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

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 P0A50: Starter/Generator Overspeed

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 an overspeed condition.

Conditions for Running the DTC

The engine is running.

Conditions for Setting the DTC

Absolute motor speed is greater than 21,000 RPM for 10 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 motor overspeed.

Reference Information

Schematic Reference

Hybrid Controls Schematics

Connector End View Reference

Hybrid Controls Connector End Views

Electrical Information Reference

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

Scan Tool Reference

Scan Tool Data List

Circuit/System Testing

Caution: To help avoid personal injury, additional precautions must be taken prior to working on the generator control module or the generator starter. After removing the 36V battery cables from the generator battery, remove both engine wiring harness connectors from the generator control module. Wait at least 5 minutes and then remove the generator control module cover. Verify voltage levels at all 36V, 12V, and 3-phase connections, are less than 3 volts using a DMM before proceeding.

  1. Disconnect the MGU C2 and the SGCM C2 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 check these circuits for short to ground.
  3. If any of the circuits are not within specification, repair the open/high resistance or short to ground.
  4. Reconnect MGU connector C2.
  5. Test the circuits in the MGU to SGCM Specification Table to ground as indicated below.
  6. If all circuits test normal, replace the MGU.
  7. Reconnect the MGU C2 connector and disconnect the SGCM C2 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 any of the circuits are not within the specified range, replace the SGCM.

SGCM to MGU Terminal Reference

MGU Connector/Terminal

SCGM Connector/Terminal

Minimum Resistance

Maximum Resistance

Harness Continuity Test.

B

C2/7

--

less than 1 ohm

C

C2/6

--

less than 1 ohm

D

C2/4

--

less than 1 ohm

E

C2/17

--

less than 1 ohm

F

C2/20

--

less than 1 ohm

G

C2/19

--

less than 1 ohm

H

C2/5

--

less than 1 ohm

MGU to SGCM Specification

MGU Connector/Terminal

Signal Name

Minimum Resistance

Maximum Resistance

Measurements made to ground. SGCM disconnected and MGU connected.

D

REZ-

1.6 K ohms

2 K ohms

H

REZ+

1.6 K ohms

2 K ohms

C

REZ3

Over 1 M ohm

--

B

REZ4

Over 1 M ohm

--

E

REZGS (GND)

--

less than 5 ohms

G

REZ1

Over 1 M ohm

--

F

C2/5

Over 1 M ohm

--

SGCM to MGU Specification

SGCM Connector/Terminal

SGCM Connector/Terminal

Minimum Resistance

Maximum Resistance

MGU disconnected and SGCM connected.

C2/4

C2/5

20.1 ohms

26.7 ohms

C2/6

C2/19

32.4 ohms

43.0 ohms

C2/7

C2/20

32.9 ohms

43.5 ohms

C2/4

C2/6

Over 1 M ohm

--

C2/4

C2/7

Over 1 M ohm

--

C2/6

C2/7

Over 1 M ohm

--

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

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