Circuit Faults - Hummer H1 Service Manual

12-4 Electrical System
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
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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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