8 DIAGNOSTIC TOOLS AND ACCESSORIES 571
Fill in the numbers for the formula:
I = 12 V ÷ 2 ohms
I=6amps
The bulb in this circuit operates at 6 amps and is rated
at 6 amps. With 12 volts app lied, 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.
Vo ltage 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. All voltage drops in a
circuit are cumulative. Corroded termina ls,
loose connect ions, da maged wires or other
similar conditions create undesirable voltage
drops that decrease the voltage available across
the key compone nts in the circuit. Increased
resistance will decrease current flow in the circuit,
preventing other components from operating
at peak efficiency. A sma ll drop across wires
(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 e lectronic circuits:
• Voltmeter
• Ohmmeter
• 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. Make sure the
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 des ign ed
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
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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