Aftermarket Heated Seat Pads and OEM Switches.

Kleynie

Kleynie

A2OC Donor
Some may remember, a couple of years back, we had the seats in Adams A2 covered in leather/Alcantara, at the same time we had aftermarket carbon heated pads installed. Since then we have struggled to get the pads to work with the OEM heated seat rollers.
The back rest and the base of the seat had two separate heated pads, so we have two wires from the backrest and two from the base. What we had done wrong was wire these in series instead of in parallel, this effectively halved the voltage to each pad! Doh! The pads must be wired in parallel.
We knew that the original Audi pads use a thermistor to control the heat, these basically act as a sensor in each seat to sense the heat depending on the roller switch setting. We had an original loom pre-installed which presents 4 wires to each seat, two for the thermistor and two for the power to the heating pads.
We had a diagram that proved to be wrong, see attached, and this may have mislead us for a while, it stated that a 47K Ohm resistor should be installed in parallel with a 10K Ohm thermistor, whilst this worked, it only worked slightly on number 4 of the roller switches.
A bit of googling and many test runs on the bench and we finally nailed it. The thermistor circuit did not require a fixed resistor, it simply required a 6.8K Ohm thermistor as in the below link:


We soldered the thermistor to two lengths of wire, heat shrunk the exposed wires and inserted it into the seat base. It is wired as per the diagram, just not with the fixed 47K Ohm resistor. It now works a treat.

So for anyone doing a heated seat install, this has made it a whole lot easier, the carbon heating pads are very cheap, the hard bit will be getting the climate panel with the switch rollers - thanks to A2Steve for ours.

Please let us know if you have any questions if doing this install.

Ian & Adam.


This is the drivers seat:

Photo 13-12-2018, 18 36 34.jpg
PHOTO-2018-10-03-13-46-33.jpg
 

Attachments

  • Sitzheizung_nachruesten.pdf
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Last edited:
Hi,

I have been able to retrofit a set of carbon mats on a different make car and make the aftermarket controller work with the original rotary selector switches.

If memory serves me right all these work by varying the resistance at the input of a small inverter, i.e., 12v DC to square wave AC of a varying pulse width. Depending on the duty cycle of the pulse you get varying levels of power to the mats, i.e., heat. I cannot recall a thermistor, but a thermal fuse; inside the mats at the very back of the mat. This turns the power off when triggered and resets when cool. If I remember correctly the resistance values of the aftermarket selector where such you could not get 100% duty cycle, thus destroying the mats.

The trick was to match the resistance values of the OEM switches to the aftermarket controller. What I did was replace the potentiometer inside the OEM switches to match the resistance values of the aftermarket switch and rearrange the tracks on the small PCB to match different resistance succession,i.e., increasing resistance for increasing the duty cycle of vice-versa.

The aftermarket controller had 10 settings for heat whereas the OEM only had 5 which was not a problem whatsoever.

Maybe these findings help.

Evros
 
The A2 OEM heated seat pads definitely have a thermistor as we have a set in the garage that we used for our investigations.

The roller switches do not vary the output to the heating elements, they simply turn on and off depending on the resistance from the thermistor.

Its very simple and certainly not as complicated as modern cars.

Ian
 
devans said:
The leather looks good , would you be able to advise who covered the seats in leather?
Click to expand...
These guys:

Automotive - Transcal

Automotive The finest human craftsmanship meets innovative technology, with the end-product both flawless and attractive. From leather car seats to the dashboard; door panels to steering wheels; and gear knobs to travel accessories. With consistent quality and innovative methods, it is obvious...
transcal.co.uk transcal.co.uk
 
I fitted a set of carbon heater pads in some black sports seats that I had in my A2 before I fitted the current leather heated sports seats
I can honestly say that the hardest part (and most painful for weeks) was the fitting of the pads into the seats. Those hog rings still give me nightmares
Had I known that you were having trouble with the wiring side of things I could have helped as I knew the thermistor value and no resistor required just a shame that I didn’t know you where struggling with the lack of or more to the point some mis information

Cheers. Paul


Sent from my iPhone using Tapatalk
 
Has, or can anyone produce a "How To" showing the steps needed to remove existing seat covers, the fitting of the heaters, the refit of the covers and how to wire everything up. That would be a very interesting post.
 
audifan said:
Has, or can anyone produce a "How To" showing the steps needed to remove existing seat covers, the fitting of the heaters, the refit of the covers and how to wire everything up. That would be a very interesting post.
Click to expand...

