Hummer H2. Manual - part 1091

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

  

Follow these instructions in order to avoid an incorrect diagnosis because of connections: 

z

If testing the vehicle with the battery cables still connected, wiggle the J 42000 clips on the terminal bolt. 
This may cut through any coating or through any oxidation that may be present on the bolt. See Special 
Tools and Equipment . 

Even new bolts contain a protective coating that may insulate or cause a resistance in the test circuit. 

z

If correct connections to the battery terminal bolts in the vehicle are in doubt, perform the following 
steps: 

1. Disconnect the negative battery cable.  

2. Disconnect the positive battery cable.  

3. Install the test adapters on the terminals.  

4. Follow the instructions for testing a removed battery.  

z

If the tester displays a REPLACE BATTERY or BAD CELL-REPLACE result for a battery tested in the 
vehicle with the battery cables connected, perform the following steps: 

1. Disconnect the negative battery cable.  

2. Disconnect the positive battery cable.  

3. Install the tester adapters. 

4. Follow the instructions for testing a removed battery.  

5. Replace the battery only if the second test shows a REPLACE BATTERY or BAD CELL-

REPLACE result. 

Use the test code from the second test for any warranty purposes. 

z

Use the correct terminal adapters. 

Do not use any common bolts or a combination of bolts, of nuts, and of washers as adapters when testing 
the battery. 

connections to the battery terminals. A failure to obtain the correct 
connections during the test may result in a failed test on a good battery. 
See Special Tools and Equipment .  

z

Use the Out of Vehicle test for each battery when testing a vehicle with 
dual batteries.  

IMPORTANT: Always write the test code displayed by the tester on the repair order 

for any warranty purposes. The number is a unique code that 
describes the test data for a particular battery at a particular time. 
The test code may occasionally repeat when you retest the same 
battery. More often, each test will result in a different code. Use the 
test code from the second, or Out of Vehicle test.

 

2004 Hummer H2 

2004 ELECTRICAL Engine Electrical - Hummer H2