At last I have real heat from the seat. I spent a long time chasing this system down, thanks in part to Jaguars inaccurate wiring diagram. But the final fix was to remove the thermostat.
At first it didn't work because there was a broken wire under the seat. Then although it should have worked it was impossible to tell using the temp sensors in my buttocks. I made some leads to hook up my ammeter to the fuse (#18, rh heelboard). To my surprise, both front heaters drew current, but only for 30 seconds until the thermostat opened.
After much searching I found the the thermostat in the bottom seat cushion, near the back. By removing the seat back cover, and unhooking the seat cushion cover at the back, I was able to dig out the offending piece, seen here-


Luckily it is wired in with spade connectors, so the easy fix is to plug in a suitable fuse (15 Amps was my choice, since I never saw more than 7.5 amps draw) and assemble the seat.
It's 17 Celsius today, but tomorrow morning ought to be the real test. At any rate, the heat is controlled by the switch now, not a pointless thermostat.

 

Are you saying no more luke warm seats? Is this the way to have nice toasty seats like the ones in my Linclon?

Any worries that they'll get to hot?

 

I will say that anyone doing this mod does so at their own risk. I did find that after 10 minutes I turned it off because it was uncomfortably warm. I don't think that it would cause a problem, but who knows. Our BMW x5 has seats that you could cook hamburgers on, so I wouldn't worry too much.
The initial current draw was about 7 amps, but it dropped off quickly to about 2 amps, where it stabilized. I'll post more as I get used to it, but I'm really happy to get this done, as winter is coming fast.
Now if only the windshield heat worked............

 

Rob,

that's an amazing discovery!!!
I spent hours of tracing down the faulty seat heater of my passenger seat.
The passenger seat only will get luke warm.
(The wife gave me lots of trouble because she likes her b.. very toasty.)

So I will follow your route and exchange the little thermostat with a fuse as well.

Thanks again,

David

 

It might be a better choice in the long run to replace the bi-metal thermostat. I am not sure what the ratings are, but I am guessing it is a normally closed unit (NC) with an open temp around 160F. Here is an example, around 20 bucks:

http://www.newark.com/airpax/c53gab1...tch/dp/34C8790

 

I'm going to track down a Rheostat capable of the 7 amp inrush current at 12 volts so I can adjust the seat temp to my backsides liking.

I'll post the part number, source and location where I install it as soon as I'm done.

I have a nice long trip tomorrow and it's going to be cold in the morning so I'm going to get busy on this now.

Ken

 

I prefer my steaks to the medium side.

Here's a guide for cooking meat:

Rare (saignant) cold red center; soft 125–130°F

Medium rare (à point) warm red center; firmer 130–140°F

Medium (cuit) pink and firm 140–150°F

Medium well (bien cuit) small amount of pink in center 150–155°F

Well done gray-brown throughout; firm >160°F

FWIW, I checked the temps in my Lincoln with the temp control set to max. I used a laser thermometer and took numerous readings while sitting in the seat. I had some errands to run and took readings when stopped at a light. For the readings I lifted my leg and got temps from 106 to 115 degrees F on the surface of the seat. The area of the seat surrounding the body part was a high of 125.4 degrees F on the surface but quickly dropped into the high 80 degree range the further away I got from the body.

That tells me the Lincoln seat (at it's max) is set at around 125 F and that the body acts like a heat sink pulling the temp down 10 degrees or so.

 

I did a bit more research and found from the part numbers that it is supposed to open at 29 degrees C, or 84 Fahrenheit. Seems ridiculously low, if you ask me. Interestingly, Volvos use the same style of thermostat too. Mine will open after 10 seconds of holding it between my finger and thumb, so it's pretty useless for a heater.
Personally I going to leave it out altogether, since finding just the right setting could be a long and expensive process of trial and error.
If you want to know more, check this site- http://www.elfaelectronics.com.ua/cg...912-66&lng=eng
You can also Google Elmwood, the manufacturer, a division of Honeywell.
The numbers on it are L29c M20994 20/98. On the back it says Elmwood T177 10 (6) A 250 V.
So you have the amperage and voltage rating if you want to search for one.