I did not take any photos when I did mine
Basically you split the base from the back
The base cover clips around the bottom edge, unclip it and you can peel the cover up the sides of the base
The cover is retained to the base using hog rings these are like small key rings but only about 10mm in diameter
The hog ring is there to attach the seat cover to the seat base. Both have spring steels wires embedded in them
The hog rings need to be opened to allow the separation of the cover and base
On re assembly the hog ring needs to be put in place and crimped to close the gap. Easier said then done when the hog ring is quite hard and you are working 2” down a slot in the foam seat base. I tried hog ring pliers but found them too bulky to be used
I’ve heard of people using tie wraps instead of hog rings but that’s not the way I role but I do concede that it may well work and would have saved the skin on my fingers

The back of the seat needs the removal of the frame that holds the net in place then it’s basically the same as the base

Paul


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Thanks Paul. Time to do each seat? How are they wired in to OEM and aftermarket looms? Genuine climates or cubbies with the heated switches are like the proverbial rocking horse poo.
 
The PDF attached at the top has the full instructions.
I remember there being a discussion about the NTC values necessary elsewhere and I'm reasonably sure this has to do with the duty cycle required and expected and the varying heat outputs of the options in terms of mats. As in a 45W mat vs 90W will have a different resistance and therefore potentially require a different NTC / thermistor to work "correctly". You're talking 4A vs 8A and therefore half the resistance and depending on what exactly the roller switch is expecting, that will change the way the circuit works.

- Bret
 
It took me about 4 hours to fit the heater to the passenger side seat bottom
I was hoping it would be quicker for the back but this was about the same time so that was Saturday gone
Sunday’s to do the drivers seat and from memory it was about the same times
I did swap the seat bases such that any wear on the drivers side base was now the passenger sides base. I also repaired the foam where it had fallen apart using an SE seat foam that was spare and some foam friendly spray glue and a hot wire bow to cut the foam (don’t breath the fumes)
I chose to modify the centre console drivers side wing to utilise the blank switch position were the eco switch would be on the 1.2 to take the heated seats controller and also changed the blue led to a red led to match in with the A2’s colour scheme
When it was done it looked totally OEM
I had no option to use the Audi seat switches because I have likely got the only A2 with a double din dash and manual heating so no cubby hole to fit the heater switches and no option to fit the heater switches into the heating control area
I did buy a pair of switches from the A8 and had them wired up to the pads before fitting with the thermistor but in the end the heater control that came with the Chinese heater pads was smaller and did both sides in one switch and looked totally oem one the led was changed
I sold the seats on when I upgraded to grey leather heated sports seats but they are still in daily use with a mate of mine and work perfectly. The lack of temperature control was never a problem as the 1 to 5 settings worked perfectly
From memory on setting 5 there was a 4amp draw per seat but you would not have 5 selected for very long
On a short 7mile commute with office clothing and a winter jacket on in the middle of winter I used to leave them on setting 3 on a longer commute say an hour or more setting 2 or 1 was more normal
I must admit I would not go back to none heated seats in a tdi A2 due to the long warm up times especially when not cold enough for the webasto to kick in, the heated seats Warming your back and bum make up for the lack of cabin temp. Now if only I could find a suitable heated steering wheel

