574 8 DIAGNOSTIC TOOLS AND ACCESSORIES
Figure 556 Checking voltage drop
To check the voltage drop across a load device,
connect the positive lead of the v oltmeter to the
positive side of the device and the negative meter
lead to the negative side of the device.
With the device operating, this will measure the
voltage drop a cross the device. With only one device,
all of the voltage should be dropped at the device. In
any circuit, the voltage applied will equal the voltage
dropped in the circuit. If this circuit only dropped 9 V
across the load, it indicates the wires and connections
dropped 3 V, indicating excessive circuit resistance.
Ammeter
An ammeter measures current flow (amperage) in a
circuit. Amperes (or amps) are units of electron flow
that indicate how many electrons are passing through
the circu it. An amp is the unit of measu rement for the
current flow in the circuit.
Ohm’s Law states that the current flow is equal to the
circuit voltage divided by the total circuit resistance
(I = E ÷ R). Therefore, increasing the voltage also
increases the current flow. Any decrea se in resista nce
will also increase the current flow.
At normal o perating voltage, most circu its hav e a
characteristic amount of current flow (current draw).
Current draw can be measure d with an a m meter.
Valuable diagnostic information can be provided
byreferringtoaspecified cu rrent draw rating for a
component (electrical device), measuring the current
flow in the circuit, and then comp aring the two
measurements (the specified current draw versus the
actual measurement).
Figure 557 Installing the ammeter
An ammeter is connected in series with the load,
switches, resistors, etc., so that all of the current
EGES-270-1
Read all safety instructions in the "Safety Information" section of this manual before doing any procedures.
Follow all warnings, cautions, and notes.
© August 2008 Navistar, Inc.
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