Multimeters - Ford Ranger Maintaince And Repair Manual

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
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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 ohmmeter's 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.
Never use an ohmmeter to check the resistance of a component or wire while there is voltage applied to the
circuit.
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HOW TO USE THIS BOOK
WARNING
TEST LIGHTS