GM Service Manual Online
For 1990-2009 cars only
Table 1: SGCM to MGU Terminal Reference Table
Table 2: MGU to SGCM Specification Table
Table 3: SGCM to MGU Specification Table

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 Descriptor

DTC P0A4B: Starter/Generator Position Sensor Circuit

Circuit/System Description

The motor generator unit (MGU) position sensor is used for determining motor control of the MGU. The motor control consists of the angular position and speed of the MGU. The rotor is bolted on the MGU shaft, the stator is bolted on the MGU bracket and the sensor circuit is soldered on the motor control board in the starter/generator control module (SGCM). The SGCM monitors the position sensor circuit to detect a converter error (loss of speed signal or tracking).

Conditions for Running the DTC

The engine is running.

Conditions for Setting the DTC

R/D converter error (loss of signal or tracking) for 100 ms

Action Taken When the DTC Sets

Stop PWM (MGU shutdown)

Conditions for Clearing the DTC

Clears 30 seconds after cycle wake up signal.

Diagnostic Aids

    • Possible internal sensor short to ground, short to voltage or open/high resistance.
    • Possible MGU or SGCM fault
    • Possible wiring harness short to ground, short to voltage or open/high resistance.

Reference Information

Schematic Reference

Hybrid Controls Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Starter/Generator 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

  1. Disconnect the MGU X2 and the SGCM X2 connectors. Refer to Generator with Starter Replacement and Generator Control Module Replacement.
  2. Test the circuits in the SGCM to MGU Terminal Reference Table as indicated below. Also test these circuits for short to ground.
  3. If any of the circuits are not within specification, check for an open/high resistance or short to ground.
  4. Reconnect SGCM connector X2.
  5. Test the circuits in the MGU to SGCM Specification Table to ground as indicated below.
  6. If any of the circuits are not within the specified range, replace the SGCM.
  7. Reconnect the MGU X2 connector and disconnect the SGCM X2 connector. Refer to Generator with Starter Replacement and Generator Control Module Replacement.
  8. Test the circuits in the SGCM to MGU Specification Table as indicated below.
  9. If all circuits test normal, replace the MGU.

SGCM to MGU Terminal Reference Table

MGU Connector/Terminal

SCGM Connector/Terminal

Minimum Resistance

Maximum Resistance

Harness Continuity Test

B

X2/7

--

less than 1 ohm

C

X2/6

--

less than 1 ohm

D

X2/4

--

less than 1 ohm

E

X2/17

--

less than 1 ohm

F

X2/20

--

less than 1 ohm

G

X2/19

--

less than 1 ohm

H

X2/5

--

less than 1 ohm


MGU to SGCM Specification Table

MGU Connector/Terminal

Signal Name

Minimum Resistance

Maximum Resistance

Measurements made to ground. SGCM connected and MGU disconnected.

D

REZ-

1.6k ohms

2k ohms

H

REZ+

1.6k ohms

2k ohms

C

REZ3

Over 1 M ohm

--

B

REZ4

Over 1 M ohm

--

E

REZGS (GND)

--

less than 5 ohm

G

REZ1

Over 1 M ohm

--

F

X2/5

Over 1 M ohm

--


SGCM to MGU Specification Table

SGCM Connector/Terminal

SGCM Connector/Terminal

Minimum Resistance

Maximum Resistance

MGU connected and SGCM disconnected.

X2/4

X2/5

20.1 ohms

26.7 ohms

X2/6

X2/19

32.4 ohms

43.0 ohms

X2/7

X2/20

32.9 ohms

43.5 ohms

X2/4

X2/6

Over 1 meg ohm

--

X2/4

X2/7

Over 1 meg ohm

--

X2/6

X2/7

Over 1 meg ohm

--

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Generator with Starter Replacement
    •  Control Module References for SGCM replacement, setup, and programming