Things to check when Heated Seats Fail
Before I go any further I need to say I haven't completed the repair of my heated seats yet as I'm waiting for a new Lower cushion element to arrive from Alfa via Italy ( £108 inc vat for the lower pad ) But I'm confident I'm on top of the problem and have a better understanding of the heated seats and possible problems.
I also need to apologise as my writing style isn't up too much and my teaching ability is non existent so any amendments you feel need to be made please message me.
Here's a not so quick guide how to work out what problem you have and solution you need.
12 volt power supply
So far I have not removed seats etc. All the below can be done without removing the seats but by unclipping parts of the upholstery from the chair.
Before you start anything the first thing's to check will be the fuses. The passenger and driver seat each have their own fuse which in my sports wagon is located in the boot fuse box to the right inside the storage compartment.
If both seats are not working it could also be worth checking the relay which is also located in the boot fuse box in location "t21". If you're unsure how to check if a relay's working you can search it on youtube. ( you will need a multimeter and 12 volt power supply )
Okay, so you've checked the fuses and the relay and either or both of the seats do not work.
First of all it's worth understanding it a bit better so you can determine what fault you have.
Your car has a heated seat element in both the lower cushion and the back rest. The element is basically a thin wire sandwiched between fabric which gets warm when a current is passed through it. As we'll as the elements the lower cushion also has a thermo cut out switch which cuts the circuit when the seat is hot enough to protect the elements from burning out . ( The back rest element does not have the cut out switch as it's fed from the lower cushion )
Assuming the fuses and relay are fine you can determine potential problems:
Q1. Bottom cushion doesn't heat up but the back rest does.
A1. This would indicate the bottom element has an open circuit ( burnt out ) but the thermo cut out is still working
Q2. Neither bottom of back rest heat up.
A2. We need to gain access to the various plugs located at the back of the seat to understand what needs to repaired / replaced.
To start move the seat as far forward and as high up as you can to gain access.
1. Floor plug ( D070B ). This is located in front of the Air Bag plug so be gentle when you unplug it as disturbing the air bag plug will cause a warning light on the dash board ( this can be turned off by disconnecting the battery for 10 minutes it will remain stored in the ECU until reset )
The car needs to be running to test you're getting power into the seats via the floor plug.
The floor plug in a black oblong plug with four pins, two either end. Using your multimeter you can test the two pins nearest the dashboard should have a 12 volt circuit whilst touching either of the two ground pins at the back of the plug. If you have power at the floor plug move on.
2. You now need to unclip some of the backing upholstery of the seat to gain access to plugs etc. Under the seat you'll find four clips which can easily be unclipped from the underside and pulled apart down the sides ( the side parts are like a tongue and groove plastic squeegee which pulls apart ) to reveal a black fabric parts of the seat which cover's the plugs we need to check. Now you need to pull the lower cushion leather towards you to gain enough slack to pull the black fabric over the four white nylon / plastic prongs which hold it into place under the seat.
Hopefully you've managed to find the plugs by now without hitting your head too many times on the chair runners.
Heated Seat Module is a black box as shown in the second photo. ( M170 ). If only one of your seats do not work you can always swap the modules to check if it's faulty or not but as I've traced power further up the wiring loom I've assumed mine is working.
3. Lower Cushion Element / Thermo switch plug ( O040A ). I'm still waiting for a working lower cushion element to arrive from Alfa so this is an educated guess.
Using your multimeter you can now test the element and thermo switch for continuity ( checking both circuits are intact )
I've guessed the two blue wires are the thermo switch. Using your multimeter test the circuit between the two wires. If your multimeter beeps the thermo switch is still functional. Now test the remaining two "element" wires. Again, if it beeps the element is fine however, I'm guessing one if not both are faulty. ( There is a work around to get the back rest working again if it's still intact but I'll talk about that in a minute ).
If your thermo switch is still functional but the bottom element is faulty it can be repaired and there's a couple of Youtube video's which can be used to help with this. As yet I have not removed the element from the seat so I cannot help with the removal / repair at the moment but once I remove my faulty one I'll try.
4. Back rest element ( O040B ). This is a 2 pin connector which is fed from the bottom cushion thermo switch power. You test this for continuity in the same way as you tested the bottom cushion. If you find you have continuity like I did you can test it by running a 12 volt power supply through it ( if anything the back rest getting warm will at least give you hope ). Please note the wiring loom from O040B continues onto the side Air bag so please be careful.
5. Hopefully by now you've worked out what your problem is. My seat wasn't working because both the thermo switch and bottom cushion element had blown however, I found if I complete the circuit on the plug which is fed by the thermo switch I can at least get the back rest to heat up as normal however, please remember you have no cut out switch protecting it so do not leave it on too long or it may burn out.
Last edited by oliverk; 18-05-13 at 22:01.