Heated Seats Not Working - Help!

Mike_Mac · February 1, 2011

This is now driving me nuts! Both my heated seats don't work and I've tried everything I can think of, with no joy.

(Car's a 93' Mk1 GS300 BTW).

The fuse is fine.

The switch has been swap tested with another with no joy. Both switches lit up fine and showed 14V flowing to the correct wire when the relevant switch was thrown.

The seats have been swapped with a different set that were working when removed from the other car, with no joy.

The wires that go from the switch to each seat have been checked - both have 14V going through them at the seat multi-plug when the switch is on - no joy.

Both seats other electrical bits work fine and their common power and negative feeds are fine, showing 14V flowing.

I've heard that if one seat has gone the other won't work either (don't know if it's true, but can't see how it can be TBH), but I've tried both new seats in different combinations with the old ones with no joy.

This is really doing my head in and has wasted over two weeks of fiddling in my spare time. Can anyone think or know of anything I've missed - a relay, or similar buried elsewhere in the car, something else? Or if anyone knows of a decent Lexus specialist somewhere near Wilts that would be useful?

Getting close to doing a Basil Fawltey!!! :D

TigerFish · February 1, 2011

If the lights in the switches light up ok then the fuse is fine anyway.

I can't find anyway that one seat can have any effect on the other seat. They both have seperate supplies from the switch, and both the seats, and the switch illuminations have different earth points. So unless there are a load of earths not connected, I can't see any connection at all from one side to the other. Each seat seems to have two halves to the heater circuit. A thermostat controls the overall temp of the whole seat, and a second controls the lower temp range. I assume it is like a safety feature, making sure if the temp goes too high, the whole heater is switched off. If the seat back harnes is not connected to the seat base harness, then that would stop the heater completely, as the circuit goes through the base, then the back and then to earth.

Are you sure they are not working? Did you feel the other seats working before removal from the other car? On my mk1, it seemed to take forever for the seats to heat up, on my mk2, they are much, much faster, almost instant.

Have you tested continuity through the seats?

On the seat base plug,

At seat temp below 30°C, there should be continuity between pin 1 and 3.

At seat temp below 30°C, there should be continuity between pin 2 and 3.

At seat temp above 50°C, there should be no continuity between pin 1 and 3.

At seat temp above 40°C, there should be continuity between pin 2 and 3.

On the seat back,

pins 1 and 3 should have constant continuity

pins 2 and 3 should have constant continuity

Pin 3 to earth should have constant continuity

Other than that, I can't think what else to check.

Mike_Mac · February 2, 2011

Thanks for that TigerFish - I only have my last GS Mk1 to go on and its seats got noticeably warmer reasonably quickly. This one just doesn't seem to change.

I didn't feel the other seats working, but it's unlikely that both would be duff, never mind one.

The problem is when you're holding your hand against the leather to check, it will warm up from bodyheat anyway!!

I've got the backs off them just now anyway, so will check continuity as you said - especially the back harness. After that I'm going to start applying a live source to the wires in the seat to see if something's wrong there.

Thanks for the reply.

Mike_Mac · February 2, 2011

Also - If anyone in the Salisbury/Wilts or Exeter/Devon areas has a Mk1 with working seats and doesn't mind - I'd really appreciate being able to come over and check your seats vs mine. It would only take about 30 minutes max? :)