Heated backrest wire question - BMW Luxury Touring Community
 
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post #1 of 10 Old Oct 27th, 2007, 6:51 pm Thread Starter
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Heated backrest wire question

I am trying to put a switch in so my wife can have the heated seat on but the heated backrest off. What wire would I need to interup. Is the brown the ground wire and if I cut it would this turn off the backrest power. Thanks for the the help

forgot to say there is a gray, black and brown wire.



Tom

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post #2 of 10 Old Oct 27th, 2007, 7:04 pm
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Is it 2 wires?

I just bought my LT and haven't really looked into the wiring, but it shouldn't be complicated. If 2 wires go to the unit you should be able to interrupt either one and the circuit will be broken.

I assume you're just putting in a simple on off switch. If that's the case it shouldn't make any difference where in the loop it goes.

Seems like basic wiring to me.

*Update* This is also assuming that the backrest and seat are on different loops. If they're not then this won't work. Breaking the circuit anywhere would turn both off. Again, I haven't looked that closely at how the wiring works.

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post #3 of 10 Old Oct 27th, 2007, 7:05 pm Thread Starter
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has 3 wires because there is a high and low

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post #4 of 10 Old Oct 28th, 2007, 12:15 pm
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Here's the schematic from the Clymer manual. If you want to interrupt just one circuit (high or low), you'll have to experiment to determine which wire goes with which circuit, and then put the switch on that wire. If you want to interrupt both circuits (backrest heater on/off no matter which setting the seat heater is on), then put the switch on the brown wire. If you want to get really fancy you could run both the black and gray wires from the seat to a potentiometer, with only the "high" backrest heat circuit coming out of the potentiometer. Then, no matter whether your passenger had the seat circuit on high or low, she/he could dial in the backrest heat from off to high. I think. Maybe. On second thought, consult someone who knows something about electric circuits and heater elements before doing anything you have read in this post.
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post #5 of 10 Old Oct 28th, 2007, 1:50 pm Thread Starter
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Thanks. I thought the brown was ground but the wire diagram I have is in german and I don't read german anymore. I just want to stop the power to on or off. Thank again. Tom

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post #6 of 10 Old Oct 29th, 2007, 10:36 pm
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According to the Clymer schematic posted, there are 3 wires going to the backrest. The way to shut off all power to the seat back would involve a double pole switch, turning off both BLACK and GRAY wires from the seat to the seat back.

There are 2 heating elements in each seat portion. 'High' gives full power (black wire) to one of the elements both seat and back. 'Low' turns both elements on (gray wire), sharing the power, so they are not as hot as in 'High' setting.

A switch to control only 1 wire to the seat back would not completely shut off the seat-back, and may cause the heating elements to become hotter than what the they can handle safely.

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post #7 of 10 Old Oct 30th, 2007, 9:49 pm
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Quote:
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On second thought, consult someone who knows something about electric circuits and heater elements before doing anything you have read in this post.
Aha. Ask and ye shall receive. Thanks, Sparky. After I read your post I re-examined the schematic, and I think I understand what you said. However, for the sake of curiosity, if Tom1200 wants both back circuits to be off at the same time, would a single pole switch on the brown ground wire not work?

And what do you think about putting a rheostat or potentiometer somewhere in the circuit? There are times when "High" is too hot but "Low" needs a boost.

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post #8 of 10 Old Oct 31st, 2007, 2:45 am
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Quote:
Originally Posted by Tom1200
I am trying to put a switch in so my wife can have the heated seat on but the heated backrest off. What wire would I need to interup. Is the brown the ground wire and if I cut it would this turn off the backrest power. Thanks for the the help

forgot to say there is a gray, black and brown wire.



Tom
The heated passenger backrest has a thermostat - somewhere - that switches off automatically when it reaches 122F. From what I remember it will stay off until the seat base switch is toggled to off, then to high or low - where it will reheat to 122F, then off again


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post #9 of 10 Old Oct 31st, 2007, 12:39 pm
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Quote:
Originally Posted by was
if Tom1200 wants both back circuits to be off at the same time, would a single pole switch on the brown ground wire not work?
A single pole on the brown wire at the seat back will still let one of the 2 heating elements get power (in through the black wire, out through the gray wire) and will increase the heating in one of the seat elements, when the switch is in the 'low' setting. Not sure if the seat's heating elements could take the increased heating for long. My owner's manual says to use the 'high' setting for quickly getting the seat warm, and to set it to 'low' afterwards. Also, switching the brown wire would only disable heat to the back rest when the switch is in the 'high' setting.
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And what do you think about putting a rheostat or potentiometer somewhere in the circuit? There are times when "High" is too hot but "Low" needs a boost.
A rheostat would have to be a high-power one. The way a rheostat works is to steal some of the power, and get hot itself, so less power gets through. A PWM (Pulse-width-modulation) circuit would be better. This is what is used in 'heat troller' controllers used for heated garments.

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post #10 of 10 Old Oct 31st, 2007, 3:59 pm
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Why not just unplug the backrest. It is a removeable item and has a connector. There is also a barrel connector under the rear seat where its wires pass through (it may be shared with some other circuits in the top case though).

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