342 Section 3 Fundamentals of Electricity and Electronics
Short Circuit
A short circuit is caused by a conductor acciden-
tally touching ground or another conductor. Shorts
are caused by wire insulation damage. There are
several classifi cations of shorts, and each causes dif-
ferent symptoms.
A hot wire short to ground produces low circuit
resistance and extremely high current. This usually
blows the fuse or circuit breaker. The fuse or circuit
breaker protects the circuit from excess current dam-
age caused by the hot wire short. See Figure 22-8A.
A short in an unprotected section of a circuit can
cause an electrical fi re when excess current overheats
a conductor and melts its insulation. The melted
insulation then allows other conductors to come into
contact. As a result, several wires can begin to burn.
This can happen in a matter of seconds.
A grounded circuit is caused by a ground wire
shorted to ground. This usually has no notice-
able effect. The short just provides another path to
ground. See Figure 22-8B.
Figure 22-9 shows how a short in different loca-
tions in a circuit can cause different symptoms. As
shown in Figure 22-9A, a short after a load does not
blow the fuse. There is still a resistance unit (bulb)
between the power source and short to limit cur-
rent. However, since the short is between two loads,
it shunts current away from the second load. The
second load (bulb) will not function. The current
seeks the path of least resistance through the short
to ground.
Figure 22-9B shows how a short before a switch
can prevent a circuit from being shut off. A short
before the grounded switch allows current in the
circuit, even with the switch off.
Figure 22-9C shows an example of a wire-to-
wire short. The conductors in two wires are touch-
ing because of damaged insulation. In this case, the
short could make both loads (bulbs) function even
when the switch to one of the loads is turned off.
Intermittent Problems
An intermittent problem is one that occurs only
under certain conditions. The problem may be pres-
ent one time but may not show up later. This makes
testing diffi cult because the symptoms may not
occur when you are troubleshooting.
Many intermittent problems are caused by poor
electrical connections. For example, two terminals in a
connector may be loose. Most of the time, the two ter-
minals make contact and the circuit functions normally.
However, when the vehicle hits a bump, the jolt may
separate the loose terminals, causing the circuit to mal-
function temporarily. An intermittent failure can result.
Using Testing Devices
This section of the chapter summarizes the use of
electrical testing devices, including jumper wires,
test lights, and multimeters. To be an effective tech-
nician, you must know how to use this equipment.
Figure 22-7. Some sensors, such as this oxygen sensor,
produce an output of less than one volt. Any resistance in the
circuit leading to the sensor could produce false signals for the
computer and upset engine or other system operation.
Figure 22-8. These are examples of short circuits. A—When a hot
wire is shorted to ground, it causes very high current that blows
the fuse. Both bulbs fail to function because of the open fuse. B—
When a ground wire is shorted to ground, it does not affect the
circuit. The short just provides a second path for ground.
Both bulbs off
No resistance
in circuit
Circuit not
affected
A
B
Short in
ground
Fuse
blows
Hot Wire Short to Ground
Shorted Ground or Grounded Circuit