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Ask Michael Warfield Your Own Question
Michael Warfield
Michael Warfield, Factory Mercedes-Benz Technician
Category: Mercedes
Satisfied Customers: 3130
Experience:  Mercedes-Benz Master Certified technician
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The passenger seat heater in my 2012 350ML only works part

Customer Question

Hello! The passenger seat heater in my 2012 350ML only works part of the time. Ideas?
JA: Have you checked the coolant level? Has there been any leaking from the hoses?
Customer: No leaking or obvious problems. When i push to button to heat the seat, it comes on about 50% of the time.
JA: What is the model of your '12 Mercedes?
Customer: 350ML
JA: Are you fixing your vehicle yourself? What have you tried so far?
Customer: Trying to! Ha! I take it to the dealer for major things. I haven't tried anything to fix this problem thus far. DIdn't bother with it in the summer :)
JA: Anything else you want the mechanic to know before I connect you?
Customer: Nope. Thanks!
Submitted: 22 days ago.
Category: Mercedes
Expert:  Michael Warfield replied 22 days ago.

Hi, my name is ***** ***** will be assisting you today! I will need some more information to provide an accurate answer/diagnosis. Have you experienced any warnings or messages on the instrument cluster? Any other functions of the vehicle that may not work properly such as the passenger seat heater?

Customer: replied 22 days ago.
No messages or warnings. It's only the passenger seat heater that only works about 1/2 the time.
Customer: replied 22 days ago.
Text please
Expert:  Michael Warfield replied 22 days ago.

Thank you for that information. I am working on a solution currently

Customer: replied 22 days ago.
Thanks!
Expert:  Michael Warfield replied 22 days ago.

Ok, I studied the function schematic. I would like to inform you on how I came to my answer so you could fully understand how your system works. The left and right seat heater switches are part of the upper control panel. When you push either button, the request information is sent out over the network to the rear sam control unit. The rear sam is responsible for many functions such as brake lighting etc... but one of them is to send the switch request signals back out over the network to each respective seat control unit. Mercedes does this because in the event of low battery voltage/alternator failure, it enables consumer shutoff protection which prevents non essential electrical consumers (seat heaters, etc...) from taking away power from vital engine and safety systems. Then each control unit would supply power to the seat heaters it is directly connected to. So we can rule out the rear sam because it works just fine for the driver's side and the network because it is up and running. We can also rule out consumer shut off protection because the driver's seat heater worksSo the only other possibilities are the passenger seat control unit has a hardware or software fault, or it is detecting a electrical resistance/amperage consumption problem and it shutting down the circuit. This would store a fault code in the passenger seat control unit and would not relay a message to the driver. The fault would only be available to the person conducting a diagnostic test for faults. If there are some fault codes stored, a replacement of the necessary seat heater pad would be necessary, usually about $500-700 depending on which one. If there are no faults, that would lead me to believe there is a hardware (burned out contact) or software failure in the seat control unit. Software should be installed to see if the problem is resolved. You could also perform electrical testing at the output side of the control unit to confirm the module is actually sending power to the seat heaters. There is a mercedes bulletin related to your problem and it says to check the wiring of the seat for chaffing and/or shorting. In my experience, such very minor chaffing and shorts cause intermittent operation and do always set fault codes. Here is the picture of the seat on where to check for the short/chaffing problem. I hope you were helped with my answer and thanks for being patient. Don't forget to rate me at the end of this chat!

Expert:  Michael Warfield replied 22 days ago.

I meant to say very minor chaffing and/or short circuits don't always set fault codes.