 

Wasn't trying to turn anyone into a rump roast .

Also, I wonder how hot it can get with no thermostat?

Agreed, 125F is probably a safer number.

 

The thermostat is inside the heater "assembly" which seems to be a cloth and foam sandwich lying just under the leather. You need to carefully cut the cloth to get to the thermostat. It's tough cloth, by the way.
The choice seems to be to remove it or replace it, and if so what value? 84 F is pretty cool, so what would you try, and how many would you buy before you got the right one? Is mine operating at the rated 29 C? There's too many variables for me to bother with another one.
For example, I found the thermostat one day, and drove the car for a couple more before I had time to fix it. In those two days, the seat was warmer, although not good enough. I had moved the thermostat a bit, probably enough that my tender gluteus maximus wasn't right on top of it anymore, giving more heat until it opened.
The only downside I can see is if the passenger seat heat was on with no one sitting there. Then it may get fairly hot, but enough to cause a problem? I wouldn't think so.
The drivers seat getting too hot isn't an issue since you just turn it off when when your buns are hot (not cross) (sorry, couldn't resist)

The rear seats (which may do if I have lots of time on my hands) are not a problem since they run through a timer anyway. How silly is that? A timer and a thermostat. Some days I think the nanny State and the lawyers have way too much influence.

 

Just a quick note on what I found today after digging through four seats ...

The seats from a 1999 XJ8 have the single thermostat in the center just like the one in your pic.

The seats from a 2001 XJ8 have two thermostats on either side of center and the connections are soldered.

Since my car had the soldered ones I cut the t-stats out and soldered the wires together.

Mmmmmm nice and toasty.... now. I'll take some temps later but so far they don't seem hotter than the Lincoln was.

 

Had a long drive today and the highest temp I recorded under my legs was 113.6 degrees. I normally like my seats set on "Roast" mode in the Lincoln but these were actually to hot.

I picked up a couple of rheostats today and will look at making the temp adjustable. As soon as I'm sure I have something that will work I'll post.

 

1999 XJ8 - Single Thermostat

Did the mod today.
Let's see if it satifies the wifey...

 

When you say the centre, where do you mean? Did you have to remove the whole seat cover to get at them? I'm interested in doing this mod to my 01 XJ8.
Ta, S

 

I did not have to pull the whole cover off. They were at the rear edge of the cushion just like the photo's show in the first posts. They were spaced about 2-3 inches from the center line of the seat. One on each side.

 

FYI, my 98 turns the heat off after 10 - 11 minutes, so there's a timer built in the control box. So your buns won't burn since it's too hot after a few minutes and if you forget the passenger seat it will shut off by itself.

 

My Jaguar seat heaters work fine. It's my Lincoln seat driver's heater that do not. I have posted on all the Lincoln forums, which get responses a day to a week later if at all. Anyone know why my driver's seat heater doesn't work?

 

I finished off the job today. I fixed the passenger seat in the front, then did the rears. The rear seats are pretty easy since they come out very easily, and now you can work on it at the kitchen table. The "hog" clips they refer to are just heavy staple items, basically folded bits of metal. These hold the seat cushion on. Just pry them off with a screwdriver and put them back on with pair of pliers to squeeze them tight.
Toasted buns all round, now!
Add that to the new windshield (the old one was sand blasted and the heat didn't work) and she's all set for winter.

 

I know its summer at the moment but I just wondered if there was any more news on this?

Sinister 1: Did you manage to fit a rheostat?

Now is a good time for me to do this mod as I have my XJ8 off the road at the moment.

 

Interesting discussion. I am a new Jag owner...1998 XJ8. Is it supposed to have heated seats or was that an option? Just wondering. Bob