GM Service Manual Online
For 1990-2009 cars only

Diagnostic Instructions

    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptors

DTC B3920 01: Group 1 Seat Motors Common Circuit Short to Battery

DTC B3920 02: Group 1 Seat Motors Common Circuit Short to Ground

DTC B3921 01: Group 2 Seat Motors Common Circuit Short to Battery

DTC B3921 02: Group 2 Seat Motors Common Circuit Short to Ground

Diagnostic Fault Information

Circuit

Short to Ground

High Resistance

Open

Short to Voltage

Signal Performance

Adjustable Pedal Motor Forward Control

B3920 02

B1335 00, B1335 0B

3

B3920 01

B1335 42, B1335 4B, B3606 5A

Adjustable Pedal Motor Rearward Control

B3920 02

B1335 00, B1335 0B

3

B3920 01

B1335 42, B1335 4B, B3606 5A

Driver Seat Front Vertical Motor Down Control

B3920 02

B1335 00, B1335 0B

1

B3920 01

B1335 42, B1335 4B, B2355 5A

Driver Seat Front Vertical Motor Up Control

B3920 02

B1335 00, B1335 0B

1

B3920 01

B1335 42, B1335 4B, B2355 5A

Driver Seat Rear Vertical Motor Down Control

B3920 02

B1335 00, B1335 0B

1

B3920 01

B1335 42, B1335 4B, B2365 5A

Driver Seat Rear Vertical Motor Up Control

B3920 02

B1335 00, B1335 0B

1

B3920 01

B1335 42, B1335 4B, B2365 5A

Driver Seat Horizontal Motor Forward Control

B3920 02

B1335 00, B1335 0B

1

B3920 01

B1335 42, B1335 4B, B2375 5A

Driver Seat Horizontal Motor Rearward Control

B3920 02

B1335 00, B1335 0B

1

B3920 01

B1335 42, B1335 4B, B2375 5A

Driver Seat Recline Motor Forward Control

B3920 02

B1335 00, B1335 0B

1

B3920 01

B1335 42, B1335 4B, B1825 5A

Driver Seat Recline Motor Rearward Control

B3920 02

B1335 00, B1335 0B

1

B3920 01

B1335 42, B1335 4B, B1825 5A

Driver Seat Lumbar Motor Up Control

B3921 02

B1340 00, B1340 0B

2

B3921 01

B1340 42, B1340 4B, B1860 5A

Driver Seat Lumbar Motor Down Control

B3921 02

B1340 00, B1340 0B

2

B3921 01

B1340 42, B1340 4B, B1860 5A

Driver Seat Lumbar Motor Forward Control

B3920 02

B1335 00, B1335 0B

2

B3920 01

B1335 42, B1335 4B, B1850 5A

Driver Seat Lumbar Motor Rearward Control

B3920 02

B1335 00, B1335 0B

2

B3920 01

B1335 42, B1335 4B, B1850 5A

Tilt Steering Wheel Motor Down Control

B3921 02

B1340 00, B1340 0B

4

B3921 01

B1340 42, B1340 4B, B2880 5A

Tilt Steering Wheel Motor Up Control

B3921 02

B1340 00, B1340 0B

4

B3921 01

B1340 42, B1340 4B, B2880 5A

1. Power seat motor inoperative

2. Lumbar motor inoperative

3. Adjustable pedal motor inoperative

4. Tilt steering wheel actuator inoperative

Circuit/System Description

The memory seat module (MSM) controls the driver seat, adjustable pedal, and tilt steering wheel motors via half bridges that are divided into 2 groups that are connected to separate power rails internal to the module. The MSM connects all of the motor control circuits to a common reference point whenever they are not in operation. This reference point is biased to approximately 2.5 V. The MSM checks to see if this reference voltage is shorted to ground or battery before enabling any seat, adjustable pedal, or steering wheel motor.

Conditions for Running the DTC

The MSM must be powered.

