Circuit Description
The powertrain control module (PCM) output (L) terminal to the alternator determines when the alternator will be turned off. The alternator 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 modulated (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 the engine is not running. The telltale will turn off after the engine is running and the alternator is putting out the desired amperage.
Conditions for Running the DTC
| • | The engine speed is less 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 85 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:
| • | A poor connection at the PCM or the generator. Inspect the harness connectors for the following conditions. Refer to Testing for Intermittent Conditions and Poor Connections and Wiring Repairs in Wiring Systems: |
| - | Backed-out terminals |
| - | Improper mating |
| - | Broken locks |
| - | Improperly formed or damaged terminals |
| - | Poor terminal-to-wire connection |
| • | A damaged harness. Inspect the wiring harness for damage. If the harness appears to be OK, disconnect the PCM, turn the ignition ON, and observe a digital multimeter (DMM) connected to the generator circuits at the PCM harness connector while moving the connectors and the wiring harnesses related to the generator control circuit. A change in the voltage indicates the location of the fault. Refer to Testing for Intermittent Conditions and Poor Connections and Wiring Repairs in Wiring Systems. |
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 the Generator PWM. Is the Generator PWM within the indicated range? | 5 - 95 % | Go to Diagnostic System Check - 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 in Engine Electrical. | |
5 | Test the (L) circuit for an open or a short to ground and repair 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 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 short to ground and repair 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 if necessary. Refer to Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | ||
9 | Test for a poor connection at the PCM and repair 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 |
