Can't locate this code on my list of C5 codes.

 

Here is a description along with the associated diagnostic table:

DTC P1637
Circuit Description
The powertrain control module (PCM) uses the generator turn on signal circuit to control the generator. A high side driver within the PCM allows the PCM to turn the generator ON and OFF. When Generator operation is desired, the PCM sends a 5 volt signal to the voltage regulator via the generator turn on signal circuit. This causes the voltage regulator to begin controlling the generator field circuit. Once the Generator is enabled by the PCM, the voltage regulator controls generator output independently of the PCM. Under certain operating conditions, the PCM can turn Off the generator by turning Off the 5 volt signal on the generator turn on signal circuit. The PCM 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 PCM to set the DTC.

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 3000 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 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 Fail 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.

Diagnostic Table (5 columns reduced to 1 column)
Step
Action
Value
Yes
No

Schematic Reference: Starting and Charging Schematics

Connector End View Reference: Engine Controls Connector End Views in Engine Controls-5.7L, Power and Grounding Connector End Views in Wiring Systems, or Engine Electrical Connector End Views

1
Did you perform the Engine Electrical Diagnostic System Check?
--
Go to Step 2
Go to Diagnostic System Check - Engine Electrical

2
Install a scan tool.
Turn ON the ignition, with the engine OFF.
Wait 5 seconds.
Start the engine.
With a scant tool, monitor the DTC information for DTC P1637 in Engine Controls.
Does the scan tool indicate that DTC P1637 has passed?
--
Go to Testing for Intermittent and Poor Connections in Wiring Systems
Go to Step 3

3
Turn Off the engine.
Disconnect the generator harness connector.
Start the engine.
Measure the voltage between the generator turn on signal circuit and ground.
Does the voltage measure near the specified value?
5.0 V
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 in Wiring Systems.

Did you find and correct the condition?
--
Go to Step 11
Go to Step 5

5
Test the generator battery positive voltage sense circuit for a high resistance or open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?
--
Go to Step 11
Go to Step 6

6
Test the generator battery positive voltage output circuit for a high resistance or open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?
--
Go to Step 11
Go to Step 7

7
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 11
Go to Step 9

8
Inspect for poor connections at the harness connector of the PCM. Refer to Testing for Intermittent and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?
--
Go to Step 11
Go to Step 10

9
Replace the generator. Refer to Generator Replacement .

Did you complete the replacement?
--
Go to Step 11
--

10

Important
The replacement PCM must be programmed.


Replace the PCM. Refer to Powertrain Control Module (PCM) Replacement in Engine Controls 5.7L.

Did you complete the repair?
--
Go to Step 11
--

11
Use the scan tool in order to clear the DTCs .
Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.
Does the DTC reset?
--
Go to Step 3
System OK

 

I appreciate the response but I'm no electrical engineer so if you could give the cliff notes version that would be great.

 

This isn't really EE level stuff. The PCM turns the generator on and off. It checks generator status by monitoring a voltage. If that voltage isn't what is expected the DTC is set. The fault can be in the generator (alternator) wiring, the alternator or the PCM. The diagnostic tells you to check the wiring and voltage measurements at the alternator.
Bill