Remember, if you are testing a wire/component that doesn't have 12V+ (so it isn't hot), then you have to use the Ohm function (measures resistance/continuity) on the multi-meter.
The difference is that when using the Volt function (when you are measuring a hot wire), you are relying on the 12V+ power from the vehicle's battery to take the measurement (and the circuit is completed by grounding the black (-) lead of the meter...then current can flow through the device).
When you use the Ohm function, the meter itself is providing the power to take the measurement. So, for example, to test that a wire is making good ground to the vehicle's chasis, then you would connect the red(+) lead from the meter to the wire end closest to the component it is supposed to ground, and then connect the black(-) lead from the meter to the vehicle chasis (to complete the ground circuit). The meter sends out just enough current (if it is a good circuit) to be able to see itself. Sometimes you have to use a 'jumper wire' (an extension wire) to add on to either the black(-) lead from the meter (or both leads, for that matter) in order to make the lead long enough to get it where you want to make contact to close the circuit.
Don't use the Ohm function to test a 12V+ hot wire!!
The difference is that when using the Volt function (when you are measuring a hot wire), you are relying on the 12V+ power from the vehicle's battery to take the measurement (and the circuit is completed by grounding the black (-) lead of the meter...then current can flow through the device).
When you use the Ohm function, the meter itself is providing the power to take the measurement. So, for example, to test that a wire is making good ground to the vehicle's chasis, then you would connect the red(+) lead from the meter to the wire end closest to the component it is supposed to ground, and then connect the black(-) lead from the meter to the vehicle chasis (to complete the ground circuit). The meter sends out just enough current (if it is a good circuit) to be able to see itself. Sometimes you have to use a 'jumper wire' (an extension wire) to add on to either the black(-) lead from the meter (or both leads, for that matter) in order to make the lead long enough to get it where you want to make contact to close the circuit.
Don't use the Ohm function to test a 12V+ hot wire!!