Hummer H1 (2006+). Manual - part 333

12-8

Electrical System

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General

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

Parallel Circuits

The HUMMER H1 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 fusible link
is compromised, the fusible link will melt open.A special insulation prevents wire fires, and swells when heated to indicate the po-
sition 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.

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 current rises above the fuses’ rating, the metal will melt and separate, leav-
ing 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 H1 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 controllers 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 control-
lers 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 circuit.
Primary controllers are used to operate the secondary controllers 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 re-
sistance 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).

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 Electrical System  12-9

Short

A short circuit happens when the current bypasses part of the normal circuit. This bypassing is usually caused by wire pinching 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 usually
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 decrease in
current flow that can affect the proper operation of an electrical load. 

Figure 12-1:   Open Circuit

Figure 12-2:    Short Circuit

Figure 12-3:   Grounded Circuit

12-10

Electrical System

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DIAGNOSTIC TOOLS

Digital Multimeter

A digital multimeter (J–39200) is required to safely test for electrical malfunctions on the HUMMER H1. 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 component to 

fail.

• Sharp test light probes may break wire strands, causing circuit failure.

• Breaks in the insulation allow moisture and contaminants to enter connectors and components, increasing the chances for 

corrosion. Even a small increase in resistance can give false readings from a sensor to an electronic component.

A digital multimeter performs all the tests a test light can perform with a greater degree of accuracy. In addition, a multimeter 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 multimeter 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 multime-
ters are fuse-protected when measuring current. However, some meters are not protected and therefore can be damaged by exces-
sive 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 select-
ing the next smallest scale, 20 volts, the display will read 12.8 volts, a more accurate measurement. Position 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 dam-
age 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 component. Adjust the multimeter ohm setting until an acceptable reading is observed.
Verify the reading with the specification.

Figure 12-4:   Amperage Test

GROUND

BATTERIES

BREAKER

SWITCH

(CLOSED)

MULTIMETER

LOAD (I.E. LIGHT, MOTOR,

SOLENOID, ETC.)

GROUND

CIRCUIT

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 Electrical System  12-11

Figure 12-5:   Voltage Drop Test

Figure 12-6:   Ohm Test

CIRCUIT MAINTENANCE AND REPAIR

All electrical connections must be kept clean and tight. Loose or corroded connections may cause a discharged battery, weak start-
ing, dim lights, or possible electrical system damage.

Wires must be replaced or repaired if insulation becomes burned, cracked, or deteriorated. When replacing a wire, it is important
that the same gauge size wire be used. Refer to wiring diagram for proper wire gauge sizes. Never replace a wire with one of a
smaller size or replace a fusible link wire with a wire of a larger size. It should also be noted that fusible link wire utilizes a special
insulation covering. When replacing a fusible link wire, the replacement wire should be the type in accordance with SAE J156.
Further, fusible link wire should never be shortened or spliced. If a repair is necessary, entire fusible link wire must be replaced
with one of the proper gauge size, length and insulation type.

Any wire repair must maintain the waterproof integrity of the vehicle. Any splice located below the 30 in. (76 cm.) fording level or
in a high splash area must be waterproof and heavy duty adhesive wall shrink tubing should be used as a minimum in these areas.

Each harness or wire must be held securely in position to prevent damage to insulation caused by vibrating and chafing.

NOTE: Before performing any wire repair, disconnect battery ground cable.

Ground Point legend

G1 - Engine ground-Intake Manifold

G2 - Ground Buss Exterior Fuse Box

G3 - Body Ground

G4 - Instrument Panel Ground Buss Interior

G5 - Frame Ground

GROUND

BATTERIES

BREAKER

SWITCH

(CLOSED)

MULTIMETER

GROUND

CIRCUIT

GROUND

BATTERIES

BREAKER

SWITCH

(OPEN)

MULTIMETER

LOAD

GROUND

CIRCUIT