Paul


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depronman said:
It took me about 4 hours to fit the heater to the passenger side seat bottom
I was hoping it would be quicker for the back but this was about the same time so that was Saturday gone
Sunday’s to do the drivers seat and from memory it was about the same times
I did swap the seat bases such that any wear on the drivers side base was now the passenger sides base. I also repaired the foam where it had fallen apart using an SE seat foam that was spare and some foam friendly spray glue and a hot wire bow to cut the foam (don’t breath the fumes)
I chose to modify the centre console drivers side wing to utilise the blank switch position were the eco switch would be on the 1.2 to take the heated seats controller and also changed the blue led to a red led to match in with the A2’s colour scheme
When it was done it looked totally OEM
I had no option to use the Audi seat switches because I have likely got the only A2 with a double din dash and manual heating so no cubby hole to fit the heater switches and no option to fit the heater switches into the heating control area
I did buy a pair of switches from the A8 and had them wired up to the pads before fitting with the thermistor but in the end the heater control that came with the Chinese heater pads was smaller and did both sides in one switch and looked totally oem one the led was changed
I sold the seats on when I upgraded to grey leather heated sports seats but they are still in daily use with a mate of mine and work perfectly. The lack of temperature control was never a problem as the 1 to 5 settings worked perfectly
From memory on setting 5 there was a 4amp draw per seat but you would not have 5 selected for very long
On a short 7mile commute with office clothing and a winter jacket on in the middle of winter I used to leave them on setting 3 on a longer commute say an hour or more setting 2 or 1 was more normal
I must admit I would not go back to none heated seats in a tdi A2 due to the long warm up times especially when not cold enough for the webasto to kick in, the heated seats Warming your back and bum make up for the lack of cabin temp. Now if only I could find a suitable heated steering wheel

Paul


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Click to expand...

You can make one using nicrhome wire, fiberglass tape and an inverter but the only real trouble is to make sure that the slip ring can take the current.

Evros
 
there are available but only in four-spoke. There's a list on the German forum. And the slip rings can deal with the currents, but you will need a specific slip ring too.

- Bret
 
depronman said:
I fitted a set of carbon heater pads in some black sports seats that I had in my A2 before I fitted the current leather heated sports seats
I can honestly say that the hardest part (and most painful for weeks) was the fitting of the pads into the seats. Those hog rings still give me nightmares
Had I known that you were having trouble with the wiring side of things I could have helped as I knew the thermistor value and no resistor required just a shame that I didn’t know you where struggling with the lack of or more to the point some mis information

Cheers. Paul


Sent from my iPhone using Tapatalk
Click to expand...
Paul, this is exactly why we posted this information, we could not find anything on line about it at the time and assumed that no-one had done it.

Ian
 
bretti_kivi said:
The PDF attached at the top has the full instructions.
I remember there being a discussion about the NTC values necessary elsewhere and I'm reasonably sure this has to do with the duty cycle required and expected and the varying heat outputs of the options in terms of mats. As in a 45W mat vs 90W will have a different resistance and therefore potentially require a different NTC / thermistor to work "correctly". You're talking 4A vs 8A and therefore half the resistance and depending on what exactly the roller switch is expecting, that will change the way the circuit works.

- Bret
Click to expand...
For the A2 the NTC value is the same whatever heated pad you use. The roller switches simply provide 12v or no volts, there is nothing inbetween, a simple on or off. The thermistor only dictates the temperature range at which the switch turns the pad on and off.
 
Morning Adam,

I’ve been reading through this, although it all makes sense it actually doesn’t mean that much to me.

The background to my issue is that I have factory fitted Twist Leather heated sport seats, the drivers rarely works (only twice in my ownership) and guess it can be a loose wire/damaged wire. I’d like to repair it and will solder the damaged wire if able.

If the factory heating element is not worth repairing I do actually have a set of carbon seat pads from my last A2 which were never fitted.

Once I’ve become brave enough to remove the seat covering would you be able to assist with a simplified talk through of how to wire up the 2 carbon heat pads with the correct thermistor you mentioned.

I did click on the link you added earlier but I read it as the minimum order was 2000 items at £0.33 each along with £15.95 delivery. Do you now have 1999 spare?

Kind regards,

Tom
 
Tom, yes its very easy and we can send you a thermistor as we do have a few spare (not 1999 though).

