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Old 30th November 2019, 22:31   #1
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Default Earthing point for heated seats?

Hi all,

my heated seats have become intermittent, but work fine out of the car with 12v directly applied.....

After extensive fault finding, switch and fuse replacements etc, I suspect now that I have an earthing problem. As the 75 has a multitude of earthing points, and before I tear my car apart looking for them, can anybody please direct me to the most likely earth?

Thanks!
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Old 1st December 2019, 16:28   #2
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According to the Haynes manual earth point are at base of the A pillars. If you have a multi meter you can use the continuity setting to confirm that, but I'd assume that's where it will be.
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Old 1st December 2019, 18:26   #3
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Failing finding an existing earth point just make your own to a nearby part of the cars body.
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Old 2nd December 2019, 10:15   #4
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Earthing point found, sadly no corrosion as a 'smoking gun'. All cleaned up and re-torqued up, but still an intermittent power supply to the seat, (intermittent meaning 7-10 seconds). Anyone like to explain why the seats are switched with an earth rather and positive?
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Old 2nd December 2019, 11:37   #5
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Quote:
Originally Posted by Falcon Flyer View Post
As the 75 has a multitude of earthing points, and before I tear my car apart looking for them, can anybody please direct me to the most likely earth?
The earth cables from the switches are connected to pins 15 & 16 of the 20 pin header (sealed connector) at the bottom of the centre console, along with several other earth cables. It is a cable from that header that is connected to the earthing point at the base of the 'A' pillar.

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Old 3rd December 2019, 11:33   #6
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Originally Posted by Falcon Flyer View Post
Anyone like to explain why the seats are switched with an earth rather and positive?
I don't know James but looking at the wiring diagram in RAVE the seat heaters have three switches in series. First the ignition switch supplies the +12v, then there's a thermostat inside the seat and the final circuit to earth takes place through the operational switch on the centre console.

When you say that the supply is intermittent, every 7 to 10 seconds, are you measuring this with the supply disconnected from the element or with the whole circuit in operation?

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Old 11th December 2019, 06:48   #7
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OK. I've taken the seats out and confirmed that 12v applied directly to the element circuits get the seats toasty warm.

I have located the heated seat grounding point at the bottom of the NS A pillar, and cleaned the crimped rings with a wire brush.

At the switch end, black is earth, and the yellow shows +12v, (I guess that's the permanent (ignition) live feeding through the seat thermistors?

When I measure resistance across the switch in operation, it shows 0.8 ohms between black and yellow. I have tried different switches, they all do the same. (oh, and I've even shorted the black to yellow, bypassing the switch........NOTHING!)

I have applied a new earth for the switch, no change.

There must be a common fault here, as both seats do not work, but I cannot fathom out why!
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Old 11th December 2019, 09:58   #8
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Each switch and corresponding seat are on separate circuits, each on their own 12v feed from the ignition switch, and their own Earth cable to the connector at the bottom of the centre console.
Orange/White is the 12v feed from the ignition switch, with a sealed joint in that cable to each seat for the temperature sensor and seat heaters.
Yellow is the return from them to the switch.
Black is the Earth connection to the switch.
When the switch is first operated, the Yellow is connected to Earth, the sensor will also be on and the seat heaters operate. When the sensor switches off, the 12v feed is lost, and the electronics in the switch, sensing the open circuit, switches off, and stays off - although the switch itself may still be in the 'on' position.

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Old 11th December 2019, 11:20   #9
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James,

I'm beginning to wonder whether you do have a fault at all. When I activate my heated seat straight after a cold start I don't feel warmth immediately. This is, of course, when the leather is extremely cold. As you say that both seats are affected, and as Ray has pointed out the two circuits are separate except for a common earth which you have temporarily replaced, it seems beyond the realms of possibility that both your heated seats have developed the same fault at the same time.

Attempting resistance measurements through the switch won't help. It's not a mechanical latching switch. As Ray says, it contains electronics, probably a timer similar to the HRW switch. The orange/white wire to pin 2 of the switch connector provides an uninterrupted supply for that and the LED.

With a cold seat and the ignition on you should see 12v on the yellow connection to the switch. When you operate the switch and the LED illuminates, your meter should show 0v. The seat should begin to heat but it won't be instantaneous.

Simon
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Old 11th December 2019, 18:56   #10
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Yeah, I definitely have a fault. The seats out of the car, connected to my 12v battery charger are toasty warm.

I did change the thermistors a few years ago following a thread entitled "Heated seats, how to improve them" by Matt1960. After that, the seats were perfect.....I just had to reset the switch timer circuit.

Sometime, during the summer, something has failed, and both seats definitely do not get warm....
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