Envoy 2WD L6-4.2L (2008)
Circuit/System Description
The memory seat module (MSM) controls the voltage supply and the ground circuits to the seat heater elements. When a heated seat function is
commanded active, the MSM will switch battery voltage to the heater element supply circuits, and ground is provided through low side drive control
circuits. During heated seat operation both the seat back and cushion heater elements are supplied battery voltage. The MSM grounds the appropriate
control circuits for back only or back and cushion heating modes, and opens or closes the active control circuits as necessary in order to maintain the
desired seat temperature. The MSM relies on inputs from thermistors located in the driver and passenger seat backs to control heated seat temperatures.
The thermistors are 2 wire sensors supplied with a 5-volt referenced signal circuit and low reference circuit from the MSM. Resistance through the
thermistors varies with temperature causing the heated seat sensor signal circuit voltage levels to decrease as the seat back temperatures rise
Circuit/System Verification
1. Ignition ON, observe the scan tool DDM, PDM Heated Seat Switch and Back Only Heat Switch parameters while pressing and releasing each of
the switches. The readings should change between Active and Inactive.
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If not the specified value, replace the affected door module.
2. Observe the scan tool MSM Seat Heat Zone and Seat Temp. Setting parameters while operating the heated seat in all operating modes. The
readings should change in relation to the heated seat switch operation.
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If not the specified value, replace the MSM.
3. If the heated seat switch inputs test normal, refer to Heated Seat Control Circuit Test.
Circuit/System Testing
1. Disconnect the harness connector at the affected seat heater element.
2. Connect a test lamp between the control circuit terminal A and the control circuit terminal B.
3. Command the heated seat ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.
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If the test lamp is always ON, test the control circuit terminal A for a short to voltage. If the circuit tests normal, replace the MSM.
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If the test lamp is always OFF, test the control circuit terminals A and B for a short to ground or an open/high resistance. If the circuit tests
normal, replace the MSM.
4. If all circuits test normal, test or replace the affected seat heater element.
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure. See: Powertrain Management/Computers and Control
Systems/Testing and Inspection/Diagnostic Trouble Code Tests and Associated Procedures/Verification Tests and Procedures