Ohmmeters - Continued

Wheeled Vehicle Electrical Systems
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An ohmmeter is actually an ammeter that measures current flow in thousandths of an ampere.

This type of ammeter is referred to as a milliammeter. Most ohmmeters consist of the

milliammeter, a battery, some form of current-limiting resistance unit, and the case. In most

ohmmeters, the resistance unit is actually two resistors. One is a fixed resistor and the other is a

variable resistor (rheostat). The simple ohmmeter has two test leads connected to two terminals.

The ohmmeter has many uses. By measuring resistance, it locates shorted or open circuits and

checks circuit continuity (zero resistance). For example, consider a wire that is broken somewhere

inside a wiring harness. You can see the ends of the wire where they connect in the circuit, but

the rest of the wire is hidden inside the harness. The ohmmeter can check the continuity of the

wire and prove if it is broken.

Use the following procedures when beginning a test with the ohmmeter:

Remove the ohmmeter test leads from the case and connect them to the two terminals on

the ohmmeter. If the wire to be tested is in a vehicle, turn off all electricity in the

vehicle's electrical system. THIS IS IMPORTANT. The ohmmeter has its own low-

voltage battery, and the meter will be burned out if higher voltage enters it.

The next step is to zero the ohmmeter. When the meter is not in use, the pointer or

hand will be on the infinity mark. To zero the meter, touch the test lead ends together to

complete a circuit. If the meter hand does not move to zero, turn the zero-ohm

adjustment knob until it does. This changes the resistance of the variable resistor.

You will find as the meter ages that you have to adjust the zero-ohm adjustment knob

more and more to make up for the weaker battery. When the hand will not come back

to zero with any adjustments, the battery is too weak and must be replaced.

When the ohmmeter is zeroed, connect one lead to each end of the wire to be tested. If

the wire is not broken, the tested wire will complete the circuit through the ohmmeter.

The hand will move to zero. If the wire is broken, the circuit will not be completed. The

hand will not move.