1991 1999 ford explorer chilton User Manual

							
current were to reach the load in the circuit, the surge could burn it out or severely damage it. It can also
overload the wiring, causing the harness to get hot and melt the insulation. To prevent this, fuses, circuit
breakers and/or fusible links are connected into the supply wires of the electrical system. These items are
nothing more than a built-in weak spot in the system. When an abnormal amount of current flows through the
system, these protective devices work as follows to protect the circuit:
Fuse-when an excessive electrical current passes through a fuse, the fuse blows (the conductor
melts) and opens the circuit, preventing the passage of current.
· 
Most vehicles use one or more fuse panels. This one is located on the drivers side kick panel
Circuit Breaker-a circuit breaker is basically a self-repairing fuse. It will open the circuit in the same
fashion as a fuse, but when the surge subsides, the circuit breaker can be reset and does not need
replacement.
· 
Fusible Link-a fusible link (fuse link or main link) is a short length of special, high temperature
insulated wire that acts as a fuse. When an excessive electrical current passes through a fusible link,
the thin gauge wire inside the link melts, creating an intentional open to protect the circuit. To repair
the circuit, the link must be replaced. Some newer type fusible links are housed in plug-in modules,
which are simply replaced like a fuse, while older type fusible links must be cut and spliced if they
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652 PROTECTIVE DEVICES 
						

							melt. Since this link is very early in the electrical path, its the first place to look if nothing on the
vehicle works, yet the battery seems to be charged and is properly connected.
CAUTION
Always replace fuses, circuit breakers and fusible links with identically rated components. Under no
circumstances should a component of higher or lower amperage rating be substituted.
SWITCHES & RELAYS
Switches are used in electrical circuits to control the passage of current. The most common use is to open and
close circuits between the battery and the various electric devices in the system. Switches are rated according
to the amount of amperage they can handle. If a sufficient amperage rated switch is not used in a circuit, the
switch could overload and cause damage.
The underhood fuse and relay panel usually contains fuses, relays, flashers and fusible links
Some electrical components which require a large amount of current to operate use a special switch called a
relay. Since these circuits carry a large amount of current, the thickness of the wire in the circuit is also
greater. If this large wire were connected from the load to the control switch, the switch would have to carry
the high amperage load and the fairing or dash would be twice as large to accommodate the increased size of
the wiring harness. To prevent these problems, a relay is used.
Relays are composed of a coil and a set of contacts. When the coil has a current passed though it, a magnetic
field is formed and this field causes the contacts to move together, completing the circuit. Most relays are
normally open, preventing current from passing through the circuit, but they can take any electrical form
depending on the job they are intended to do. Relays can be considered remote control switches. They allow
a smaller current to operate devices that require higher amperages. When a small current operates the coil, a
larger current is allowed to pass by the contacts. Some common circuits which may use relays are the horn,
headlights, starter, electric fuel pump and other high draw circuits. HOW TO USE THIS BOOK
SWITCHES & RELAYS 653 
						

							
Relays are composed of a coil and a switch. These two components are linked together so that when one operates, the
other operates at the same time. The large wires in the circuit are connected from the battery to one side of the relay
switch (B+) and from the opposite side of the relay switch to the load (component). Smaller wires are connected from the relay coil to the control switch for the circuit and from the opposite side of the relay coil to ground
Click to enlarge
LOAD
Every electrical circuit must include a load (something to use the electricity coming from the source).
Without this load, the battery would attempt to deliver its entire power supply from one pole to another. This
is called a short circuit. All this electricity would take a short cut to ground and cause a great amount of
damage to other components in the circuit by developing a tremendous amount of heat. This condition could
develop sufficient heat to melt the insulation on all the surrounding wires and reduce a multiple wire cable to
a lump of plastic and copper.
WIRING & HARNESSES
The average vehicle contains meters and meters of wiring, with hundreds of individual connections. To
protect the many wires from damage and to keep them from becoming a confusing tangle, they are organized
into bundles, enclosed in plastic or taped together and called wiring harnesses. Different harnesses serve
different parts of the vehicle. Individual wires are color coded to help trace them through a harness where
sections are hidden from view.
Automotive wiring or circuit conductors can be either single strand wire, multi-strand wire or printed
circuitry. Single strand wire has a solid metal core and is usually used inside such components as alternators,
motors, relays and other devices. Multi-strand wire has a core made of many small strands of wire twisted
together into a single conductor. Most of the wiring in an automotive electrical system is made up of
multi-strand wire, either as a single conductor or grouped together in a harness. All wiring is color coded on
the insulator, either as a solid color or as a colored wire with an identification stripe. A printed circuit is a thin
film of copper or other conductor that is printed on an insulator backing. Occasionally, a printed circuit is HOW TO USE THIS BOOK
654 LOAD 
						

