Hot feet

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BjornV
Posts: 38
Joined: Thu Feb 08, 2007 8:33 pm
Location: Södertälje/Sweden

Hot feet

Postby BjornV » Sun Nov 27, 2011 9:51 pm

Hello everyone.

My question is about hot feet.
The car in Question a 94 V12 coupé. The air-con is not working because of a leaking compressor.

The thing is that the car has a tendency to allways blow hot air down on my feet. More or less regardless of the position of the temerature knob. Sometimes, it blows un-heated air down though.
It seems tom me that something is partially stuck. Does anyone know where the problem might be located? On the facia I can get un-heated air like expected.
Björn V
XJS 6.0 V12 -94
Morgan +8 -98

PaulGover
Posts: 425
Joined: Mon Jul 19, 2010 8:06 pm

Re: Hot feet

Postby PaulGover » Mon Nov 28, 2011 5:26 pm

I have a '91 version so it might not be the same, but I'd look first at the cabin temperature sensor, fitted below the glove box/passenger air bag. It's a fan sucking air over a thermistor. If either stop working, the aircon will think the cabin is freezing, and continually blow hot air. If not that, then probably something horrible in the linkage directing airflow. In mine it's mostly vacuum pipes, but I think later ones are electric motors.

AFAIR the "supplementary information" CD from Jaguar Heritage has some extensive aircon diagnostic procedures for the older equipment, and probably carries over.
1981 XJ6 Series III basket case
1991 XJS V12 "facelift"
2008 XJ TDVi

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BjornV
Posts: 38
Joined: Thu Feb 08, 2007 8:33 pm
Location: Södertälje/Sweden

Re: Hot feet

Postby BjornV » Sat Dec 03, 2011 8:47 pm

That was interesting. I think I know what that temperature sensor is. Is it possible to test it? After reading more on a Jaguar Heritage CD, I find that a possible problem is the lower flap motor. That does not seem impossible to get to. I wonder if it is possible to test it if taken out of the car?
Björn V
XJS 6.0 V12 -94
Morgan +8 -98

PaulGover
Posts: 425
Joined: Mon Jul 19, 2010 8:06 pm

Re: Hot feet

Postby PaulGover » Sun Dec 04, 2011 7:28 pm

I think one of the standard tests is to start the car with the air con on producing heat. Push the cigar lighter in and let it heat up. Hold it in front of the vent for the temperature sensor. If it works correctly, the heat from the lighter should make the air con cycle round until it blows cold air. If you have the appropriate covers off, you should see the motors moving the flaps.

The temperature sensor is a resistor whose value changes as it gets hotter. As far as I remember, the JDHT disks tell you the appropriate values for a range of temperatures. You can remove the trim panel and disconnect the wiring, and then test it with a multimeter.
1981 XJ6 Series III basket case
1991 XJS V12 "facelift"
2008 XJ TDVi

User avatar
BjornV
Posts: 38
Joined: Thu Feb 08, 2007 8:33 pm
Location: Södertälje/Sweden

Re: Hot feet

Postby BjornV » Wed Dec 21, 2011 7:49 pm

Thank you for the tip. I will test the temperature sensor.

What I have done so far is to test the electric motor for the lower flap. If connected to 12V the motor works, so that is not the problem.
Björn V
XJS 6.0 V12 -94
Morgan +8 -98


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