03DelcoRemy Diagnostic Manual130004222020

delcoremy.com Page 5 CLAMP-ON TYPE AMMETER DP-1011 FIGURE 1-6. Clamp-On Type Ammeter 1-20. MEASURING RESISTANCE WITH THE OHMMETER. The ohmmeter is used to measure electrical resistance. The unit of measure is the ohm. The ohmmeter is connected across the unit or portion of the circuit for which the resistance is to be measured. The ohmmeter has its own power source, usually a small battery, which causes current to flow through the circuit to be measured. Since the meter power source (battery) voltage is known, the current is inversely proportional to the resistance of the circuit. The meter automatically computes and reads out the resistance: E (voltage) divided by I (current) equals R (resistance). 1-21. OHMMETER SELECTION. As with voltmeters and ammeters, both analog (needle) and digital ohmmeters are available. Digital ohmmeters are preferred for the same reasons as the others. Additionally, most digital ohmmeters have a diode scale which can be used to test diodes and transistors. 1-22. OHMMETER USE. Figures 1-7 through 1-9 illustrate some typical uses of the ohmmeter. CAUTION Typical heavy duty electrical system voltage (12 or 24-volts) could produce a current flow through the ohmmeter that could damage the meter. Never connect an ohmmeter to a circuit that is under voltage. Always disconnect the battery ground cable before using the ohmmeter. 1-23. As shown in Figures 1-7 and 1-8, the ohmmeter also is useful as a continuity meter. An infinite ohmmeter reading (See Figure 1-7) indicates an open circuit (no continuity). A very low (approaching zero) reading (See Figure 1-8) indicates continuity. Figure 1-9 illustrates some digital ohmmeter applications. FIGURE 1-7. Analog Ohmmeter Showing Open Circuit NOTE Most digital meters have a diode scale ( ) which must be used to check diodes or transistors rather than the ohm scale. 1-24. APPLYING AN ELECTRICAL LOAD WITH A CARBON PILE. (See Figure 1-10) The carbon pile is a variable resistor designed to carry high current. A carbon pile, along with a voltmeter and ammeter, is normally an integral part of a modern battery tester. In addition to load testing batteries, carbon piles are used to test starter and alternator circuits. FIGURE 1-8. Analog Ohmmeter Showing Continuity TOUCHING GROUND OHMMETER OPEN OHMETER DP-1012 TRUCK FRAME DP-1013