							
sandwiched between two sheets of plastic for more protection and flexibility. A complete printed circuit,
consisting of conductors, insulating material and connectors for lamps or other components is called a printed
circuit board. Printed circuitry is used in place of individual wires or harnesses in places where space is
limited, such as behind instrument panels.
Since automotive electrical systems are very sensitive to changes in resistance, the selection of properly sized
wires is critical when systems are repaired. A loose or corroded connection or a replacement wire that is too
small for the circuit will add extra resistance and an additional voltage drop to the circuit.
The wire gauge number is an expression of the cross-section area of the conductor. Vehicles from countries
that use the metric system will typically describe the wire size as its cross-sectional area in square
millimeters. In this method, the larger the wire, the greater the number. Another common system for
expressing wire size is the American Wire Gauge (AWG) system. As gauge number increases, area decreases
and the wire becomes smaller. An 18 gauge wire is smaller than a 4 gauge wire. A wire with a higher gauge
number will carry less current than a wire with a lower gauge number. Gauge wire size refers to the size of the
strands of the conductor, not the size of the complete wire with insulator. It is possible, therefore, to have two
wires of the same gauge with different diameters because one may have thicker insulation than the other.
It is essential to understand how a circuit works before trying to figure out why it doesnt. An electrical
schematic shows the electrical current paths when a circuit is operating properly. Schematics break the entire
electrical system down into individual circuits. In a schematic, usually no attempt is made to represent wiring
and components as they physically appear on the vehicle; switches and other components are shown as simply
as possible. Face views of harness connectors show the cavity or terminal locations in all multi-pin
connectors to help locate test points.
CONNECTORS
Three types of connectors are commonly used in automotive applications-weatherproof, molded and hard
shell.
Hard shell (left) and weatherproof (right) connectors have replaceable terminals
Weatherproof-these connectors are most commonly used where the connector is exposed to the
elements. Terminals are protected against moisture and dirt by sealing rings which provide a
weathertight seal. All repairs require the use of a special terminal and the tool required to service it.
Unlike standard blade type terminals, these weatherproof terminals cannot be straightened once they
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WIRING & HARNESSES 655 
						

							are bent. Make certain that the connectors are properly seated and all of the sealing rings are in place
when connecting leads.
Molded-these connectors require complete replacement of the connector if found to be defective.
This means splicing a new connector assembly into the harness. All splices should be soldered to
insure proper contact. Use care when probing the connections or replacing terminals in them, as it is
possible to create a short circuit between opposite terminals. If this happens to the wrong terminal
pair, it is possible to damage certain components. Always use jumper wires between connectors for
circuit checking and NEVER probe through weatherproof seals.
· 
Hard Shell-unlike molded connectors, the terminal contacts in hard-shell connectors can be replaced.
Replacement usually involves the use of a special terminal removal tool that depresses the locking
tangs (barbs) on the connector terminal and allows the connector to be removed from the rear of the
shell. The connector shell should be replaced if it shows any evidence of burning, melting, cracks, or
breaks. Replace individual terminals that are burnt, corroded, distorted or loose.
· 
Weatherproof connectors are most commonly used in the engine compartment or where the connector is exposed to the elements
Test Equipment
Pinpointing the exact cause of trouble in an electrical circuit is most times accomplished by the use of special
test equipment. The following describes different types of commonly used test equipment and briefly explains
how to use them in diagnosis. In addition to the information covered below, the tool manufacturers
instructions booklet (provided with the tester) should be read and clearly understood before attempting any
test procedures.
JUMPER WIRES CAUTION
Never use jumper wires made from a thinner gauge wire than the circuit being tested. If the jumper wire is of
too small a gauge, it may overheat and possibly melt. Never use jumpers to bypass high resistance loads in a
circuit. Bypassing resistances, in effect, creates a short circuit. This may, in turn, cause damage and fire.
Jumper wires should only be used to bypass lengths of wire or to simulate switches.
Jumper wires are simple, yet extremely valuable, pieces of test equipment. They are basically test wires which
are used to bypass sections of a circuit. Although jumper wires can be purchased, they are usually fabricated
from lengths of standard automotive wire and whatever type of connector (alligator clip, spade connector or HOW TO USE THIS BOOK
656 CONNECTORS 
						

