My seats don't seem to heat at all. Both switches light up normally. Steering wheel heats. Help.

 

If they are the same setup as the XK8, the seat heaters only operate when the temperature drops to near freezing regardless of the switch position. The Driver's Handbooks don't make this clear.



The above section from the X350 handbook I've outlined in red warns about the heaters not operating but doesn't explain the ambient temperature limitations.

Graham

 

I guess that explains why mine work great when the car is cold but seem to stop working after the car is warm for awhile.

 

Used to my F350. Seat really gets hot.
The Jag does warm, but not very much.
I start off using it and by the time my hands are burning, I turn it all off. Kinda wish there were separate, individual controls...

 

Just razzin ya n9921x. Maybe its just a matter of swapping the t-stat to the wheel and/or seats for raising/lowering the set point. Now, "where is that sucker located?" is the question.

 

Matt,
I like your idea. With my luck I'd screw it up and neither would forever work.
My car came from Ft Worth. White Vdp. Park Place.
I love it.

 

Glad our metroplex could provide you with a good vehicle. VDPs are so nice, ohh the leg room in back! Makes me want to sell the house, rent an apt, and hire a driver for me. (thats the only way I could afford one)

Try the t-stat idea, and let us know!

 

Its controlled by a thermistor thats in the seat with the heater pad.
You can see this clearly in the electrical diagrams

Just play around with resistor the values in parallel to raise the temp

Cheers
34by151

 

Thanks for the tip, unfortunately, I've never completely understood electricity...got a C in my electrical engineering class, and I think that was being generous. I can't even calculate or figure out the right resistor value to keep an LED bulb from remaining lit in a 12V system when the key is pulled.

 

To keep it simple

A resistor in series increases the value
A resistor in parallel decreases the value
What you need is one in parallel, ie lower the resistance

Sill keeping it simple
If you increase the resistance you lower the voltage and vise versa

How the "Temp Sensor" works. Its a variable resistor.
Think of it like the volume knob on a stereo but instead of you turning the knob its done by changes on temp. Thats a what a termistor is, just a variable resistor that changes value accordion to temp

How does this work:
If he FEM has 3 wires going to the seat, the seat has 4 the extra one going to earth

From the FEM one wire goes to the heater pad and then to earth
The other 2 wires are the thermistor or temp sensor

So what you need to do is install a variable resistor on the 2 wires connected to the FEM
These are on connector SD28-14 and SD28-13. I used a 1M one, but anything over 1M will do.

See here
1M 16mm Potentiometer Linear Single Gang (B) - Jaycar Electronics

You can also use this which is what I used
Resistance Wheel - Jaycar Electronics


Changing the settings:
Have the variable resistor first set to the highest resistance so it does not affect the settings.
Turn on the seat heater on the highest setting.
Wait for it to get up to the normal temp
Now wind back the variable resistor and the seat heat will go up.
Give it a few minutes to catch up its not instant
Once you get it to where you like, leave to be your all set

You can leave the variable resistor as is, just put some silicone on the shaft so it does not move due to vibrations over time. Alternatively. I just used a multimeter to read the resistance then purchased a resistor close to that value and added it in permanently

Cheers
34by151

 

My question remains... Is it electrically possible to loose heat on all four seats at the same time, while the buttons still illuminate? Where would the common fault exist? I'm especially curious, because fuses are placed into separate front and rear modules. The sophisticated electrical advice is fun to read, but I'm not looking to rewire or modify any electrical components. I just want to ensure that everything is working as it should have from the factory. I've read the manual notice that states the seats might not work with higher ambient, body, or garage, temperatures. I drove the car in sub freezing temperatures and thought it was only my imagination warming the leather. It's funny to read other posts about the wife wishing the seats were hotter, that's exactly what prompted my search for these facts.

 

I figured I should follow up on this: my seats work as they should. They definitely provide heat. They are very warm, but they do not provide scorching heat like some manufacurers provide. The steering wheel does get hotter than the drivers seat.

 

The temp is limited by 2 things

1. The setting on the switch as indicated by the led bar
2. The Temp sesor

So once the seat gets to the max temp it custs off the heater pads.
The only way to increase the max heat setting is to alter the temp sensor in each seat.

IE a lower signal means it gets to a higher temp before cutoff.
You are just fooling the FEM by giving it less feedback

Lowing the sensor increases the cutoff of all 3 positions
Unfortunately this is the cutoff Jag have set and its not able to be changed by any other means

All the details on how are in post 10

Cheers
34by151

 

Graham:
Is it possible for the front seat heaters to work in 40F ish weather and the rear seats will not work, but the lights go on due to the temperature limitation, or perhaps is there another problem? I also took off the rear control on the back end of the center console and I do not see any connection to the left most plug which I believe has 8 pins on it. Perhaps there is something disconnected that needs to be connected. I am not the original owner of my 2004 XJR. I also checked the fuse in the trunk (boot) and found fuse number 13 to be OK.

thanks any others who may have thoughts on the matter

RyeJag