A DMM should be used instead of a test lamp in order to test for voltage in high impedance circuits. While a test lamp shows whether voltage is present if the impedance is low enough, a DMM indicates how much voltage is present. In other words, if there is not enough current, the test lamp will not illuminate even though voltage is present.
There are many different ways to test car and truck electrical circuit. The test that needs to be done depends on the problem, the circuit, and the expected results.
Click below for descriptions of some of these tests.
How To Use A Volt / Ohm Meter