Hummer H2. Manual - part 1161

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 10-11 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 

4

3. Connect test lamp to battery 

positive voltage.  

4. Turn ON the ignition, with the 

engine OFF.  

5. Probe the F-Terminal in the 

generator connector.  

6. Observe the GEN - F Terminal 

Signal parameter in the PCM data 
list.  

Is the GEN - F Terminal Signal 
parameter near the specified value?

100%

Go to Charging System 

Test (CS/AD/SI 

Generators)

Go to Step 5

5

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

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

Replace the PCM. Refer to Powertrain 
Control Module (PCM) Replacement 
in Engine Controls - 4.8L, 5.3L or 6.0L. 
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

 

2004 Hummer H2 

2004 ELECTRICAL Engine Electrical - Hummer H2

  

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 10-11 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 

z

The ignition is ON.  

z

System voltage between 9.0-18.0 volts.  

Conditions for Setting the DTC 

z

An improper voltage level has been detected on the generator L-terminal circuit.  

z

The above conditions are present for at least 30 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 

z

The DTC becomes history when the conditions for setting the DTC are no longer present.  

z

The history DTC clears after 40 malfunction free warm-up cycles.  

z

The PCM receives a clear code command from the scan tool.  

DTC P1637 

Step

Action

Value

Yes

No

Schematic Reference:Starting and Charging Schematics 
Connector End View Reference: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

1. Install a scan tool.  

2. Start the engine.  

3. With a scan 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 

Conditions and Poor 

Connections in Wiring 

Systems

Go to Step 3

1. Turn ON the ignition, leaving the 

 

2004 Hummer H2 

2004 ELECTRICAL Engine Electrical - Hummer H2

  

SYMPTOMS - ENGINE ELECTRICAL 

z

Perform Diagnostic System Check - Engine Electrical before using the Symptom Tables in order to 

3

engine OFF.  

2. Disconnect the generator harness 

connector.  

3. Measure the voltage between the 

generator turn on signal circuit and 
ground.  

Does the voltage measure near the specified 
value?

10-11 

volts

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 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 

(CS/AD/SI 

Generators)

6

Inspect for poor connections at the harness 
connector of the PCM. Refer to Testing for 
Intermittent Conditions and Poor 
Connections and Connector Repairs in 
Wiring Systems. 
Did you find and correct the condition?

-

Go to Step 8

Go to Step 7

7

Replace the PCM/ECM. Refer to 
Powertrain Control Module (PCM) 
Replacement in Engine Controls 4.8L, 
5.3L, and 6.0L.Is action complete?

IMPORTANT:

The replacement PCM must be 
programmed.

-

Go to Step 8

-

8

1. Use the scan tool in order to clear the 

DTCs.  

2. Operate the vehicle within the 

Conditions for Running the DTC as 
specified in the supporting text.  

Does the DTC reset?

-

Go to Step 2

System OK

IMPORTANT: The following steps must be completed before using the symptom tables.

 

2004 Hummer H2 

2004 ELECTRICAL Engine Electrical - Hummer H2

  

verify that all of the following are true: 

{

There are no DTCs set.  

{

The control module(s) can communicate via the serial data link.  

z

Review the system operation in order to familiarize yourself with the system functions. Refer to one of 
the following system operations: 

{

Battery Description and Operation  

{

Starting System Description and Operation  

{

Charging System Description and Operation  

Visual/Physical Inspection 

z

Inspect for aftermarket devices which could affect the operation of the Starting and Charging Systems. 
Refer to Checking Aftermarket Accessories in Wiring Systems.  

z

Inspect the easily accessible or visible system components for obvious damage or conditions which could 
cause the symptom.  

Intermittent 

Faulty electrical connections or wiring may be the cause of intermittent conditions. Refer to Testing for 
Intermittent Conditions and Poor Connections in Wiring Systems. 

Symptom List 

Refer to a symptom diagnostic procedure from the following list in order to diagnose the symptom: 

z

Starter Solenoid Does Not Click  

z

Starter Solenoid Clicks, Engine Does Not Crank  

z

Engine Cranks Slowly  

z

Battery Inspection/Test (Non-HP2)  

z

Charge Indicator Always On  

z

Charge Indicator Inoperative  

z

Charging System Test (CS/AD/SI Generators)  

z

Generator Noise Diagnosis  

BATTERY INSPECTION/TEST (NON-HP2) 

Diagnostic Aids 

CAUTION: Refer to Battery Disconnect Caution in Cautions and Notices.

IMPORTANT:

z

The battery test using the J 42000 Battery Tester requires correct 

 

2004 Hummer H2 

2004 ELECTRICAL Engine Electrical - Hummer H2