Using The Digital Multimeter; Test Meters - Navistar International DT 466 Diagnostic/Troubleshooting Manual

Fill in the numbers for the formula:
I = 12 V ÷ 2 ohms
I = 6 amps
The bulb in this circuit operates at 6 amps and is rated
at 6 amps. With 12 volts applied, the bulb will glow at
the rated output level (candlepower rating). However,
• If the voltage applied is low (low battery), the value
of E is lower, current flow will be less, and the bulb
will glow less brightly.
• If connections are loose or the switch is corroded,
the circuit resistance will be greater (value of R
will be larger), the current flow will be reduced,
and the bulb will glow less brightly.
Voltage drops are important for the following reasons:
• High voltage drops indicate excessive resistance.
For example, if a blower motor runs too slowly
or a light glows too dimly, the circuit may have
excessive resistance. Voltage drop readings can
isolate problems in parts of a circuit (corroded or
loose terminals, for example).
• Too low of a voltage drop indicates low resistance.
For example, if a blower motor runs too fast, the
problem could be low resistance in a resistor pack.
• Maximum allowable voltage drop under load
is critical, especially for more than one high
resistance problem.
circuit are cumulative.
loose connections, damaged wires or other
similar conditions create undesirable voltage
drops that decrease the voltage available across
the key components in the circuit.
resistance will decrease current flow in the circuit,
preventing other components from operating
at peak efficiency.
(conductors), connectors, switches, etc., is
normal because all conductors have some
resistance, but the total should be less than
10% of the total voltage drop in the circuit.

Using the Digital Multimeter

The following electrical test equipment should be
available for testing electronic circuits:
• Voltmeter
• Ohmmeter
Read all safety instructions in the "Safety Information" section of this manual before doing any procedures.
8 DIAGNOSTIC TOOLS AND ACCESSORIES
All voltage drops in a
Corroded terminals,
Increased
A small drop across wires
Follow all warnings, cautions, and notes.
© August 2008 Navistar, Inc.
• Ammeter
• Jumper wires
• Test lights

Test Meters

Figure 551 Typical Test Meters
Test meters come in a variety of models. Any working
model will be adequate for simple tests. However,
accurate readings are important.
test meter is of high quality. The Fluke 88 Digital
Multimeter (DMM) is recommended because it has
very little current and a high impedance (resistance)
of 10 megaohms (10 MΩ).
CAUTION: Only use a high impedance digital
multimeter when troubleshooting an electronic circuit.
Do not use any kind of battery powered test light.
Battery test lights can damage an electronic control
circuit.
NOTE: Some devices in an electronic control system
are not capable of carrying an appreciable amount of
current. Therefore, test equipment must be designed
to not damage any part the electronic control system.
Do not use analog meters unless specified. Analog
meters use too much current to test an electronic
control system.
EGES-270-1
571
Make sure the