Hummer H1 (2002+). Manual - part 194

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Electrical System 12-3

05745159

DIAGNOSTICS

The following section of the service manual deals with electri-
cal diagnostics. The section consists of three main tools:

Diagnostic charts

Wiring diagrams

Connector legend

Diagnostic Charts

Diagnostic charts are designed to take you through individual
steps to locate a problem. The steps start at a point in the elec-
trical system and progress to find the problem without over-
looking items that may be contributing to the problem. In most
cases the chart will refer to a wiring diagram, and connector
callouts, both of which are provided for reference. The charts
are designed to find problems which are current, not intermit-
tent. These charts do not include preliminary visual checks.
The preliminary checks must be done prior to using the charts
for diagnosis. If the checks are not done, a simple problem
such as a blown fuse may be overlooked.

Wiring Diagrams

Wiring diagrams give a visual representation of the vehicle
wiring the diagnostic charts refer to. The wiring diagrams in
this section are broken down into systems. The diagrams will
include any part of the electrical system which pertains to the
system being diagnosed. Refer to the connector legend for con-
nector identification of system connectors.

Connector Legend

The connector legend is a reference library of all the vehicle con-
nectors that are called out in the wiring diagrams. Included are
drawings of both male and female connector halves called out
with pin/cavity arrangement, and a listing of all included circuits.

Preliminary Checks

The diagnostics in this section are designed to find problems
that are not obvious. Before starting any formal diagnostic
chart, preliminary checks must be performed. Preliminary
checks should include:

• Visual inspection of all components

• Inspection of any fuses related to the system

• Connector inspection

• Inspection of electrical harnesses

• Charging system and batteries

These types of checks are not included in the diagnostic chart.
In fact, most of the charts rely on the fact that you, the techni-
cian, has performed these initial inspections.

Diagnostic Strategy

Whenever a vehicle is being diagnosed for a problem, a strat-
egy should be used. The following charts are strategy based,
meaning they all follow the principle of starting simple and

working step by step to more complex tests. This allows prob-
lems which may be small in nature to be found quickly, and not
overlooked. NEVER skip steps in a diagnostic chart. Each step
relies on a previous step for correct diagnosis. Avoid random
diagnostics and parts replacement which can lead to long, ex-
pensive diagnostic times and may not reveal the problem.

Intermittent Failures

ALL of the charts in this section are for use on current failures.
Do not attempt to use these charts to diagnose a problem unless
you are sure the problem currently exists. If the problem is in-
termittent, parts will be unnecessarily replaced, or no problem
will be found.

BASIC ELECTRICAL CIRCUITS

WARNING: When removing battery cables, disconnect
ground cable first. Ensure all switches are off before
disconnecting battery ground cable

General

An electrical circuit is a number of electrical devices which are
connected in a loop from a positive voltage source (battery
positive) to a negative ground source.

Parallel Circuits

The Hummer electrical system is a parallel circuit. In a parallel
circuit, the electrical devices form more than one current path
to and from the power supply. The supply voltage is the same
in each path.

Circuit Components

A normal circuit path starts at the power supply (battery system
or alternator). Next in line is the circuit protection: fusible link,
fuse, or circuit breaker. The circuit load, such as lights, motors,
or solenoids completes the circuit to the ground system.

Circuit protection devices

Fusible Links

A fusible link is a section of wire, usually two gauge sizes
smaller than the circuit it protects. If the current rating of a fus-
ible link is compromised, the fusible link will melt open.A spe-
cial insulation prevents wire fires, and swells when heated to
indicate the position of the open in the wire.

Circuit Breakers

Circuit breakers are electrical mechanical devices that will act
as a fuse to prevent excess current flow in a circuit. Unlike
fuses, the mechanical opening of contacts stops current flow.
The contacts will reset in a short period of time. This process
will repeat until the current excess is stopped.

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Electrical System

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Fuses

The most common protector in the vehicle electrical circuit is a
fuse. A fuse is a metallic connector within a circuit that is made
of a low melting point metal that acts as a “weak link”. If cur-
rent rises above the fuses’ rating, the metal will melt and sepa-
rate, leaving an open. The fuse is surrounded by a non
flammable plastic covering, and will limit current to a specific
amperage. Fuses can be found in range from 1A to 30A, and be
mini or maxi types. All fuses used on the HUMMER® are
blade type.

Circuit Controllers

Circuit controllers are used to turn the current off and on in a
circuit. Controllers can be mechanical or solid state.

Solid state controllers combine the use of semi conductors
along with electromechanical devices to control current in a
circuit. Solid state controllers are typically associated with
computers, and engine control systems. Most solid state con-
trollers are specific to a purpose.