							
pin connector) that is required for the particular application being tested. In cramped, hard-to-reach areas, it
is advisable to have insulated boots over the jumper wire terminals in order to prevent accidental grounding. It
is also advisable to include a standard automotive fuse in any jumper wire. This is commonly referred to as a
fused jumper. By inserting an in-line fuse holder between a set of test leads, a fused jumper wire can be
used for bypassing open circuits. Use a 5 amp fuse to provide protection against voltage spikes.
Jumper wires are used primarily to locate open electrical circuits, on either the ground (-) side of the circuit or
on the power (+) side. If an electrical component fails to operate, connect the jumper wire between the
component and a good ground. If the component operates only with the jumper installed, the ground circuit is
open. If the ground circuit is good, but the component does not operate, the circuit between the power feed
and component may be open. By moving the jumper wire successively back from the component toward the
power source, you can isolate the area of the circuit where the open is located. When the component stops
functioning, or the power is cut off, the open is in the segment of wire between the jumper and the point
previously tested.
You can sometimes connect the jumper wire directly from the battery to the hot terminal of the component,
but first make sure the component uses 12 volts in operation. Some electrical components, such as fuel
injectors or sensors, are designed to operate on about 4 to 5 volts, and running 12 volts directly to these
components will cause damage.
TEST LIGHTS
A 12 volt test light is used to detect the presence of voltage in a circuit
The test light is used to check circuits and components while electrical current is flowing through them. It is
used for voltage and ground tests. To use a 12 volt test light, connect the ground clip to a good ground and
probe wherever necessary with the pick. The test light will illuminate when voltage is detected. This does not
necessarily mean that 12 volts (or any particular amount of voltage) is present; it only means that some
voltage is present. It is advisable before using the test light to touch its ground clip and probe across the
battery posts or terminals to make sure the light is operating properly.
WARNING
Do not use a test light to probe electronic ignition, spark plug or coil wires. Never use a pick-type test light to
probe wiring on computer controlled systems unless specifically instructed to do so. Any wire insulation that
is pierced by the test light probe should be taped and sealed with silicone after testing.
Like the jumper wire, the 12 volt test light is used to isolate opens in circuits. But, whereas the jumper wire is
used to bypass the open to operate the load, the 12 volt test light is used to locate the presence of voltage in a HOW TO USE THIS BOOK
JUMPER WIRES 657 
						

