Heated seats - troubleshooting
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Thread: Heated seats - troubleshooting

  1. #1
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    Heated seats - troubleshooting

    HEATED SEAT TROUBLESHOOTING

    Car: Passat Wagon B5 1.8T my 1997 (AEB engine, Highline w/ leather-alcantara sport seats), factory-installed seat heaters

    I have the following situation: Seat heater on driver side is completely non-functional, while the passenger seat heater works fine. Will go through the steps following the current flow.

    Some related topics

    Diagnosing heated seats - How to remove dash switch

    How to fix your heated seats and other seat related stuff - Good guide by TwoManyPassats

    Tools needed:

    - T40 Torx bit
    - T25 Torx bit
    - Socket wrench (1/4" is good for the torx screws and 1/2" is most probably the size of M8 triple square socket)
    - Flathead screwdriver
    - Philips ph2 screwdriver
    - Multimeter with resistance, continuity and voltage measurement
    - M8 Triple square socket
    - About 15cm of shrink tube w/ glue (from 3mm to 1mm is fine for this job, but nothing bigger)
    - Spray glue (I used small pieces of the heater element fabric as patches and spray glued them on)
    - If you want to be exact, also a needle and some thread for sewing the heater element back together

    1. Check fuses

    - There are 2 fuses related to heated seats,
    * #5 10A mini-fuse for dashboard lighting, including heated seat control switch lights
    * #44 30A regular fuse for seat heater current.

    2. Check dashboard control switch operation

    - Simplest way to check operation is to try the switch on with a seat that is known to have a working heater (thanks soundguybob)
    ** This is due to the fact that the switch is transistor-controlled, it will not pass voltage to the heater element if either the element or the thermistor are faulty.
    - Check that you get 12V from pins 6 and 4 (directly from fuse #44)

    3. Remove seat (NOTE: SIDE AIRBAG! USE PRECAUTION)

    ** NOTE - MEASUREMENTS CAN BE DONE ALSO WITHOUT REMOVING THE SEAT!

    - Slide seat to front, remove plastic covers from both sides above the rails behind the seat
    - 2x T40 Torx screws in the front of the seat rail, a plastic clip pin in middle - 20Nm, so your 1/4" socket wrench does the job
    - Remove battery negative cable
    - Slide seat back to detach seat from the guide rails
    - Tilt backrest all the way to front
    - Remove seat heater green 6-pin connector
    - Ground yourself by touching the door striker or other chassis part
    - Remove side airbag connector (yellow)
    - Place electrical tape over the airbag connector in the seat frame and to the harness side connector
    - Wiggle the seat out of the front door

    4. Check backrest heater element continuity and resistance

    - Remove the backrest heater connector (black / white) from seat underside
    - Measure continuity between the two pins on the white side
    - Resistance in a working backrest element - measured 1.2 ohms from my backrest, so should be fine. If there is a greater resistance within the heater element, you probably do have a partially damaged wire somewhere
    - No continuity / Infinite resistance
    -> remove backrest
    -> search for burn marks and twists in wire
    -> go through in sections with a multimeter /w alligator jaws and small needles if no apparent break point is not found

    5. Check seat heater element continuity and resistance

    - Measure continuity and resistance between the green seat heater connector and the black connector you just removed from holder with the backrest harness
    ** Green connector pin 4 (sole pin with two blanks beside it, black / yellow wire) - black connector pin 2 (blue / brown wire)
    - Resistance in a working backrest element - measured 1.2 ohms from my seat, so should be fine. If there is a greater resistance within the heater element, you probably do have a partially damaged wire somewhere
    - No continuity / Infinite resistance
    -> remove upholstery
    -> search for visible break points, burn marks or twists in wire
    -> go through in sections with a multimeter /w alligator jaws and small needles if no apparent break point is not found

    ** Repairing the seat element does not necessarily require removing the backrest or even taking the whole seat off the car, but taking the seat from the car and backrest removal makes troubleshooting and repair a lot more convenient as you can put the whole seat on your workbench or table.

    6. NTC-thermistor operation

    - Check resistance between green connector pins 1 and 2, should be less than 10kOhms if the seat surface temperate is around 20°C
    -> If infinite, either the thermistor is broken or there is a broken wire
    - Remove seat upholstery and rip off the heater element from the rear of the cushion so that you can remove the metallic holder in the wire loom
    - Open up the element from the rear end until you see the two shrink tubes which have a black/white wire connecting to a blue wire and a brown wire connecting to a blue wire
    - Open the shrink tubes and measure continuity and resistance between green connector pins 1 and the brown wire solder point, also measure the black/white wire with the green connector pin 2
    -> If resistance is infinite and there's no continuity, you got a broken wire between the green connector and the solder point -> replace
    - Next measure resistance between the blue wires going to the thermistor -> should be less than 10kOhms if the heater element surface temperate is around 20°C
    -> If infinite, either the thermistor is broken or the blue wires have a snap somewhere
    - Desolder the blue wires from brown and black/white wires
    - Remove the heater element a bit further, the thermistor is right under your bunghole when sitting
    - Carefully pry the element underside open around the thermistor and remove the thermistor with its wiring
    - Now measure the thermistor resistance from the solder points on the thermistor, should be less than 10kOhms if surrounding air temperate is around 20°C
    -> If infinite, go get yourself a new button-style NTC-thermistor from your local electronics shop, value should be around 6,8kOhms for correct operation
    -> If you get a proper resistance from the thermistor, replace both wires
    *** I think if you remove the thermistor, you should replace the blue wires anyway as they are really weak, just to avoid having to go through this again in after a short while.

