$28 seat heat pad replacement DIY

[Turbo_Bimmer]

Like many of us here, I got the burning butt problem with a defective heating mat.
Since BMW doesn't sell the heating pad but the complete leather/pad combo which is very expensive, I had to find a way to fix that seat.

I had to find a pad that physically fits but also had the same power as the BMW one and the same resistance on the temperature sensor. This was not as easy as I tought. Most heating pad manufacturers make them in a shape to fit the OE seats, but unfortunatly, a model for the X5 was not offered. I had to settle for a square model that fit the center section of the seat (not the bolsters). This particular model was for a Ford Explorer... The wattage was higher than the BMW one (100W vs 50W) and the sensor resistance was different, but I decided to give it a try.

Since the control system in the X5 needs the temperature sensor to be connected and in the good resistance range, I had no choice than to re-use the BMW sensor. What I did, is to insert the new heating pad just under the leather, on top of the BMW heat pad. I kept the BMW sensor wires connected as before, and for the heating element wires, I cut the green/violet wire going to the BMW mat, and connected it to the new mat. I kept the ground wire connected since it is also used by the temperature sensor.

The BMW heat pad is sewed to the leather, but on the edges only. I cut an opening in the pad near an edge to insert the new pad. I decided to cut on one side and a bit on the front, to be sure not to cut the sensor wires, which go out of the BMW pad from behind.

Does it work?
A LOT! the 100W mat comes to hot temperature in no time. Before, the BMW mat could take up to 5 minutes before I could feel some heat. Now, it is like 15 seconds. But, there is a problem here. Since the heating pad is at the top layer, and the sensor is at a lower layer (in the BMW mat underneath), the time it takes for the heat to reach the sensor and adjust the power, is too long, and the heat becomes almost unbearable. After a while, if you can support the high heat spikes, (3 minutes), the system will stabilse and everything is fine.
I drove the X5 one month like that, and found that annoying. I decided to install a power resistor in serie to drop some voltage. I calculated that a 0.47 ohm / 50W resistor would drop enough voltage to let the heat pad operated at 70W. I ordered the resistor from Digikey, and installed it under the seat, screwed to the metal base (with a drop of heatsink compound).
Now, since the heat builds up slower, the heat has time to transfer to the sensor, and the control is more stable, and the 70W instead of the 50W of the BMW unit, makes it come to temperature faster.

So now, I have a working heating seat again!

The parts used are:
1. Dorman heat pad #641-203 ordered from a EBay vendor for $28
2. Power resistor 0.47 ohms, from Digikey.ca #A102181-ND $4.50
3. Some wires, soldering iron, electrical tape.
* EDIT * : I later switched to a 0.68 ohms resistor because the heating was too much with the heat mat directly under the leather. Digikey #: A102449-ND

Step by step:
(instruction to disassemble the seat can be found in the online TIS:
SpaghettiCoder)

Dorman heat pad. A pair of hog ring pliers (useless, broke on first use... cast tin metal!) and some rings are included with the pad.
I used two pairs of long nose pliers to open/close the rings.



Those are the rings that need to be opened. Some are in a tight spot. The whole process is physically demanding on hands.



The leather/heat pad/insulation are all attached together. This is the part that BMW will sell you if you dish over $1000.



I cut the insulation first. One side and a bit at the front.



Then, the heat pad needs to be cut also. You will be cutting through the heating elements, but since no more power will go into them, no problem there. Just be carefull not to cut at the back of the pad since the heat sensor wires are in that area and not too deep to cut into the leather.



Insert the new pad. Feed the cable first, into an opening toward the back of the seat cushion. There is already a cable there from the BMW mat. Insert it so the double sided tape faces down.



Carefully center the mat, then remove the double sided tape protection paper and stick the pad to the BMW pad. There is also a tape near the back, but it is hard to reach, so I didn't stick it.



Once the pad is securly in place, fold back the mats, insulation, and use some duct tape to hold everything in place.




Reassemble the seat cushion, back, side controls and trims like shown in the TIS manual.
Unclip the yellow cable connector holder. Remove the connector for heating seats (the right one on the picture with the green/violet wire)



The cable from the new pad has two yellow wires, they are the heating element. The red ones are for the sensor, but since it is not the same range of resistance, it won't work, so, just leave the wires aside and taped. (The BMW sensor at room temperature is around 12K, and the sensor in the Dorman heat pad is round 1.7K)




One of the yellow wire from the new heat pad goes to the green/violet wire from the harness. The picture shows a direct connection, but later I installed a resistor in serie, so, the green/violet wire goes to the resistor, and the other side of the resitor goes to the yellow wire. Now, the BMW pad is not part of the circuit anymore. The other yellow wire goes the the brown wire. Just remove enough insulation on the brown wire to be able to solder the yellow wire.


Prepare the power resistor by soldering two length of wires on both ends, and tape the connections. Spread a bit of heatsink compound on it's back. Mark and drill two small holes under the seat and screw the resistor to the seat bottom. Route the two wires following the bottom of the seat, using tye wraps to secure them, so they won't move and get caught in the adjusting mechanism. Solder them to the green/violet and yellow wires.


Reassemble the plug into the yellow housing, Snap the housing in place. Verify that no wire can be caught when the seat is adjusted, and no sharp edges rub on the wires. Bolt the seat back in place and go for a ride to test your work. Using Ignition ON to test won't work, even if the green LEDs are on beside the switch. At least on my truck it was like that.

Enjoy!

[X5SND]

Nice man! I'm sure plenty of people will find this as a suitable solution to the infamous problem that has long plagued us. Well done!

[Whitecat]

Impressive Turbo ! Very good work and well explained....and at low cost

Thanks

[SlickGT1]

Dude nice job, and even nicer price.

[Skyline]

Nice write up...with a great result.

> Forum mods...this should be added to the How To's.

[davintosh]

That's pretty awesome. My e53 doesn't have heated seats, but my e32 does, and the bottom on the driver's side doesn't work. I'll have to give this a go. Thanks for taking the time to write it up!

[Turbo_Bimmer]

Thanks guys. If this can help some people fix their seat, I'm happy.
I've learned so much from others on this forum, it's my turn to contribute.

[stewey]

Nice write up! I'm planning to do this myself. Just curious, how did you determine the specs for the factory warmers? Or the dorman ones for that matter (I don't see them on their site).

[Turbo_Bimmer]

Originally Posted by stewey

Nice write up! I'm planning to do this myself. Just curious, how did you determine the specs for the factory warmers? Or the dorman ones for that matter (I don't see them on their site).

Thanks,
I measured the resistance of the bottom heater of the passenger seat (working one). Since the Dorman one has lower resistance, I had to add the power resistor to reduce the voltage at the heater.

Just a note on the value of the resistor. With the 0.47R the heating was too much. I then switched to a 0.68R resistor. In my case the pad is directly under the leather, maybe if someone installs it under the insulation the 0.47R would be ok. Anyways, changing the resistor is not a big jog.

Search for 641-203 in EBay and you will find a vendor.
Good luck.

[pezho405]

Originally Posted by X5SND

Nice man! I'm sure plenty of people will find this as a suitable solution to the infamous problem that has long plagued us. Well done!

Mine burned through the seat and made a hole. if i turn it on and look in the hole i see sparks and stuff