GM Service Manual Online
For 1990-2009 cars only

Object Number: 515390  Size: SF
Engine Controls Component Views
Engine Data Sensors-MAF, EVAP Vent, EVAP Purge, EGR
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice

Circuit Description

The PCM Output (L) terminal to the alternator determines when the alternator will be turned OFF. It is on under all normal vehicle operating conditions except during low RPM conditions. The alternator is turned off during low RPM conditions to remove excess load from the engine.

The PCM Input Field (F) terminal from the alternator provides a 400 Hz PWM signal to the PCM. The duty cycle of the F terminal mimics the duty cycle of the field coils in the alternator, Thus, the F terminal duty cycle indicates the relative electrical load on the alternator.

The alternator telltale lamp will illuminate any time the ignition is on and engine is not running. The Telltale will go out after the engine is running and the alternator is putting out the desired amperage.

Diagnostic Aids

Check for the following conditions:

    •  Poor connection at the PCM or generator -- Inspect the harness connectors for the following conditions :
       - Backed out terminals
       - Improper mating
       - Broken locks
       -  Improperly formed or damaged terminals
       - Poor terminal to wire connection
       -  Refer to Testing for Intermittent Conditions and Poor Connections Connector Repairs and Wiring Repairs .
    •  Damaged harness -- Inspect the wiring harness for damage. If the harness appears to be OK, disconnect the PCM, turn the ignition on and observe voltmeter connected to the Generator circuits at the PCM harness connector while moving connectors and wiring harnesses related to the Generator control circuit. A change in voltage will indicate the location of the fault. Refer to Connector Repairs and Wiring Repairs in Wiring Systems.

Test Description

The numbers below refer to the step numbers on the diagnostic table:

  1. This vehicle is equipped with a PCM which utilizes and Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is being replaced, the new PCM must be programmed.

Step

Action

Values

Yes

No

1

Did you perform the Powertrain On-Board Diagnostic System Check?

--

Go To Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2

Does the scan tool indicate that the generator PWM is within the indicated range?

5%-95%

Go To Diagnostic System Check - Engine Electrical in Engine Electrical.

Go to Step 3

3

  1. Turn OFF the ignition.
  2. Disconnect the generator.
  3. Connect a DMM between the (L) terminal at the harness connector and a known good ground.
  4. Start the engine.
  5. Command the generator ON with the scan tool while monitoring the DMM.

Does the voltage change from the first indicated value to the second indicated value?

0 V

5 V

Go To Step 4

Go to Step 5

4

  1. Turn ON the ignition , with the engine OFF.
  2. Connect a test lamp to battery positive voltage, and repeatedly touch the (F) terminal at the generator harness connector while monitoring the generator PWM on the scan tool.

Is the reading affected?

--

Go To Diagnostic System Check - Engine Electrical in Engine Electrical.

Go to Step 7

5

Test the (L) circuit for an open or short to ground and repair if necessary. Refer to Wiring Repairs in Wiring Systems.

Was a problem found?

--

Go To Step 11

Go to Step 6

6

Test the (L) circuit for a short to voltage and repair if necessary. Refer to Wiring Repairs in Wiring Systems.

Was a problem found?

--

Go To Step 11

Go to Step 9

7

Test the (F) circuit for an open or short to ground and repair if necessary. Refer to Wiring Repairs in Wiring Systems.

Was a problem found?

--

Go To Step 11

Go to Step 8

8

Test the (F) circuit for a short to voltage and repair if necessary. Refer to Wiring Repairs in Wiring Systems.

Was a problem found?

--

Go To Step 11

Go to Step 9

9

Check for a poor connection at the PCM and repair if necessary. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.

was a problem found.

--

Go To Step 11

Go to Step 10

10

Important:: Replacement PCM must be programmed.

Replace the PCM. Refer to Powertrain Control Module Replacement/Programming .

Is action complete?

--

Go To Step 11

--

11

  1. Review and record scan tool Failure Records data.
  2. Clear DTCs
  3. Operate the vehicle within Failure Records conditions as noted.
  4. Using a scan tool, monitor Specific DTC info for DTC P0121.

Does the scan tool indicate DTC P0121 failed this ignition?

--

Go To Step 2

System OK