Conditions for Setting the DTC

B3920 01

The voltage on any power rail 1 motor control circuit exceeds 2.78 V for 100 ms.

B3920 02

The voltage on any power rail 1 motor control circuit is below 1.47 V for 100 ms.

B3921 01

The voltage on any power rail 2 motor control circuit exceeds 2.78 V for 100 ms.

B3921 02

The voltage on any power rail 2 motor control circuit is below 1.47 V for 100 ms.

Action Taken When the DTC Sets

The MSM will not allow operation of any motor on the power rail for which the DTC was set.

Conditions for Clearing the DTC

    • The current DTC clears when the malfunction is no longer present and the power mode changes to OFF then back to ACC or RUN.
    • A history DTC will clear after 50 consecutive fault-free ignition cycles have occurred.

Reference Information

Schematic Reference

    •  Driver Seat Schematics
    •  Passenger Seat Schematics
    •  Adjustable Pedal Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Memory Seats Description and Operation

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Testing

B3920 01 or B3920 02

  1. Ignition OFF, disconnect the X1 harness connector at the MSM.
  2. Ignition ON, verify that a test lamp does not illuminate between the control circuits listed below and ground:
  3. • Front vertical motor terminal 12
    • Front vertical motor terminal 13
    • Rear vertical motor terminal 15
    • Rear vertical motor terminal 16
    • Seat horizontal motor terminal 9
    • Seat horizontal motor terminal 10
    If the test lamp illuminates, test the control circuit for a short to voltage.
  4. Test for infinite resistance between the control circuits listed below and ground:
  5. • Front vertical motor terminal 12
    • Front vertical motor terminal 13
    • Rear vertical motor terminal 15
    • Rear vertical motor terminal 16
    • Seat horizontal motor terminal 9
    • Seat horizontal motor terminal 10
    If not the specified value, test the control circuit for a short to ground.
  6. Ignition OFF, disconnect the X4 harness connector at the MSM.
  7. Ignition ON, verify that a test lamp does not illuminate between the control circuits listed below and ground:
  8. • Lumbar horizontal motor terminal 12
    • Lumbar horizontal motor terminal 13
    • Seat recline motor terminal 15
    • Seat recline motor terminal 16
    • Adjustable pedal actuator terminal 9
    • Adjustable pedal actuator terminal 10
    If the test lamp illuminates, test the control circuit for a short to voltage.
  9. Test for infinite resistance between the control circuits listed below and ground:
  10. • Lumbar horizontal motor terminal 12
    • Lumbar horizontal motor terminal 13
    • Seat recline motor terminal 15
    • Seat recline motor terminal 16
    • Adjustable pedal actuator terminal 9
    • Adjustable pedal actuator terminal 10
    If not the specified value, test the control circuit for a short to ground.
  11. If all circuits test normal, replace the MSM.

B3921 01 or B3921 02

  1. Ignition OFF, disconnect the X4 harness connector at the MSM.
  2. Ignition ON, verify that a test lamp does not illuminate between the control circuits listed below and ground:
  3. • Lumbar vertical motor terminal 1
    • Lumbar vertical motor terminal 2
    • Steering wheel tilt actuator terminal 7
    • Steering wheel tilt actuator terminal 8
    If the test lamp illuminates, test the control circuit for a short to voltage.
  4. Test for infinite resistance between the control circuits listed below and ground:
  5. • Lumbar vertical motor terminal 1
    • Lumbar vertical motor terminal 2
    • Steering wheel tilt actuator terminal 7
    • Steering wheel tilt actuator terminal 8
    If not the specified value, test the control circuit for a short to ground.
  6. If all circuits test normal, replace the MSM.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Front Seat Adjuster Replacement
    •  Front Seat Recliner Actuator Motor Replacement
    •  Front Seat Lumbar Support Replacement
    •  Brake Pedal Assembly Replacement
    •  Control Module References for the MSM replacement, programming and setup