Mechanical controllers are the most common type, and can be
seen as switches or relays. Switches are a primary controller
while relays are typically secondary controllers.

Primary mechanical controllers are very simple contacts that
are either open or closed, and can be changed state. Primary
controllers are usually limited in their ability to handle large
current flows due to the restriction of size.

Secondary mechanical controllers are used in conjunction with
the primary controllers to handle larger current draws in a cir-
cuit. Primary controllers are used to operate the secondary con-
trollers from a remote location, placing less or no restriction on

the size of a secondary controller. The secondary controller is
usually a relay.

Circuit Faults

The following are the four electrical fault conditions that cause
a malfunction in a circuit: open, short, short to ground, and
high resistance connection

Open

An open circuit occurs whenever there is a break in the circuit
continuity. The break can be caused by a connector disconnect,
a broken wire, or a defective component (Figure 12-1).

Short

A short circuit happens when the current bypasses part of the
normal circuit. This bypassing is usually caused by wire pinch-
ing or chaffing. Usual symptoms are inappropriate activation
or deactivation of a load device (Figure 12-2).

Short To Ground

A grounded circuit is also a short circuit, except the current
flows directly to ground with very little restriction. This is usu-
ally caused by wire pinching or chaffing against the frame or
body (Figure 12-3).

High Resistance Connection

A high resistance connection is an electrical connection that is
corroded or loose. High resistance connections cause a de-
crease in current flow that can affect the proper operation of an
electrical load.

Figure 12-1: Open Circuit

Figure 12-2: Short Circuit

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Electrical System 12-5

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Figure 12-3: Grounded Circuit

DIAGNOSTIC TOOLS

Digital Multimeter

A digital multimeter (J–39200) is required to safely test for
electrical malfunctions on the Hummer. Due to the complexity
of the electrical system, a test light should not be used to test
electrical circuits.

• Test lights do not have current limiting capabilities. The

use of a test light may actually cause an electronic com-
ponent to fail.

• Sharp test light probes may break wire strands, causing

circuit failure.

• Breaks in the insulation allow moisture and contami-

nants to enter connectors and components, increasing
the chances for corrosion. Even a small increase in resis-
tance can give false readings from a sensor to an elec-
tronic component.

A digital multimeter performs all the tests a test light can per-
form with a greater degree of accuracy. In addition, a multime-
ter can be used to test for current in a circuit.

DIAGNOSTIC TESTS

NOTE:

Follow all manufacturer’s recommendations when

testing for current. All multi-meters have a maximum current
rating. Not all multimeters contain a fuse that protects the mul-
timeter from excess current draw.

Amperage Test

Use caution when testing for current. Always check multimeter
owner's manual for maximum current to be tested. Most multi-
meters are fuse-protected when measuring current. However,
some meters are not protected and therefore can be damaged
by excessive current. Position multimeter leads after closed
switch and before load (Figure 12-4).

Voltage Test

Multimeters have a number of different voltage scales to
choose from. Always use the lowest scale possible to test the

circuit. For example, if you select the 200 volt scale and you
are testing for battery voltage, most multimeters will display
12 volts. By selecting the next smallest scale, 20 volts, the dis-
play will read 12.8 volts, a more accurate measurement. Posi-
tion multimeter leads on each side of the load (Figure 12-5).

OHM Test

CAUTION:

Before using a multimeter, ensure the circuit is not

energized. Even a small voltage applied to an multimeter will
damage it.

Continuity - Testing for continuity in a circuit requires the use
of the lowest ohm scale available. Position the multimeter
leads on each side of the circuit or component being tested. A
reading of less than one ohm is acceptable continuity
(Figure 12-6).

Resistance - To test for resistance, first touch the meter leads
together to ensure that the meter zeros out, then position the
leads of the multimeter on each side of the circuit or compo-
nent. Adjust the multimeter ohm setting until an acceptable
reading is observed. Verify the reading with the specification.

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Electrical System

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Figure 12-4: Amperage Test

Figure 12-5: Voltage Drop Test

Figure 12-6: Ohm Test

GROUND

BATTERIES

BREAKER

SWITCH

(CLOSED)

MULTIMETER

LOAD (I.E. LIGHT, MOTOR,

SOLENOID, ETC.)

GROUND

CIRCUIT

GROUND

BATTERIES

BREAKER

SWITCH

(CLOSED)

MULTIMETER

GROUND

CIRCUIT

GROUND

BATTERIES

BREAKER

SWITCH

(OPEN)

MULTIMETER

LOAD

GROUND

CIRCUIT

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