Land Rover Defender: Heater Mats

DIAGNOSIS AND TESTING

PRINCIPLES OF OPERATION

For a detailed description of the Heater Mats, refer to the relevant Description and Operation section in the workshop manual.

INSPECTION AND VERIFICATION

CAUTION: Diagnosis by substitution from a donor vehicle is NOT acceptable. Substitution of control modules does not guarantee confirmation of a fault, and may also cause additional faults in the vehicle being tested and/or the donor vehicle.

NOTES:

• If a control module or a component is at fault and the vehicle remains under manufacturer warranty, refer to the Warranty Policy and Procedures manual, or determine if any prior approval program is in operation, prior to the installation of a new module/component.
• When performing voltage or resistance tests, always use a digital multimeter that has the resolution ability to view 3 decimal places. For example, on the 2 volts range can measure 1mV or 2 K Ohm range can measure 1 Ohm. When testing resistance always take the resistance of the digital multimeter leads into account.
• Check and rectify basic faults before beginning diagnostic routines involving pinpoint tests.

1. Verify the customer concern
2. Check the operation of other seats in the row (if more than one seat in the row has failed, this could be a software related issue)
3. Visually inspect for obvious signs of damage and system integrity (damage to seat trims including cuts, damage to wiring harnesses).

Visual Inspection

4. If an obvious cause for an observed or reported concern is found, correct the cause (if possible) before proceeding to the next step
5. Check the JLR claims submission system for open campaigns. Refer to the corresponding bulletins and SSMs which may be valid for the specific customer complaint and complete the recommendations as required
6. If the cause is not visually evident, verify the symptom and refer to the Symptom Chart, alternatively check for Diagnostic Trouble Code(s) (DTC)s and refer to the DTC Index

SEAT HEATER MAT DIAGNOSTICS

Each seat has two heating elements: one in the base (the seat heater module, comprising the seat cushion heating element and an integral heat sensor), and one in the back (backrest heating element). The seat cushion and backrest seat heater elements connect to the HVAC Control Module, although the elements are electrically wired in series (each drawing proportion of battery voltage). As well as power and ground circuits, there is a Local Interconnect Network (LIN) circuit between the seat heater module and the HVAC Control Module. The seat heater module monitors the performance of both seat cushion and backrest heaters and reports the status of the seat heating system through the LIN connection to the HVAC Control Module. Any faults detected in the seat cushion and/or backrest heaters should trigger a DTC to be set by the HVAC Control Module. The table below lists the potential symptoms that may be exhibited by the seat heater system and the steps that should be taken to rectify the faults.

SEAT HEATER MAT DIAGNOSTICS AND ASSOCIATED SYMPTOMS

SEAT HEATER MAT FAULTS - FURTHER DIAGNOSTIC GUIDANCE

• Advice On Pinpointing Which Heater Mat Is Faulty - Diagnosis Of Seat Heater Elements Using Resistance Measurements Of Backrest Heater Mat - In the case of suspected seat heater element failures, the exact location of the failure (seat cushion or backrest heater element) should always be confirmed by electrical resistance tests before replacement. Do not replace components based only on temperature performance. Heater mat failures can be located through resistance measurements of the backrest heater mat (Note: the resistance of the seat cushion heater mat cannot be measured directly because one pin is hid by the seat cushion cover over the unit). Use the following procedure to check the resistance of the backrest heater mat:
  • Disconnect the backrest heater mat.
  • Use a digital multimeter to measure the backrest heater mat resistance @ 20ºC (note: this should not be done when the seat heaters have been used recently, as a previous heating cycle can increase the resistance leading to an inaccurate reading).
  • Refer to the table below for expected resistance values. If the measured resistance is outside this tolerance, then replace the backrest seat heater mat. If the resistance is within the specified tolerance, the seat cushion heater mat should be replaced.
• Seat Heater Sensor Faults - Each seat has two heating elements: one in the base (seat cushion), and one in the back (backrest). There is only one heater sensor in each seat, which is located in the base. This sensor controls both heating elements. If there is a fault with the sensor signal, neither element will operate. If the signal fault is traced to the sensor itself (in the seat cushion heater mat), there is no need to replace the backrest heater mat.
• Simultaneous Failure of Both Seat Heater Mats Is Unlikely - The seat cushion and backrest seat heater elements connect to the HVAC Control Module, but the control module only acts as a splice box and the elements are electrically wired in series (each drawing proportion of battery voltage). The only way for one element to operate while the other is faulty is if one is the elements has a short-circuit fault. Under normal circumstances, where short-circuit faults are present these will be detected by the HVAC Control Module which will switch off both elements. Similarly, if the sensor (in the seat cushion element) is faulty, the HVAC Control Module will not apply voltage to the elements and, if one element is open-circuit, then no voltage can be supplied to the other element.

  Given this, while it may often be the case that both elements cease to operate, it is extremely unlikely that both heater mats are faulty. Consequently, if a fault is confirmed with one of the seat heater mats, do not also replace the other.

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