Circuit Description
The powertrain control module (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 in order to remove excess load from the engine.
The PCM input field (F) terminal from the alternator provides a 400 Hz pulse width modulation (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 lamp will go out after the engine is running and the alternator is distributing the desired amperage.
Conditions for Running the DTC
| • | The engine speed is less than 1000 RPM to run the low duty cycle test. |
| OR |
| • | The engine speed is more than 1300 RPM to run the high duty cycle test. |
Conditions for Setting the DTC
| • | The (L) terminal voltage is low while the PCM is commanding the generator ON. |
| • | The above condition is present for 10 seconds. |
OR
| • | The (F) terminal PWM is less than 10 percent for 30 seconds. |
| OR |
| • | The (F) terminal PWM is more than 95 percent for 60 seconds. |
Action Taken When the DTC Sets
The PCM stores conditions which were present when the DTC set as Failure Records only. This information will not be stored as Freeze Frame Records.
Conditions for Clearing the MIL/DTC
| • | The DTC becomes history when the conditions for setting the DTC are no longer present. |
| • | The history DTC clears after 40 malfunction free warm-up cycles. |
| • | The PCM receives a clear code command from the scan tool. |
Diagnostic Aids
Inspect for the following conditions:
Important: Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.
| • | Faulty terminal connection--Inspect the harness connectors for the following conditions: |
| - | Backed out terminals |
| - | Improper mating |
| - | Broken locks |
| - | Improperly formed or damaged terminals |
| - | Faulty terminal to wire connection--Use a corresponding mating terminal in order to test for proper tension. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. |
| • | Damaged harness--Inspect the wiring harness for damage. |
| If the harness inspection does not reveal a problem, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. |
| • | Clean and secure connections at the PCM and the engine grounds. |
If the DTC is determined to be intermittent, reviewing the Failure Records can be useful in determining when the DTC was last set.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On Board Diagnostic (OBD) System Check? | -- | ||
2 | Using the scan tool, monitor Generator PWM. Does the scan tool indicate that the Generator PWM is within the indicated range? | 5% - 95% | Go to Diagnostic System Check - Engine Electrical or Diagnostic Starting Point - Engine Electrical in Engine Electrical | |
3 |
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? | 0.0 V 5.0 V | ||
4 |
Is the Generator PWM display affected? | -- | Go to Diagnostic System Check - Engine Electrical or Diagnostic Starting Point - Engine Electrical in Engine Electrical | |
5 | Test the (L) circuit for an open or a short to ground and repair the circuit if necessary. Refer to Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | ||
6 | Test the (L) circuit for a short to voltage and repair the circuit if necessary. Refer to Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | ||
7 | Test the (F) circuit for an open or a short to ground and repair the circuit if necessary. Refer to Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | ||
8 | Test the (F) circuit for a short to voltage and repair the circuit if necessary. Refer to Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | ||
9 | Test for a faulty connection at the PCM and repair the connection if necessary. Refer to Testing for Continuity , and Testing for Intermittent Conditions and Poor Connections in Wiring Systems. Did you find and correct the condition? | -- | ||
|
Important: : The replacement PCM must be programmed. Replace the PCM. Refer to Powertrain Control Module Replacement/Programming . Is the action complete? | -- | -- | ||
11 |
Does the scan tool indicate DTC P0620 failed this ignition? | -- | System OK |
