Ohmmeter - Navistar International DT 466 Diagnostic/Troubleshooting Manual

flows through the meter.
current flow only when the circuit is powered up and
operating. The DMM is fused to measure up to 10
amps using the 10 A connection point.
Before measuring current
approximately how many amps are in the circuit
to correctly connect the ammeter. The estimate of
current flow can easily be calculated. The resistance
of the light bulb is 2 ohms.
current flow will be 6 amps (6 amps = 12 V ÷ 2
ohms). If the fuse is removed and an ammeter is
installed with the switch closed, 6 amps of current will
be measured flowing in the circuit. Notice that the
ammeter is installed in series so that all the current in
the circuit flows through it.
WARNING: To avoid serious personal injury
or possible death, always make sure the power is
off before cutting, soldering, removing circuit
components, or before inserting the digital
multimeter for current measurements.
small amounts of current can be dangerous.
Excessive current draw means that more current
is flowing in a circuit than the fuse and circuit were
designed to handle. Excessive current draw will
open fuses and circuit breakers, and will also quickly
discharge batteries. An ammeter can diagnose these
conditions.
Reduced current draw will cause a device (an
electric window motor, for example) to operate poorly.
Increased circuit resistance will cause lower current
flow (often due to loose or corroded connections).

Ohmmeter

CAUTION: To prevent damage to the test meter, only
use the ohmmeter on circuits when the power is OFF.
Power from 12 V systems may damage the meter.
The ohmmeter measures resistance (ohms) in a
circuit. Ohmmeters use a small battery to supply
voltage and current flow through the circuit being
tested. Based on Ohm's Law, the ohmmeter
calculates resistance in the circuit by measuring
the voltage of the meter battery and the amount of
current flow in the circuit. Range selection and meter
adjustment are not necessary with the DMM.
Read all safety instructions in the "Safety Information" section of this manual before doing any procedures.
8 DIAGNOSTIC TOOLS AND ACCESSORIES
The ammeter measures
flow, determine
Applying Ohm's law,
Even
EGES-270-1
Follow all warnings, cautions, and notes.
© August 2008 Navistar, Inc.
Figure 558 Measuring resistance
Resistance measurements are used to determine
the resistance of a load or conductors, the value of
resistors and the operation of variable resistors.
To measure the resistance of a component or a
circuit, remove power from the circuit. Isolate the
component or circuit from other components and
circuits so that the meter current (from probe to
probe) only flows through the selected component or
circuit. When measuring the resistance of the load,
most of the current flow from the meter will go through
the indicator lamp because it has less resistance.
Remove one connector to the load. It is not always
apparent when a component must be isolated, so it
is a good practice to isolate a component or circuit
by disconnecting one circuit. Place the ohmmeter
leads across the component or circuit to display the
resistance in ohms. When checking a sensor or
variable resistor such as the fuel level gauge, heating
the element or moving the arm should move the meter
through a range of resistance that can be compared
to a specification.
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