Circuit Description
The engine control module (ECM) uses the generator turn-on signal circuit to control the voltage output of the generator. When a different voltage is desired, the ECM will change the pulse width modulation (PWM) signal to the voltage regulator via the generator turn-on signal circuit. This causes the voltage regulator to the voltage output of the generator. The ECM has fault detection circuitry which monitors the state of the generator turn-on signal circuit. If the fault detection circuit senses a voltage other than what is expected, this DTC will set. The voltage regulator also contains fault detection circuitry. If the regulator detects a problem, the regulator will ground the generator turn-on signal circuit, pulling the voltage low. This also causes the ECM to set the DTC.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P1668 Generator L-Terminal Control Circuit
Conditions for Running the DTC
Key ON Test
| • | The key is in the ON position for 5 seconds. |
| • | The engine is OFF. |
Engine Run Test
| • | The engine is running. |
| • | The engine speed is less than 3,000 RPM. |
Conditions for Setting the DTC
Key ON Test
| • | The ECM detects a high signal voltage on the generator turn on signal circuit for at least 5 seconds. |
| • | The engine must be started. |
During the RUN test, the ECM detects a low signal voltage on the generator turn on signal circuit for at least 5 seconds.
Action Taken When the DTC Sets
| • | The ECM sends a class 2 message to the instrument panel cluster (IPC) to illuminate the charge indicator and/or turn on a message. |
| • | The ECM will not illuminate the malfunction indicator lamp (MIL). |
| • | The ECM will store the conditions present when the DTC set as Failure Records data only. |
Conditions for Clearing the MIL/DTC
| • | The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction. |
| • | The DTC can be cleared by using the scan tool Clear DTC Information function. |
Step | Action | Value | Yes | No |
|---|---|---|---|---|
Schematic Reference: Starting and Charging Schematics Connector End View Reference: Engine Control Module Connector End Views | ||||
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | |
2 |
Does the scan tool indicate that DTC P1668 has passed? | -- | Go to Testing for Intermittent Conditions and Poor Connections | Go to Step 3 |
3 |
Is the PWM signal within the specified value? | 15-87% | Go to Step 5 | Go to Step 4 |
4 | Test the generator turn on signal circuit for a short or open. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | -- | Go to Step 10 | Go to Step 5 |
5 | Test the generator battery positive voltage output circuit for a high resistance or open. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | -- | Go to Step 10 | Go to Step 6 |
6 | Inspect for poor connections at the harness connector of the generator. Refer to Connector Repairs . Did you find and correct the condition? | -- | Go to Step 10 | Go to Step 8 |
7 | Inspect for poor connections at the harness connector of the engine control module (ECM). Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | -- | Go to Step 10 | Go to Step 9 |
8 | Replace the generator. Refer to Generator Replacement . Did you complete the replacement? | -- | Go to Step 10 | -- |
9 |
Important: The replacement ECM must be programmed. Replace the ECM. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | -- | Go to Step 10 | -- |
10 |
Does the DTC reset? | -- | Go to Step 3 | System OK |
