GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2006 🢒 DTS 🢒 Body 🢒 Seats 🢒 DTC B3920 or B3921

DTC Descriptors

This diagnostic procedure supports the following DTCs:

DTC B3920 01: Group 1 Seat Motors Circuit Shorted to Battery or Open

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

DTC B3921 01: Group 2 Seat Motors Circuit Shorted to Battery or Open

DTC B3921 02: Group 2 Seat Motors Circuit Shorted to Ground

Diagnostic Fault Information

Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

Circuit

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

Driver Seat Front Vertical Motor Down Control

B3920 02

1

B3920 01

--

Driver Seat Front Vertical Motor Up Control

B3920 02

1

B3920 01

--

Driver Seat Rear Vertical Motor Down Control

B3920 02

1

B3920 01

--

Driver Seat Rear Vertical Motor Up Control

B3920 02

1

B3920 01

--

Driver Seat Horizontal Motor Forward Control

B3920 02

1

B3920 01

--

Driver Seat Horizontal Motor Rearward Control

B3920 02

1

B3920 01

--

Driver Seat Recline Motor Forward Control

B3920 02

1

B3920 01

--

Driver Seat Recline Motor Rearward Control

B3920 02

1

B3920 01

--

Driver Seat Lumbar Motor Down Control

B3921 02

2

B3921 01

--

Driver Seat Lumbar Motor Forward Control

B3920 02

2

B3920 01

--

Driver Seat Lumbar Motor Up Control

B3921 02

2

B3921 01

--

Driver Seat Lumbar Motor Rearward Control

B3920 02

2

B3920 01

--

Passenger Massage Up Motor Up

B3921 02

2

B3921 01

--

Passenger Massage Down Motor

B3921 02

2

B3921 01

--

1. Power seat inoperative

2. Lumbar inoperative

Circuit/System Description

The memory seat module (MSM) controls the seat, recline, lumbar, and steering column motors via half bridges that are divided into 2 groups that are connected to separate power rails 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

Power rail 2 includes the following motors:

    • Driver seat lumbar vertical motor
    • Passenger seat lumbar vertical motor
    • Steering column tilt actuator
    • Steering column telescope actuator

The MSM connects all of the motor control circuits on each 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, lumbar, or steering column motor.

Diagnostic Aids

The scan tool may reference the Memory Seat Module (MSM) as the Driver Position Module.

Conditions for Running the DTC

B3920

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

B3921

All motor outputs for power rail 2 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.

B3921 01

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

B3921 02

The voltage on any power rail 2 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 sets to history when the reference voltage is between 1.47 volts and 2.5 volts for 3 seconds and the power mode changes to 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

B3920 Group 1 Motors

  1. Ignition OFF, disconnect the (C1) harness connector at the memory seat module.
  2. Ignition ON, verify that a test 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 for front vertical motor terminal 12 and terminal 13
    • 1 ohm for rear vertical motor terminal 15 and terminal 16
    • 1 ohms for seat horizontal motor terminal 9 and terminal 10
    ⇒If less than the specified resistance, test the control circuits for a short together or shorted motor.
  5. Ignition OFF, disconnect the (C4) harness connector at the memory seat module.
  6. Ignition ON, verify that a test does not illuminate between any of the following motor control circuits and ground:
    7. • 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 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. • 10 ohms for lumbar horizontal motor terminal 12 and terminal 13
    • 1 ohm for recline motor terminal 15 and terminal 16
    ⇒If less than the specified resistance, test the control circuits for a short together or shorted motor.
  9. If all circuits test normal, replace the memory seat module.

B3921 Group 2 Motors
  1. Ignition OFF, disconnect the (C4) harness connector at the memory seat module.
  2. Ignition ON, verify that a test lamp does not illuminate between any of the following motor control circuits and ground:
    3. • Driver seat lumbar vertical motor terminal 1
    • Driver seat lumbar vertical motor terminal 2
    • Passenger seat lumbar vertical motor terminal 4
    • Passenger seat lumbar vertical motor terminal 5
    • Steering column tilt actuator terminal 7
    • Steering column tilt actuator terminal 8
    ⇒If the test lamp illuminates, test the control circuit for a short to voltage.
  3. Test for infinite ohms of resistance between the following motor control circuits and ground:
    4. • Driver seat lumbar vertical motor terminal 1
    • Driver seat lumbar vertical motor terminal 2
    • Passenger seat lumbar vertical motor terminal 4
    • Passenger seat lumbar vertical motor terminal 5
    • Steering column tilt actuator terminal 7
    • Steering column tilt actuator terminal 8
    ⇒If less than infinite, test the control circuit for a short to ground.
  4. Test for greater resistance values than specified between the following motor control circuits:
    5. • 10 ohms for driver seat lumbar vertical motor terminal 1 and terminal 2
    • 10 ohm for passenger seat lumbar vertical motor terminal 4 and terminal 5
    • 1 ohm for steering column tilt actuator terminal 7 and terminal 8
    ⇒If less than the specified resistance, test the control circuits for a short together or shorted motor.
  5. Ignition OFF, disconnect the (C1) harness connector at the memory seat module.
  6. Ignition ON, verify that a test lamp does not illuminate between any of the following actuator control circuits and ground:
    7. • Steering column telescope actuator terminal 8
    • Steering column telescope actuator terminal 7
    ⇒If the test lamp illuminates, test the control circuit for a short to voltage.
  7. Test for infinite ohms of resistance between the following actuator control circuits and ground:
    8. • Steering column telescope actuator terminal 8
    • Steering column telescope actuator terminal 7
    ⇒If less than infinite, test the control circuit for a short to ground.
  8. Test for greater than 1 ohm of resistance between the steering column telescope actuator control circuits terminal 7 and terminal 8.
    9. ⇒If less than 1 ohm, test the control circuits for a short together or shorted actuator.
  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
    •  Tilt Motor Replacement
    •  Telescope Actuator Assembly Replacement
    •  Control Module References for MSM replacement, programming and setup.