Control lamp (with a needle probe): This lamp can be used to determine the presence of voltage in a circuit. The brighter the lamp glows, the higher the voltage. The needle probe can be used to pierce the insulation of the wire being tested. The clamp on the lamp wire is attached to bare metal (housing). Be careful, the test lamp consumes a lot of power, so for measurements in electronic components (for example, in control devices) it is not suitable. In this case, you should use a LED tester.
LED voltage indicator: Depending on the design, this device can determine the presence of direct and alternating voltage from 6 to 700 volts. Voltage indication is optical, using LEDs.
Multimeter (combined electrical measuring instrument). With the help of a multimeter you can measure voltage, current (constant and variable) and resistance. As a rule, galvanic cells are used as a power source in multimeters (own power supply).
Voltage measurement: In order to, for example, measure the open-circuit voltage of a battery, the negative lead of the measuring device must be connected to the negative pole of the battery or the case. The positive lead of the device must be connected to the positive pole of the battery or to the measured circuit. If the device shows, for example, only 10.4 V, this indicates a short circuit in one of the battery cells. Check the battery voltage with the starter running - if the device shows a voltage of 5 volts, then there is something wrong with the battery.
Current measurement: In order to measure the current, it is necessary to break the electric circuit and connect the measuring device to the break point. In the process of a number of measurements when checking the condition of the car's electrical equipment, it is enough to remove the plug contact and connect the measuring device between the plug and the contact tongue. Be careful, pay attention to the measurement range of the multimeter. Such consumers as, for example, a starter consume very large currents, when measuring these currents, the device may fail.
Resistance measurement: with the help of a multimeter, you can check whether a particular wire or switch is conducting current. If the current flows without obstruction, the device shows zero resistance. If the circuit is broken at some point, the device will show infinite resistance. In addition, with the help of a multimeter, you can determine the internal resistance of parts and units.
(This publication is borrowed from the resource: «AUDImanual»)