Chapter 3 Brake Tools, Shop Equipment, and Service Information 37 Copyright by Goodheart-Willcox Co., Inc. is measured in ohms. Ohmmeters can only check an electri- cal circuit when no current is fl owing. An ohmmeter has two leads, which are connected in parallel to each side of the unit or circuit to be tested. Polar- ity (direction of current fl ow) is not important when check- ing resistance, except in the case of diodes or some computer circuits. Consult the manufacturer’s service manual before using an ohmmeter to check any computer control circuit. Most ohmmeters have selector knobs for checking various ranges of resistance values. Analog ohmmeters have a special knob to adjust the needle to zero before checking resistance. Th e needle should always be adjusted to zero whenever the range is changed. Voltmeter Voltmeters are used to check voltage potential between two points in an energized circuit. Th e circuit must have a source of electricity available before voltage can be checked. On some voltmeters, diff erent scales can be selected, depending on the voltage type and level to be measured. Always observe proper polarity when attaching a volt- meter’s leads. When a voltmeter is used on a modern vehicle with a negative ground, the negative lead should always be connected in parallel to the frame or other ground, and the positive lead to the positive part of the circuit. Ammeter An ammeter is used to check the current fl ow in a circuit. Ammeters can also be used to check the current draw of solenoids, motors, and other electrical devices, when they are suspected of drawing too much current. Most amme- ters have two terminals for the positive lead. One is used to check low amperage, usually under 1 ampere. Th e other terminal is connected to the meter through a high amper- age fuse and can be used to check circuits with current loads up to 10 amperes. Unlike ohmmeters and voltmeters, the ammeter must be connected in series to measure current. Heavy-duty ammeters are also used to check the amper- age draw of starters or other motors, and when checking the charge level of a battery. Most ammeters used in automotive service are part of a multimeter. Waveform Meters Th e pulse and waveform meter is normally used to mea- sure the control voltages applied to such devices as anti-lock brake solenoids. In most cases, the waveform frequency is controlled by the computer, and checking the waveform is a way of checking computer system operation. Abnormal waveforms or pulse rate changes in response to changes in sensor inputs indicate the input sensors, computer, and out- put devices are all working. Waveform meter functionality is usually built into sophisticated diagnostic scanners and scopes, Figure 3-26. Goodheart-Willcox Publisher Figure 3-23. Twelve-volt non-powered test light. The sharp- tipped probe fits easily into small areas. Never pierce wire insulation unless recommended by the manufacturer. Goodheart-Willcox Publisher Figure 3-24. Powered test light. If the circuit being tested is complete, the instrument will sound an audible alarm and illuminate LEDs. Goodheart-Willcox Publisher Figure 3-25. A combination multimeter for volts, ohms, and amps. This model includes a built-in inductive clamp for measuring amperage. Handle all electronic instruments with care.