a polarized magnetic shaft. This allows the motor to turn if the pump is clogged or jammed. This link should help you with the pump. Jaguar XJ6 Aux heater pump If I recall on my 95 you have a valve or two above the pump that works with the heating system.

Gus -
I was wondering about that coolant pump. Too cold now to mess with it, but it's on my list for later on. My other thought was the cabin temperature sensor, which is evidently behind the left side of the dash area. I believe a vent and hose lead to a small fan motor and thence to the actual temp sensor. Not sure how to get at that part, exactly. But again, too cold now to mess with it! I'll wait till the temps get back up into the 40s anyway.

Are you referring to the sensor mounted on the dash?

Yes, or behind it, or wherever the heck it is! How do you get at it?

I am not sure how or where it is but I got a drawing for it. I cleaned mine in my S-Type with Mass Air Flow Sensor Cleaner and it worked well. You may want to try that. I can tell you that cleaning my pump on my 95 worked well.

it's the pump.

At least I know where the heater pump is! What is the usual failure mode with these, since they don't have a direct mechanical drive?

"Failure Mode" do dat mean how it broke? Boy you weren't kidding when you said you were an engineer.

The impeller of the pump could be hanging up. Check Gus' page in the link he posted above on how to clean it.

FWIW, I'm in the same boat with heater issues too. It's 8 degrees F here and I have to drive around town in second gear to keep the revs up so that enough coolant gets to the heater core.

Looks like I'll be under the hood this weekend.

Fortunately, my Jag is not needed as a daily driver, and I can work on it under better conditions a few weeks from now. As my wife pointedly reminds me, I could work on my cars in the garage, except the garage is full of junk....(er) lawn and garden machinery.

I was actually a scientist more than an engineer. If I were a true engineer, I probably would have said "root cause" rather than "failure mode" ( ! )

FWIW, when you check your pump function note that it is not energized until the engine is running.

The cabin temperature sensor is part of the assembly that holds the remote trunk release button on the farthest left hand side of the dash at the bottom. About where your left knee is. You might try blowing it out using a can of compressed air through the grilles on each side of the switch.

Plums, do you know how to actually remove the sensor? Or so complicated as to not be worth it?