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Tom
Tom, ASE Master Certified
Category: Jeep
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Experience:  Chrysler Level 3 Technician, Chrysler Training Instructor
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2002 Jeep Grand Cherokee Limited: heated seats..a wiring diagram

Resolved Question:

Hi, I have a 2002 Jeep Grand Cherokee Limited with heated front seats. The switch for the heated seats LED's work for both high and low but no heat at the seat. I think I need a wiring diagram to get troubleshoot it since I have no idea where the wiring goes off the switch.
Submitted: 2 years ago.
Category: Jeep
Expert:  Tom replied 2 years ago.

Tom :

Hi, and welcome to Just Answer. My name is XXXXX XXXXX I will help the best I can.

Tom :

I'll post some diagrams for you and as much info as I can.

Tom :
  • If the heated seat switch LED indicators light, but the heating elements do not heat, check the power seat circuit breaker in the junction block.
  • Tom :

    NOTE: DO NOT ATTEMPT TO SWAP MEMORY OR NON-MEMORY HEATED SEAT MODULES FROM ONE VEHICLE TO ANOTHER. MOST OF THESE MODULES ARE VEHICLE FEATURE SPECIFIC AND THEREFORE NOT INTERCHANGEABLE. ALWAYS USE THE CORRECT PART NUMBERED MODULE WHEN DIAGNOSING OR REPLACING A MODULE.

    Tom :

    DESCRIPTION


    Individually controlled driver and passenger side electrically heated front seats are available factory-installed optional equipment on this model, when it is also equipped with the power seat option. The heated seat system allows both the driver and the front seat passenger the option to select one of two seat heating ranges, Low or High, or to turn the individual seat heaters Off using the heated seat switches located in the center lower bezel near the bottom of the instrument panel center stack. The heated seat switch circuit operates on ignition switched battery current supplied through a fuse in the junction block, only when the ignition switch is in the On position.


    The heated seat system consists of the following components :



    • Heated seat elements

    • Heated seat sensors

    • Heated seat module (or memory heated seat module)

    • Heated seat switches.


    The heated seat system also relies upon resources shared with other electronic modules in the vehicle over the Programmable Communications Interface (PCI) data bus network. The PCI data bus network allows the sharing of sensor information. This helps to reduce wire harness complexity, internal controller hardware, and component sensor current loads. At the same time, this system provides increased reliability, enhanced diagnostics, and allows the addition of many new feature capabilities. For diagnosis of these electronic modules or of the PCI data bus network, the use of a DRB® scan tool and the proper Diagnostic Procedures manual are recommended.


    The electronic modules that may affect heated seat system operation are as follows:



    • Body Control Module (BCM) - Refer to Body Control Module in Electronic Control Modules for more information.

    • Heated Seat Module (HSM) - Refer to Heated Seat Module in Electronic Control Modules for more information.

    • Memory Heated Seat Module (MHSM) - If the vehicle is equipped with the Memory System, refer to Memory Seat Module in Electronic Control Modules for more information.


    Refer to Power Seats Premium I/III in the Contents of Wiring Diagrams for complete circuit diagrams. Following are general descriptions of the major components in the heated seat system.

    Tom :

    OPERATION


    The heated seat system will only operate when the ignition switch is in the On position, and the surface temperature at the front seat heating element sensors is below the designed temperature set points of the system. The heated seat system will not operate in ambient temperatures greater than about 41° C (105° F). The front seat heating elements and sensors are hard wired to the Heated Seat Module (HSM) or the Memory Heated Seat Module (MHSM).


    The heated seat switches are hard wired to the Body Control Module (BCM). The BCM monitors the heated seat switch inputs, then sends heated seat switch status messages to the HSM or MHSM over the Programmable Communications Interface (PCI) data bus. The HSM or MHSM contains the control logic for the heated seat system. The HSM or MHSM responds to the heated seat switch status messages, ignition switch status messages, and the front seat heating element sensor inputs by controlling the output to the front seat heating elements through integral solid-state relays.


    When a seat heater is turned on, the sensor located on the seat cushion electric heater element provides the HSM or MHSM with an input indicating the surface temperature of the seat cushion. If the surface temperature input is below the temperature set point for the selected Low or High heated seat switch position, the HSM or MHSM energizes the integral solid-state relay, which supplies battery current to the heating elements in the seat cushion and back. When the sensor input indicates the correct temperature set point has been achieved, the HSM or MHSM de-energizes the solid-state relay. The HSM or MHSM will continue to cycle the solid-state relay as needed to maintain the temperature set point.


    The HSM or MHSM and the seat heater elements operate on non-switched battery current supplied through the power seat circuit breaker in the junction block. However, the HSM or MHSM will automatically turn off the heating elements if it detects an open or short in the sensor circuit, a short or open in the heating element circuit causing an excessive current draw, or when the ignition switch is turned to the Off position.


    See the owner's manual in the vehicle glove box for more information on the features, use and operation of the heated seat system.

    Tom :

    any luck?

    Customer:

    Thanks for the information Tom but I was hoping you could maybe shed some light on what a typical failure might be (ie: if the seat switch indicator lights then typically the heating element has failed, etc). I found a complete 2001 heated seat circuit diagram on another answer on this site so I am not sure which diagrams you posted maybe you can tell me. Also another answer said what the expected resistence of the heating elements should be so I am not sure if you can tell me that on the 02 GC Ltd with MHSM.

    Customer:

    Do you know where I can get/borrow a DRB scan tool? I know Checker Auto and other auto parts stores loan certain code readers, etc.

    Customer:

    Do you know where the Power Seat Circuit breakers is located in the junction box (under the hood or under the dash)? Is there one circuit breaker for both seats or one for each?

    Customer:

    Can you provide the information on the MHSM that your previous information referenced?

    Customer:

    Can you provide the Power Seat Premium I/III information in the wiring diagram content that shows the complete circuit diagram?

    Customer:

    Sorry not trying to be a pain but a lot of the information you first provided was general description or it referenced other information not provided (ie: MHSM, Premium I/III power seat info)

    Customer:

    I know I threw a lot of things at you but whatever you can answer would be great. I have a feeling that it might be the driver side heating element. I guess I need to find a DRB scan tool to verify it though unless you have any other ideas. Thanks again!


     

    Tom :

    I posted two wiring diagrams of the switches and the heating elements and seat module (MHSM). If they don't open let me know.

    The diagrams are the same for Premium I/III, MHSM is memory heated seat module, the same module powers the heat elements, memory and power seat function, the switches are controlled by the body control module. Since the lights work on the switches that is usuall an indication that the switches are ok. The DRB is normally only found at a Chrysler dealer it will tell you whether the switches are working and if the modules are communicating. The service info I provided was probably more confusing than useful so I apoligize for that.

    Open the diagrams and find the wires that go to the heater elements iin the seat. Test for power at the heat elements with a test light or meter. If there is no power to both elements the seat module is the common cause. If only one element is not working it is normally that element alone.

    Tom :

    Resistance of the elements is a moving target, but as long as it does have an open or infinite reisistance it should be good. Make sure when you ohm test it that you push around on the seat cushion and see if it goes open.

    Tom :

    Let me know if the diagrams from my first couple posts aren't showing up I can try them again.

    Customer:

    Tom, I had been traveling alot so didn't get any time to check the power based on your diagrams. I am planning to take a look at it today. Thanks for the help!

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