Voltmeter

Voltmeter:

Meter Scales:

The voltmeter (properly observed) will give the technician more information than the ammeter, ohmmeter, and test lamp combined. Its application for diagnosis here is to measure the electrical pressure (voltage) drop in a resistance circuit. Voltage drop is a reduction or "using up" of the voltage to push electricity through a resistance. It can be compared to the pressure of water flowing through a metering valve.

Low voltage to a lamp makes the lamp glow dimly. This can be caused by low source voltage (battery discharge or low alternator output) or by high resistance in the circuit due to a poor connection. Before making any meter measurements, it is important to review the relationship between current, voltage and resistance.

Determining voltage drop is important because it provides the following information:
^ Too high of a voltage drop indicates excessive resistance. If, for instance, a blower motor runs too slowly or a lamp glows too dimly, one can be sure there is excessive resistance in the circuit. By taking voltage drop readings in various parts of the circuit, the problem (corroded terminals, for example) can be isolated.
^ Too low of a voltage drop indicates low resistance. If, for instance, a blower motor runs too fast, the problem could be isolated to a low resistance in a resistor pack by taking voltage drop readings.
^ Maximum allowable voltage drop under a load is critical, especially if there is more than one high resistance problem in a circuit. It is important because, like all resistances, all voltage drops are cumulative. Corroded terminals, loose connections, and similar problems reduce the voltage available across the key circuit components. The current flow is reduced in the circuit, and all of the affected components operate at less than peak efficiency. A small drop across wires (conductors), switches, connectors, etc., is normal. This is due to the resistance of the conductors but should be less than 10 percent of the total drop.

When using a voltmeter:
^ Be sure to connect the positive lead to the battery side and the negative lead to the ground side of the component being checked.
^ Voltage drop occurs when electricity (current) flows through a resistance. Make sure the voltage drop being measured is only through the component being checked, not through the component and a poor connection.
^ The circuit must be operating (lamp ON or motor running, for example) to measure voltage drop.

The instrument panel voltmeter in the vehicle should also be observed for monitoring proper operation of the generator, battery, cranking motor, and cranking circuit. In this application, battery voltage drop can be monitored while the engine is cranking; after the engine is running, generator output voltage can be monitored. This can be a valuable first step before diagnosing other electrical problems.