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XC90 Heated Seat Not Working

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Old May 5th, 2018, 00:22   #1
rjtire555
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Default XC90 Heated Seat Not Working

Hi!

I'm having an issue with the heated seats in my XC90.
Using some of the other threads on here, I've debugged the issue a little.

1. The driver's seat doesn't heat, the passenger seat heats weakly
2. The fuses for both seats are okay and have been replaced
3. The push buttons on the dashboard light up when pressed
4. At the heated seat controllers under the seats, I can see GND, 12V and another wire with an intermediate voltage
5. I've bought a replacement heated seat controller from Skandix but substituting this didn't resolve the issue
6. The heat pads in the seat bases and backs each measures less than 1 ohm or less when measured with a multimeter

Does anyone know if there are any other elements in the heated seat circuit that I could try?

Does anyone know how the different levels of seat heating are communicated to the heated seat controller? Does the intermediate voltage that I'm measuring change when the buttons on the dash are pressed?

Thanks,
Joe
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Old May 5th, 2018, 07:41   #2
SwissXC90
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Default

To help you understand the system here is the description from VIDA:

Seat heaters
The seat heaters are activated using the switches on the Climate Control Module (CCM) (3/112). The function is normally switched off. When activating the seat heaters, the first position is high temperature. If the button is pressed again, the temperature will be low. The status of the indication is displayed via LEDs. The Climate Control Module (CCM) transmits information about the selected temperature to the relevant Seat Heating Module (SHM) on the relevant LIN bus. The Seat Heating Module (SHM) then controls the current to the heater pads.
Each Seat Heating Module (SHM) then transmits its present status to the Climate Control Module (CCM).

The system for the seat heaters consists of three different parts:
  • a seat heating module (SHM) which controls the function for communicating with the climate control module (CCM)
  • two heater pads
  • a temperature sensor.

The seat heating module (SHM) is under the left and right-hand front seat. The heater pads are in the seat and backrest. The temperature sensor is in the seat.

The seat heating module (SHM) is directly connected to the heater pad and temperature sensor. The seat heating module (SHM) communicates with the climate control module (CCM) via a LIN bus.

There are diagnostics for the seat heating module (SHM).

Regulating seat heat
Communication between the Climate Control Module (CCM) and each Seat Heating Module (SHM) takes place in the following way:

seat heating module (SHM), driver's side (9/12) on LIN bus 1
seat heating module (SHM), passenger side (9/13) on LIN bus 2.

The Climate Control Module (CCM) receives information from the Central Electronic Module (CEM) (4/56) about the type of seat and upholstery via the Controller Area Network (CAN). The levels for high and low temperature are adapted for the relevant seat and upholstery by the Climate Control Module (CCM). To a certain extent, the levels for high and low temperature can be adapted to the wishes of the customer using VIDA.


Physically, the SHM module has +12V, LIN bus and ground
You have confirmed all 3 wires already:
The LIN bus sits somewhere in the middle (6 to 7 volts, somewhere there), and if you see this then the bus is OK.
But you know the bus is OK because the seat heater would not turn on if not OK.

The only thing left is the 2 x seat heater pads per seat, and the temperature sensor

The temp sensor is pins B2 & B3 on the SHM
The heater pads are on C1 & C2, and on B1 & B4

The heater pads should all measure around the same resistance per pad.
Any variations and you can suspect broken heater elements in the seat heater

The temp sensor should measure a resistance (not sure how much) that varies by temperature. So measure in the morning when cold, and then midday when warm, and record the difference.

You can also diagnose the SHM using VIDA which will tell you what is wrong.

Normally, with any electronics, it is the moving parts that fail first.
And on a seat heater the moving parts are the seat heating elements, as then move when someone sits in the seat, causing stress and sometimes fractures in the heater wires.
If you have a suspect heater pad, then you need to remove the seat from the car (2 people required, heavy!) then remove the seat cover to access the heating pad.
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Old Dec 6th, 2021, 12:40   #3
paxs
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Default xc90 2017

i have same issue

Just driver seat is not heating
on screen showing like is working but not heating

Let me know if you find the problem so i can fix too!

thank you
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Old Dec 6th, 2021, 15:13   #4
Tannaton
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Default

The thread you replied to is from 2018 so they might not be reading still....

Check the following:

If you have VIDA then heated seat faults are logged through the CEM via the SHM.

To diagnose it's easier to remove the seat - just a few bolts and plugs. The resistance of each heating pad (base & back) should be less than 1 ohm - anything great then that pad is broken. The temperature sensor is in the base pad. Replacement pads cost about £80 from the dealer, they're fiddly to fit but worth it.

Very rare it's the fuse or SHM.
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Old Dec 6th, 2021, 17:18   #5
paxs
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ufff sorry saw now is old

thank you for help will try this !

thank you again
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