Measuring current

with ohmmeter:

NOTE: above all ensure that the component has been disconnected from the power supply.

Set the Multimeter to measure ohms and set it so that when the prods are touched together a reading of 0 Ω can be taken

Remove the fuse from the affected circuit (which will be burnt out) and disconnect the charge.

Connect the prod of the instrument to the terminal of the fuse nearest the charge and the other to a suitable ground point. If the ohmmeter shows a resistance of 0 Ω, or is very low, part of the circuit will have been short circuited to ground (bared, worn, pinched wire etc.). If the resistance is ∞ (infinite), then in that particular stretch the circuit is whole. The affected part can be easily found by moving the wiring to identify in which position the resistance value ceases to be ∞ (infinite).

NOTE measurements taken with a voltmeter are more accurate and if both options are possible the voltmeter could be chosen

MEASURING CURRENT

It will sometimes be necessary to take reading of the current absorption, in which case the Multimeter will not suffice. It is therefore necessary to use another instrument, for example a snap on ammeter, operating as follows:

• insert the lead of which the current is to be measured into the pincers, suitably connected to the multimeter set to measure volts (mV);
• NOTE: ensure that the flow of current (from positive towards ground) is the same as indicated on the pincers;
• take a reading in mV which coincides with the value of the current in A.

TESTING COMPONENTS:

BULBS:

NOTE: a bulb is characterized by two values, voltage and wattage rating. The resistance of the bulb is lower as its wattage increases. Example: a headlight bulb (12V-45W) will have a resistance which is much lower than an instrument panel warning light bulb (12V-3W).

To check whether a bulb is damaged or not, remove it and connect the prods of a Multimeter, set to measure ohms, to the terminals of the lamp itself. A finite resistance value (quite large as indicated above) indicates that the bulb is working while a resistance value of ∞ (infinite) signifies that the filament of the bulb is interrupted.

N.B. An easier way to check whether a bulb is damaged or not without removing it, is to connect a 12 V power source to the terminal connected to the vehicle's power supply and see if the bulb lights up.

FUSES:

A fuse is an electrical conductor the cross section of which is such that if the load passing through the cable exceeds a certain value, called fuse amperage, it will blow and interrupt the circuit.

If it is not possible to verify whether the filament is intact or not, it can be checked by connecting the prods of a Multimeter, set to measure ohms, to the terminals of the fuse. A zero value (0 Ω) indicates that the fuse is good.