Chevrolet TrailBlazer, Service Manual

Page: 352 / 677

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.
CAUTION
Never use an ohmmeter to check the resistance of a component or wire while there is
voltage applied to the circuit.
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.