For 1990-2009 cars

### Circuit Description

The PCM uses the generator field duty cycle signal circuit to monitor the duty cycle of the generator. The generator field duty cycle signal circuit connects to the high side of the field winding in the generator. A pulse width modulated (PWM) high side driver in the voltage regulator turns the field winding ON and OFF. The PCM uses the PWM signal input to determine the generator load on the engine. This allows the PCM to adjust the idle speed to compensate for high electrical loads.

The PCM monitors the state of the generator field duty cycle signal circuit. When the key is in the RUN position and the engine is OFF, the PCM should detect a duty cycle near 0 percent. However, when the engine is running, the duty cycle should be between 5 percent and 100 percent. The PCM monitors the PWM signal using a key ON test and a RUN test. During the tests, if the PCM detects an out of range PWM signal, DTC P1638 will set. When the DTC sets, the PCM will turn on the charge indicator in the IPC.

### Conditions for Running the DTC

#### Key ON Test

• | No generator, CKP sensors, or CMP sensor DTCs are set.

• | The key is in the RUN position.

• | The engine is not running.

#### Run Test

• | No generator, CKP sensors, or CMP sensor DTCs are set.

• | The engine is less than 3000 RPM.

### Conditions for Setting the DTC

• | During the ignition ON test, the PCM detects a PWM signal is out of range.

• | During the RUN test, the PCM detects a PWM signal less then 5 percent for more than 6 seconds.

### Action Taken When the DTC Sets

• | The PCM will store the conditions present when the DTC set as Fail Records data only.

• | The malfunction indicator lamp (MIL) will not illuminate.

### Conditions for Clearing the DTC

• | The Conditions for Setting the DTC are no longer present.

• | A history DTC will clear after 40 malfunction free ignition cycles.

• | The Powertrain Control Module (PCM) receives the clear code command from the scan tool.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the Engine Electrical Diagnostic System Check? | -- | Go to Step 2 | Go to Diagnostic System Check - Engine Electrical
2 |

1. Install a scan tool.
2. Start the engine.
3. With a scan tool, observe the GEN - F Terminal parameter in the PCM data list .

Does the scan tool indicate that the GEN - F Terminal parameter is within the specified range?

5-95%

Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems

Go to Step 3

3 |

1. Turn ON the ignition, leaving the engine OFF.
2. Connect a test lamp to battery positive voltage and repeatedly probe the generator field duty cycle signal circuit in the harness connector while monitoring the Generator PWM on the scan tool.

Is the Generator PWM display affected?

--

Go to Step 5

Go to Step 4

4 | Test the generator field duty cycle signal circuit for a short or open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 8 | Go to Step 6
5 | Inspect for poor connections at the harness connector of the generator. Refer to Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 8 | Go to Charging System Test
6 | Inspect for poor connections at the harness connector of the PCM. Refer to Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 8 | Go to Step 7
7 | Important: The replacement PCM must be programmed. Replace the PCM. Refer to Powertrain Control Module Replacement in Engine Controls-8.1L. Did you complete the repair? | -- | Go to Step 8 | --
8 |

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

Does the scan tool indicate that this DTC failed this ignition?

--

Go to Step 2

System OK