The OEM heated pads are attached to the seat fabric, so you may have to separate them and add your carbon pad.

Surely its worth putting a meter on the connector under the seat first and checking you have 12V to the heaters? It could be the switch at fault.
 
Kleynie said:
Tom, yes its very easy and we can send you a thermistor as we do have a few spare (not 1999 though).

The OEM heated pads are attached to the seat fabric, so you may have to separate them and add your carbon pad.

Surely its worth putting a meter on the connector under the seat first and checking you have 12V to the heaters? It could be the switch at fault.
Click to expand...
Hi Adam,

Thanks for responding. This really isn’t my areas of expertise but happy to give anything a go as it annoys me something so special just not working.

Im thinking it’s somewhere within the seat as I have swapped the switches around with no joy, the drives switch works on the passengers seat. I have also checked the harness by putting the passengers seat in the drivers side and it heats up. It’s got to be the seat itself.

The fact that I’ve had it working on 2 separate occasions leads me to think it’s a loose wire or very worn wire somewhere within the seat.


Once I have the seat covering off, what I’m hoping for is to find a stretched/weak wire where someone has knelt on the seat (I remember Tom @timmus telling me that it’s most likely the case and I told him it certainly wasn’t me, 2 minutes later I too was kneeling on the seat trying to undo the bolts for the centre tunnel) and solder repair it. I’d also like to be fully armed with everything I need, including knowledge to fit the aftermarket items should a repair not be possible.

Thanks for the offer of help sir. I will be along via PM when I get round to this task.

Kind regards.

Tom
 
Kleynie said:
For the A2 the NTC value is the same whatever heated pad you use. The roller switches simply provide 12v or no volts, there is nothing inbetween, a simple on or off. The thermistor only dictates the temperature range at which the switch turns the pad on and off.
Click to expand...

This is oversimplified. The pads used will dictate what NTC value you need to use, just like it will dictate whether it's series or parallel connection for seat base / seat back. The resistor is important as it will allow some form of current to flow even when the thermistor is open. The whole system is a simple PWM and I'm assuming that the roller reads the current that is being returned to it via the thermistor / resistor and then switches as appropriate. I would also assume that there's no current supplied if there is no resistor in place, so the full 100%-5%-100%-5% will go 100%-0%-100%-0%. That will be more of an issue at level 4 and when it's cold. There's also a limit in terms of 15A in terms of power, as there are only 1,5mm cables, so 2 90W mats are the absolute limit. It's not the KW-A2 setup where the mat simply switches off at 70C.

The longer thread with the discussion about various mats is here: https://a2-freun.de/forum/forums/topic/7059-anleitung-sitzheizung-nachrüsten ; there's no final discussion about the NTC / resistor values for different mats.
Having said that; the Waeco mats seem to indicate:
- initial mat resistance is around 10 ohms
- thermistor probably is around 2.2k at 70C and 12k at 20C, and 29k at 0C.
If you've a 47k resistor, then the implication is when there's more than 10mA being returned, the system will be switched off. I'd suspect that as the roller is increase, the current goes up that's permitted, but it will stop at some point.
Again, though - any particular mat will have variable numbers here, so YMMV.

- Bret
 
correction: 12 ohms is the total system resistance for one side with the nominal 50W Waeco mats. That leaves us with 24W per mat and a total of 48W for one side. 2A per mat.

If they're the carbon ones, we're looking at 1.6 ohms per side to start with and around 7.5A per side.

Playing some more with the Thermistor table here, https://www.yumpu.com/en/document/r...tput-table-10k-2-thermistor-output-table-bapi, the resistance calc here: https://www.allaboutcircuits.com/tools/parallel-resistance-calculator/ and the ohm's law calc here: https://www.rapidtables.com/calc/electric/ohms-law-calculator.html I'm reasonably sure there's a transistor somewhere that turns on a very small current and as long as it's below around 6mA return it will allow current to flow. And the whole thing will be done with a potentiometer dictating how low or high the current gets to be before switch-off. Elegant.
 
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