Having issues with neither of the front heated seats working on my '04 XJ8 . I've only owned the car for a month now, so I don't have much history on this. When I hit the buttons, all 3 leds turn on next to both buttons and stay on, and the heated steering wheel turns on. But I get no heat on either seat. I see this is controlled from the Front Electronic Module (FEM) so I began investigating here. Here's what I found. With ignition on and plug CR10 unplugged, I get +12V at CR10-7 and CR10-'8 which are fed from Fuse F47 in the trunk. (Good) At CR10-11, CR10-13, and CR10-14 I get good ground connection. (Good) Measuring the resistance of the heating elements from CR10-16 to ground, I get about 3 ohms. 14V/3ohm = 4.7A, 14V*4.7A = 65 watts. This sounds plausible for a heated seat. (Good?)

When I plug the harness back in, and turn on the heated seat, I get no voltage from CR10-16 or CR10-17. I assume on high, I should be seeing +12V on these pins. (Bad) One thing I have not checked is resistance of temp sensors.CR1-16 to CR1-18. Anybody know roughly what these values should be when it's 20-30 degrees F?

Assuming the temp sensors aren't out of whack, I'm thinking it must be the FEM. Anybody know if these are prone to failure? I seems kind of odd to me that they power the heated seats directly, and not through and external relay.

 

From one of my old posts-

This week while doing some other work I had the rear seat off and decided to measure the resistance of the sensor at various outside temperatures (early morning and late afternoon). Here are the resistance values and temperatures

55F - 7450 ohms
80F - 4630 ohms
85F - 3620 ohms

In order to make them warmer I added a 1K resistor in series with the sensor (temperature is limited to prevent burning in sleeping,or otherwise unaware passengers).

Adding ~1K ohm resistor in series should make Low equal to the current High, Medium equal to High plus 10F, and High equal to High plus 20F.