    7. Put the thing back together

    - Remember to re-measure heater element continuity after every step you do, just incase the heater element breaks during the reassembly (I was especially worried when putting the upholstery back as it requires quite a bit force to get the steel braces back in place)
    - Might be a good idea to confirm the thermistor resistance too when reassembling

    Disclaimer: If you fuck something up, don't blame me, I am not a professional and even professionals make mistakes, perhaps more often than I do, so I won't take responsibility of any issues you might end up with using this guide. If you are unsure about anything, brake before you break and ask for advice.

    Questions about the troubleshooting process and filling in with details is greatly appreciated and welcome!

    PICTURES











































    If you want to get some fit to the seat, you can bend the cushion frame a bit inwards, but be careful if you got a wide ass... It will not be as snug as you put it, because at least the leather upholstery does not give in that much. I bent it a little, just to match it perfectly with my butt.
    Last edited by Jimme; 11-02-2010 at 04:46 AM.
    Tmark likes this.

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  3. #2
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    Got my broken heated seat working with this walkthrough, hope someone else benefits from it too.

  4. #3
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    The switches have a small circuit in them, so the simplest test is to see if it works by testing it on a known good seat. The thermistor provides feedback to the circuit in the switch, basically telling it how warm the seat is. It works in conjunction with the setting on the front of the switch to regulate power to the heater elements. The switch will not output voltage to the elements if the connection is broken to the thermistor, or if the thermistor has failed.

  5. #4
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    Quote Originally Posted by soundguybob View Post
    The switches have a small circuit in them, so the simplest test is to see if it works by testing it on a known good seat. The thermistor provides feedback to the circuit in the switch, basically telling it how warm the seat is. It works in conjunction with the setting on the front of the switch to regulate power to the heater elements. The switch will not output voltage to the elements if the connection is broken to the thermistor, or if the thermistor has failed.
    That's what I thought when looking at the wiring diagram, just couldn't be sure if there was an easy way to probe the switch. Thanks for your input, I'll add it to the guide.

  6. #5
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    Mine was working great a couple weeks ago, then just recently stopped! Passenger seat works great still though, going to have to take a day and work it out with this guide. Thanks!

  7. #6
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    Hello Jimme,

    First off, I really appreciate your thread in regards to fixing broken heated seats or in my case, adding to a VW that never had them.

    I am searching for an NTC thermistor but wanted to confirm the ohm rating. Do I need a 6.8k ohm, 6 to 8k ohm? I've been searching on and off for a while but would like to find a quick solution. Thanks in Advance!

  8. #7
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    Forgive me if I'm beating a dead horse, but this troubleshooting is about nonworking seats, whereas my driver's side runs a lot hotter than it should; I can't find any posts about this. On 1, it feels like it should be a 3, and on 2, it feels like it should be a 5 (very toasty, as hot as it seems safe). I can't imagine putting it to 3-5. Sometimes it's worse than others. For long drives (half hour plus) it also seems as though it'll get itself nice'n warm, then drop off for a while and possibly never start heating again for the rest of the trip (unless I'm just getting used to it). Would any of the info here be relevant to that? Or is it a simple matter of replacing the driver's switch, which I assume is the control switch in the dash?

    My passenger's side heater switch was replaced by the dealer before I got the car and it works as it should.

  9. #8
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    I just wanted to say thanks for posting this information. The drivers seat on my MK1 TT was not heating at all. Following your instructions, I tested continuity for the seat elements first (both the seat pad and the seat back) and saw that they were fine, but I had no resistance when I checked the thermistor. So I pulled the seat out of the car, removed the seat pad, pull the leather cover off, and carefully cut and pulled away the cloth to expose the thermistor. I tested for continuity again right at the leads and there was none.

    For giggles, I connected a 12V 1A power adapter to the leads of the seat elements just to see if the seat would get warm and sure enough, it did. So the problem is the thermistor for sure.

    I found a local electronics store that sells thermistors, but can't find anything in between 1k ohms and 10k ohms. But the 10k ohms thermistors are rated 10k ohms at 25C, so I think at 20C, they'll be closer to the 6k ~ 8k ohms range you recommend.

    I found two that seem to be ok:
    this and this

    The 1st one has a higher mW rating, but the 2nd one has a Percentage of Resistance Tolerance of 1% vs 10% on the 1st one.

    Which one would be the better one to use?

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