							
circuit. If the test light illuminates, there is power up to that point in the circuit; if the test light does not
illuminate, there is an open circuit (no power). Move the test light in successive steps back toward the power
source until the light in the handle illuminates. The open is between the probe and a point which was
previously probed.
The self-powered test light is similar in design to the 12 volt test light, but contains a 1.5 volt penlight battery
in the handle. It is most often used in place of a multimeter to check for open or short circuits when power is
isolated from the circuit (continuity test).
The battery in a self-powered test light does not provide much current. A weak battery may not provide
enough power to illuminate the test light even when a complete circuit is made (especially if there is high
resistance in the circuit). Always make sure that the test battery is strong. To check the battery, briefly touch
the ground clip to the probe; if the light glows brightly, the battery is strong enough for testing.
A self-powered test light should not be used on any computer controlled system or component. The
small amount of electricity transmitted by the test light is enough to damage many electronic
automotive components.
MULTIMETERS
Multimeters are an extremely useful tool for troubleshooting electrical problems. They can be purchased in
either analog or digital form and have a price range to suit any budget. A multimeter is a voltmeter, ammeter
and ohmmeter (along with other features) combined into one instrument. It is often used when testing solid
state circuits because of its high input impedance (usually 10 megaohms or more). A brief description of the
multimeter main test functions follows:
Voltmeter-the voltmeter is used to measure voltage at any point in a circuit, or to measure the voltage
drop across any part of a circuit. Voltmeters usually have various scales and a selector switch to allow
the reading of different voltage ranges. The voltmeter has a positive and a negative lead. To avoid
damage to the meter, always connect the negative lead to the negative (-) side of the circuit (to
ground or nearest the ground side of the circuit) and connect the positive lead to the positive (+) side
of the circuit (to the power source or the nearest power source). Note that the negative voltmeter lead
will always be black and that the positive voltmeter will always be some color other than black
(usually red).
· 
Ohmmeter-the ohmmeter is designed to read resistance (measured in ohms) in a circuit or
component. Most ohmmeters will have a selector switch which permits the measurement of different
ranges of resistance (usually the selector switch allows the multiplication of the meter reading by 10,
100, 1,000 and 10,000). Some ohmmeters are auto-ranging which means the meter itself will
determine which scale to use. Since the meters are powered by an internal battery, the ohmmeter can
be used like a self-powered test light. When the ohmmeter is connected, current from the ohmmeter
flows through the circuit or component being tested. Since the ohmmeters internal resistance and
voltage are known values, the amount of current flow through the meter depends on the resistance of
the circuit or component being tested. The ohmmeter can also be used to perform a continuity test for
suspected open circuits. In using the meter for making continuity checks, do not be concerned with
the actual resistance readings. Zero resistance, or any ohm reading, indicates continuity in the circuit.
Infinite resistance indicates an opening in the circuit. A high resistance reading where there should be
none indicates a problem in the circuit. Checks for short circuits are made in the same manner as
checks for open circuits, except that the circuit must be isolated from both power and normal ground.
Infinite resistance indicates no continuity, while zero resistance indicates a dead short.
· 
WARNING
Never use an ohmmeter to check the resistance of a component or wire while there is voltage applied to the
circuit. HOW TO USE THIS BOOK
658 TEST LIGHTS 
						

							Ammeter-an ammeter measures the amount of current flowing through a circuit in units called
amperes or amps. At normal operating voltage, most circuits have a characteristic amount of amperes,
called current draw which can be measured using an ammeter. By referring to a specified current
draw rating, then measuring the amperes and comparing the two values, one can determine what is
happening within the circuit to aid in diagnosis. An open circuit, for example, will not allow any
current to flow, so the ammeter reading will be zero. A damaged component or circuit will have an
increased current draw, so the reading will be high. The ammeter is always connected in series with
the circuit being tested. All of the current that normally flows through the circuit must also flow
through the ammeter; if there is any other path for the current to follow, the ammeter reading will not
be accurate. The ammeter itself has very little resistance to current flow and, therefore, will not affect
the circuit, but it will measure current draw only when the circuit is closed and electricity is flowing.
Excessive current draw can blow fuses and drain the battery, while a reduced current draw can cause
motors to run slowly, lights to dim and other components to not operate properly.
· 
Troubleshooting Electrical Systems
When diagnosing a specific problem, organized troubleshooting is a must. The complexity of a modern
automotive vehicle demands that you approach any problem in a logical, organized manner. There are certain
troubleshooting techniques, however, which are standard:
Establish when the problem occurs. Does the problem appear only under certain conditions? Were
there any noises, odors or other unusual symptoms? Isolate the problem area. To do this, make some
simple tests and observations, then eliminate the systems that are working properly. Check for
obvious problems, such as broken wires and loose or dirty connections. Always check the obvious
before assuming something complicated is the cause.
· 
Test for problems systematically to determine the cause once the problem area is isolated. Are all the
components functioning properly? Is there power going to electrical switches and motors. Performing
careful, systematic checks will often turn up most causes on the first inspection, without wasting time
checking components that have little or no relationship to the problem.
· 
Test all repairs after the work is done to make sure that the problem is fixed. Some causes can be
traced to more than one component, so a careful verification of repair work is important in order to
pick up additional malfunctions that may cause a problem to reappear or a different problem to arise.
A blown fuse, for example, is a simple problem that may require more than another fuse to repair. If
you dont look for a problem that caused a fuse to blow, a shorted wire (for example) may go
undetected.
· 
Experience has shown that most problems tend to be the result of a fairly simple and obvious cause, such as
loose or corroded connectors, bad grounds or damaged wire insulation which causes a short. This makes
careful visual inspection of components during testing essential to quick and accurate troubleshooting.
Testing
OPEN CIRCUITS HOW TO USE THIS BOOK
MULTIMETERS 659 
						

