It seems either the wire is broken at the element or the element itself is bad. The only reason why I don't know for sure that the wire isn't broken is even though I can trace where the wires go through the lower pad and I can't pull any wires down through the opening, the element itself is sandwiched between the pad and the leather and I'd have to remove the hog rings that hold the piping down and those go straight through the pad. I looked for broken wire inside the heating element, but these look like the thinner carbon fiber style warmers instead of imbedded wire elements. Do these not usually fail? I can't find a single failure on the forum that didn't go back to a bad fuse block. I've even checked the warmer for resistance and it shows full open. Am I missing something? It's only the driver's seat that doesn't work. I noticed that the passenger seat only works if someone is sitting in the seat which I assume is in conjunction with the air bag sensor, but it gets nice and warm.

 

A couple of suggestions. There are DTC's for seat heater faults, but you'll likely need IDS/SDD to read them.

I don't know if this true on the X350, but most seat heaters have an inline bimetal thermostat at the heating element. On my XJS, this is what failed, and all you had to do was solder in a replacement. I know this is also a common failure point in Range Rovers and others.

The two weakest points in seat heaters seem to be either wear induced broken wires, or the bimetal thermostat. Both are easy fixes themselves, but you have to pull the seat and undo the hog rings to gain access.

The spec on the XJS thermostat is "45°C/12V/10Amp bimetallic" and I found replacements online.

 

That's exactly where I'm at. The seat heater has four wires. Two for the thermostat circuit and two larger wires for the power and ground of the heating element. The heating element itself shows an open circuit when tested at the plug under the seat. I thought I might find a broken circuit at the connector plugs at the pad, but that doesn't seem to be the case. From how far I was able to get into it, it's probably an issue in the upper pad because there was at least no visible damage or burn marks typical of a failed seat heater in the lower pad.

 

My drivers side warmer burnt out as well

Ill call it a a warmer as it's never hot enough to call it a heater!!!
Also I had modified the thermistor to make the pads warmer.
So im guessing the coils in the warmer are not that great and burn out easily, especially if you turn up the current!!

I replaced mine with a set off Ebay (that are much hotter)
As the new ones don't have a thermistor I just added one to the wiring under the pad

Haven't had a problem after that

Cheers
34by151

 

Was your pad hog ringed to the leather? I'm thinking I'll just replace it for the sake of going to the trouble of messing with it more. I do know a guy that used to do heated seats so I could have him walk me through it, I'd just hate to mess up the leather cover. The wiring couldn't be simpler in the sense of electric circuits go anyway.

 

I have the XJR so extra bolsters in the seat
I gave it to an upholsterer to take of the leather skin, and put the skin back on.

I just did the work on the pads for the same reason as you.
But I think I would tackle it now as the leather is showing age on the bolsters
Then Again Id consider replacing the leather as well

Cambo has done it so you may want to PM him.

BTW, If I was doing it again i'd also fit a massager unit to the upper seat back.
All in all im glad I burnt out the OEM pads (my fault) as they are next to useless for producing heat. In saying that its really cold here if it drop below 20degC. Below that and I reach for the seat heater so im guessing we want it hotter quicker

Cheers
34by151

 

How would you/could you add a massager? Sounds like an upgrade is comming soon, to a theater near me!

 

Here is a link to a pad kit with massager
Premium Seat Heaters With Massager

Cheers
34by151