Circuit Description
The generator has an input to the PCM called the F Terminal to indicate the percentage of total capacity that the generator is producing. This signal is detected by the PCM as a duty cycle from the generator and displayed on the scan tool as a percentage. The PCM can monitor the generators output under all conditions to determine if it is functioning normally.
When there is low demand from the electrical system on the generator, a low duty cycle percentage will be displayed. As more accessory load is placed on the generator, the duty cycle output detected by the PCM will approach 100 percent. A normally functioning generating system will never reach 100 percent as indicated on the scan tool.
Conditions for Running the DTC
The engine is operating.
Conditions for Setting the DTC
| • | The F Terminal indicates a duty cycle less than 10 percent with the engine running. |
| • | All conditions met for 6 seconds. |
Action Taken When the DTC Sets
| • | The PCM stores the DTC information into memory when the diagnostic runs and fails. |
| • | The malfunction indicator lamp (MIL) will not illuminate. |
| • | The PCM records the operating conditions at the time the diagnostic fails. The PCM stores this information in the Failure Records. |
Conditions for Clearing the DTC
| • | A last test failed, or current DTC, clears when the diagnostic runs and does not fail. |
| • | A history DTC will clear after 40 consecutive warm-up cycles, if no failures are reported by this or any other non-emission related diagnostic. |
| • | Use a scan tool in order to clear the DTC. |
Diagnostic Aids
| • | DTCs P1637 and P1638 may set at the same time depending on the failure. Also, the L-terminal circuit can cause a DTC P1638 to set. This diagnostic table diagnoses both DTCs P1637 and P1638. |
| • | Use a scan tool in order to diagnosis the generator circuits. The scan tool should display Inactive for the L terminal and 10-40 percent for the F terminal when the ignition is ON. When the engine is operating, the display should indicate that the L terminal is Active and the F terminal is more than 10 percent. |
| • | A generator fault, such as a shorted output diode, malfunctioning regulator, open or shorted rotor, or open sense lead or battery feed may cause a low battery charge. Refer to Charging System Test in Engine Electrical for further information and generator diagnosis. |
| • | For an intermittent condition, refer to Symptoms . |
Test Description
The numbers below refer to the step numbers on the Diagnostic Table.
-
This step verifies the fault is current.
-
Inspect the circuit for an open or a short to ground if the test lamp did not illuminate.
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If the scan tool displays Active, this indicates the circuit is open.
-
This step tests the F terminal for being shorted to a voltage. With the ignition ON, the F terminal should display between 10-40 percent. If the F terminal displays greater than 40 percent, inspect the circuit for a short to voltage.
-
When the engine is started the L terminal should be active.
-
This step is testing the L terminal for an open. If the L terminal displays Active when the circuit is grounded, this indicates the circuit is open.
-
This step is testing the F terminal circuit for an open or a short to ground. If the F terminal parameter goes to 99 percent the condition is within the generator.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||
|
Important: The following tests assumes that the generator belt is installed correctly and in good condition. The battery is adequately charged.
Did DTC P1637 and/or P1638 fail this ignition? | -- | |||
3 |
Did DTC P1637 and/or P1638 fail this ignition? | -- | Go to Diagnostic Aids | |
Does the test lamp illuminate for both circuits? | -- | Go to Diagnostic System Check - Engine Electrical in Engine Electrical | ||
Does the scan tool display L terminal as inactive? | -- | |||
Monitor the F terminal parameter with a scan tool in the Engine 3 Data list. Is the F terminal percentage less than the specified value? | 40% | |||
Does the scan tool display L terminal as inactive? | -- | |||
Does the scan tool display L terminal as inactive? | -- | |||
9 |
Does the DMM indicate continuity within the specified range? | 0-2ohms | ||
Did the Generator F terminal parameter change from 0 to 99 percent? | -- | |||
11 | Test the continuity of the F terminal circuit from the generator harness connector to battery ground with the DMM. Refer to Testing for Continuity in Wiring Systems. Does the DMM indicate continuity within the specified range? | 0-10ohms | ||
12 |
Does the DMM indicate continuity within the specified range? | 0-2ohms | ||
13 |
Is the F terminal percentage at the specified value? | 0% | ||
14 |
Does the scan tool display L terminal as inactive? | -- | ||
15 | Repair the open/high resistance in the generator circuit. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | -- | |
16 | Repair the short to ground in the generator circuit. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | -- | |
17 | Repair the short to voltage on the F terminal circuit. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | -- | |
18 |
Is the action complete? | -- | ||
19 |
Important: Program the replacement PCM. Replace the PCM. Refer to Powertrain Control Module Replacement . Is the action complete? | -- | -- | |
20 |
Did you find and correct the condition? | -- | ||
21 | Replace the generator. Refer to Generator Replacement in Engine Electrical. Is the action complete? | -- | -- | |
22 |
Does the scan tool indicate that this diagnostic ran and passed? | -- | ||
23 | Select the Capture Info option and the Review Info option using the scan tool. Does the scan tool display any DTCs that you have not diagnosed? | -- | Go to the applicable DTC table | System OK |