							The infinite reading on this multimeter indicates that the circuit is open
This test already assumes the existence of an open in the circuit and it is used to help locate the open portion.
Isolate the circuit from power and ground.
1. 
Connect the self-powered test light or ohmmeter ground clip to the ground side of the circuit and
probe sections of the circuit sequentially.
2. 
If the light is out or there is infinite resistance, the open is between the probe and the circuit ground.
3. 
If the light is on or the meter shows continuity, the open is between the probe and the end of the
circuit toward the power source.
4. 
SHORT CIRCUITS
Never use a self-powered test light to perform checks for opens or shorts when power is applied to the
circuit under test. The test light can be damaged by outside power.
Isolate the circuit from power and ground.
1. 
Connect the self-powered test light or ohmmeter ground clip to a good ground and probe any
easy-to-reach point in the circuit.
2. 
If the light comes on or there is continuity, there is a short somewhere in the circuit.
3. 
To isolate the short, probe a test point at either end of the isolated circuit (the light should be on or the
meter should indicate continuity).
4. 
Leave the test light probe engaged and sequentially open connectors or switches, remove parts, etc.
until the light goes out or continuity is broken.
5. 
When the light goes out, the short is between the last two circuit components which were opened.
6. 
VOLTAGE
This test determines voltage available from the battery and should be the first step in any electrical
troubleshooting procedure after visual inspection. Many electrical problems, especially on computer
controlled systems, can be caused by a low state of charge in the battery. Excessive corrosion at the battery
cable terminals can cause poor contact that will prevent proper charging and full battery current flow.
Set the voltmeter selector switch to the 20V position.
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660 OPEN CIRCUITS 
						

							Connect the multimeter negative lead to the batterys negative (-) post or terminal and the positive
lead to the batterys positive (+) post or terminal.
2. 
Turn the ignition switch ONto provide a load.
3. 
A well charged battery should register over 12 volts. If the meter reads below 11.5 volts, the battery
power may be insufficient to operate the electrical system properly.
4. 
VOLTAGE DROP
This voltage drop test revealed high resistance (low voltage) in the circuit
When current flows through a load, the voltage beyond the load drops. This voltage drop is due to the
resistance created by the load and also by small resistances created by corrosion at the connectors and
damaged insulation on the wires. The maximum allowable voltage drop under load is critical, especially if
there is more than one load in the circuit, since all voltage drops are cumulative.
Set the voltmeter selector switch to the 20 volt position.
1. 
Connect the multimeter negative lead to a good ground.
2. 
Operate the circuit and check the voltage prior to the first component (load).
3. 
There should be little or no voltage drop in the circuit prior to the first component. If a voltage drop
exists, the wire or connectors in the circuit are suspect.
4. 
While operating the first component in the circuit, probe the ground side of the component with the
positive meter lead and observe the voltage readings. A small voltage drop should be noticed. This
voltage drop is caused by the resistance of the component.
5. 
Repeat the test for each component (load) down the circuit.
6. 
If a large voltage drop is noticed, the preceding component, wire or connector is suspect.
7. 
RESISTANCE HOW TO USE THIS BOOK
VOLTAGE 661