GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2007 🢒 Lucerne 🢒 DTC 🢒 DTC B3920 or B3921

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 Shorted to Battery

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

Diagnostic Fault Information

Circuit

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

Driver Seat Front Vertical Motor Down Control

B3920 02

1

B3920 01

B2355 5A

Driver Seat Front Vertical Motor Up Control

B3920 02

1

B3920 01

B2355 5A

Driver Seat Rear Vertical Motor Down Control

B3920 02

1

B3920 01

B2365 5A

Driver Seat Rear Vertical Motor Up Control

B3920 02

1

B3920 01

B2365 5A

Driver Seat Horizontal Motor Forward Control

B3920 02

1

B3920 01

B2375 5A

Driver Seat Horizontal Motor Rearward Control

B3920 02

1

B3920 01

B2375 5A

Driver Seat Recline Motor Forward Control

B3920 02

1

B3920 01

B1825 5A

Driver Seat Recline Motor Rearward Control

B3920 02

1

B3920 01

B1825 5A

Driver Seat Lumbar Motor Down Control

B3920 02

2

B3920 01

B1860 5A

Driver Seat Lumbar Motor Up Control

B3920 02

2

B3920 01

B1860 5A

Driver Seat Lumbar Motor Forward Control

B3920 02

2

B3920 01

B1850 5A

Driver Seat Lumbar Motor Rearward Control

B3920 02

2

B3920 01

B1850 5A

1. Power seat inoperative

2. Lumbar inoperative

Circuit/System Description

The memory seat module (MSM) controls the seat, recline, and lumbar motors via half bridges that are connected to a power rail internal to the module. Power rail 1 includes of the following motors:

    • Seat front vertical motor
    • Seat rear vertical motor
    • Seat horizontal motor
    • Seat recline motor
    • Seat lumbar horizontal motor
    • Seat lumbar vertical motor

The MSM connects all of the motor control circuits on the power rail to a common reference point whenever they are not in operation. This reference point is biased to approximately 2.5 volts. The MSM checks to see if this reference voltage is shorted to ground or battery before enabling any seat, recline or lumbar motor.

Conditions for Running the DTC

All motor outputs for power rail 1 must be inactive for this DTC to set.

Conditions for Setting the DTC

B3920 01

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

B3920 02

The voltage on any power rail 1 motor control circuit is below 1.47 volts 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 and will set to history, when the voltage is between 1.47 volts and 2.5 volts for 3 seconds and the ignition is cycled OFF then back to ACC or RUN.
    • The history DTC will clear after 50 consecutive fault-free ignition cycles have occurred.

Reference Information

Schematic Reference

    •  Driver Seat Schematics
    •  Passenger Seat Schematics

Connector End View Reference

Power Seat Connector End Views

Electrical Information Reference

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

Scan Tool Reference

    •  Scan Tool Output Controls
    •  Scan Tool Data List
    •  Scan Tool Data Definitions

Circuit/System Testing
  1. Ignition OFF, disconnect the (C1) harness connector at the MSM.
  2. Ignition ON, verify that a test lamp does not illuminate between any of the following motor control circuits 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.
  3. Test for infinite resistance between the following motor control circuits and ground:
    4. • 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 less than infinite resistance, test the motor control circuit for a short to ground.
  4. Test for greater resistance values than specified between the following motor control circuits:
    5. • 1 ohm between the front vertical motor terminal 12 and terminal 13
    • 1 ohm between the rear vertical motor terminal 15 and terminal 16
    • 1 ohm between the seat horizontal motor terminal 9 and terminal 10
    ⇒If less than the specified resistance, test the control circuits for a short together or a shorted motor.
  5. Ignition OFF, disconnect the (C4) harness connector at the MSM.
  6. Ignition ON, verify that a test does not illuminate between any of the following motor control circuits and ground:
    7. • Lumbar vertical motor terminal 1
    • Lumbar vertical motor terminal 2
    • Lumbar horizontal motor terminal 12
    • Lumbar horizontal motor terminal 13
    • Seat recline motor terminal 15
    • Seat recline motor terminal 16
    ⇒If the test lamp illuminates, test the control circuit for a short to voltage.
  7. Test for infinite resistance between the following motor control circuits and ground:
    8. • Lumbar vertical motor terminal 1
    • Lumbar vertical motor terminal 2
    • Lumbar horizontal motor terminal 12
    • Lumbar horizontal motor terminal 13
    • Seat recline motor terminal 15
    • Seat recline motor terminal 16
    ⇒If less than infinite resistance, test the motor control circuit for a short to ground.
  8. Test for greater resistance values than specified between the following motor control circuits:
    9. • 8 ohms between the lumbar vertical motor terminal 1 and terminal 2
    • 4 ohms between the lumbar horizontal motor terminal 12 and terminal 13
    • 2 ohms between the recline motor terminal 15 and terminal 16
    ⇒If less than the specified resistance, test the control circuits for a short together or a shorted motor.
  9. If all circuits test normal, replace the memory seat module.

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
    •  Control Module References for MSM replacement, programming and setup